Impaginato Santilli LLAATTEE HHOOLLOOCCEENNEE EENNVVIIRROONNMMEENNTTAALL EEVVOOLLUUTTIIOONN OOFF TTHHEE UUPPPPEERR VVAALLLLEE DDEELL GGAALLLLOO ((CCEENNTTRRAALL AALLPPSS)):: AANN IINNTTEERRDDIISSCCIIPPLLIINNAARRYY SSTTUUDDYY MMaauurriizziioo SSaannttiillllii11,, MMaannuueellaa PPeellffiinnii11,, MMaarrccoo CCaacccciiaanniiggaa22,, RRoobbeerrttoo CCoommoollllii33 && CCeessaarree RRaavvaazzzzii44 1Dipartimento di Scienze della Terra, Sezione di Geologia e Paleontologia, Università di Milano, Via Mangiagalli 34, 20133 Milano (Italia); e-mail: maurizio.santilli@unimi.it; e-mail: manuela.pelfini@unimi.it 2Dipartimento di Biologia, Sezione di Botanica Sistematica, Università di Milano, Via Celoria 26, 20133 Milano e-mail: marco.caccianiga@unimi.it 3Dipartimento di Scienze dell’Ambiente e del Territorio, Università di Milano-Bicocca, Piazza della Scienza 1, 20126 Milano; e-mail: roberto.comolli@unimib.it 4CNR, Istituto per la Dinamica dei Processi Ambientali, Piazza Cittadella 4, 24129 Bergamo e-mail: cnrbg1@mediacom.it ABSTRACT Through an interdisciplinary study based on the application of geomorphological, stratigraphic, dendrochronological, pedological and phytosociological methods, the late Holocene environmental evolution of the valley bottom of the upper Valle del Gallo (upper Valtellina / upper Valle dello Spöl, Central Italian Alps), an area high in the mountains characterized by a strong environmental dynamism, has been reconstructed. The geomorphological survey and some stratigraphic sections prove that the studied area was occupied, probably from the Late-glacial period, by a lacustrine basin. Laminites, microturbidites, fluvial, deltaic and debris flow deposits formed by the pro- gradation of the surrounding fans deposited in the lake and buried it before the Middle Holocene, originating a flat primitive valley bot- tom. This original surface was subsequently carved by the regressive erosion of the main river and its tributaries which isolated, espe- cially since the Middle Ages, numerous surfaces from the more recent debris flow activity. Some fans were partially dismantled while in some depressed sectors a succession of silty deposits formed by sheetfloods accumulated. The adaptation and the reaction of the arboreal vegetation to the debris flow activity allowed us to apply dendrogeomorphological techniques thanks to which some events were dated and the recent morphologic evolution of one of the fans reconstructed. The ecological succession takes place on two types of carbonatic substratum characterising the study area: sandy gravel deriving from debris flows, silt and fine sands deriving from sheet- floods. The soils and vegetation of the upper Valle del Gallo are generally scarcely developed. The vegetation is dominated by Pinus montana forests at various degrees of evolution, following pioneer communities in which the pine plays a key role. The pedological study shows that pine forests develop on Leptosols, poor and dry grounds rich in rock fragments of the fans, only locally susceptible of evolving into Cambisols. These surfaces are frequently buried by debris flows that cover the grounds thus inducing regressive stages in the vegetational series. Yet trees often survive these events since they are able to heal their scars, straighten themselves up and to develop adventitious roots from the buried base of the stems and branches. On the contrary Fluvisols develop on sheetfloods silty deposits, where peculiar herbaceous communities predominate, adapted to a light periodic burial that does not kill the vegetation. Last but not least must have been the influence of the anthropic deforestation, that has probably contributed in a considerable way to the present landscape setting. SOMMARIO Evoluzione ambientale tardo-olocenica dell’alta Valle del Gallo (Alpi Centrali): uno studio interdisciplinare. Mediante uno studio interdi- sciplinare basato sull’impiego di metodi geomorfologici, stratigrafici, dendrocronologici, pedologici e fitosociologici, è stata ricostruita l’e- voluzione ambientale tardo-olocenica del fondovalle dell’alta Valle del Gallo (alta Valtellina/alta Valle dello Spöl - SO), un’area d’alta montagna contraddistinta da un’elevata dinamicità ambientale. Il rilevamento geomorfologico e alcune sezioni stratigrafiche documen- tano che l’area studiata era occupata, probabilmente a partire dal Tardiglaciale, da un bacino lacustre. In esso si sono depositate lami- niti, microtorbiditi, depositi deltizi, fluviali e di debris flows derivati dalla progradazione dei conoidi circostanti, che hanno interrato il lago prima della metà dell’Olocene, dando origine ad un primitivo fondovalle pianeggiante. Questa originaria superficie è stata successiva- mente intagliata dall’erosione regressiva del torrente principale e dei suoi affluenti, che ha isolato, soprattutto a partire dal Medioevo, diverse superfici dall’attività più recente dei debris flows. Alcuni conoidi sono stati parzialmente smantellati, mentre in alcuni settori depressi si è accumulata una successione di depositi limosi derivati da processi di sheetflood. Gli adattamenti e la reazione della vege- tazione arborea all’attività dei debris flows hanno consentito l’impiego di tecniche dendrogeomorfologiche mediante cui sono stati datati alcuni eventi ed è stata ricostruita l’evoluzione morfologica recente di uno dei conoidi. Sui due tipi di substrato carbonatico che caratte- rizzano l’area di studio, ghiaie sabbiose derivanti da debris flows, limi e sabbie fini derivanti da sheetfloods, ha luogo la successione ecologica. I suoli e la vegetazione dell’alta Valle del Gallo sono, in generale, scarsamente evoluti. La vegetazione è dominata da pinete a Pinus montana con gradi diversi di evoluzione, che seguono aggruppamenti pionieri in cui il pino gioca un ruolo chiave. Lo studio pedologico indica che le pinete si sviluppano su suoli poveri, scheletrici e asciutti di conoide, i Leptosols, solo localmente suscettibili di evolvere verso i Cambisols. Frequentemente queste superfici vengono sepolte da colate di detrito che ricoprono i suoli e inducono stadi regressivi nella serie vegetazionale. Tuttavia, spesso gli alberi sopravvivono a questi eventi poiché sono in grado di rimarginare le cica- trici, raddrizzarsi e sviluppare radici avventizie dalla base sepolta dei fusti e dei rami. Sui depositi limosi di sheetfloods, invece, si svilup- pano Fluvisols e prevalgono peculiari aggruppamenti erbacei adattati ad un leggero seppellimento periodico che non uccide la vegeta- zione. Non meno importante deve essere stata, infine, l’influenza del disboscamento antropico, che ha probabilmente contribuito in modo notevole all’assetto del paesaggio attuale. Keywords: Alluvial fans, Debris flows, Holocene stratigraphy, Dendrogeomorphology, Mountain pine, Alpine soils, Vegetation. Parole chiave: Conoidi alluvionali, Debris flows, Stratigrafia olocenica, Dendrogeomorfologia, Pino montano, Suoli alpini, Vegetazione. Il Quaternario Italian Journal of Quaternary Sciences 1155(2), 2002, 187-208 188 M. Santilli et alii 11.. IINNTTRROODDUUCCTTIIOONN This report deals with the late Holocene environ- mental evolution of the upper Valle del Gallo (upper Valtellina / upper Valle dello Spöl, Central Italian Alps). This area high in the mountains shows a peculiar land- scape characterized by a strong dynamic of the slopes, by the aggradation of fans mostly originated by debris flows and by a hydrographic network which is progressi- vely deepening. The upper Valle del Gallo is particularly suitable to study the frequency and the effects of some geomorphologic processes strictly associated with the atmospheric dynamics, especially with precipitation, on the environment. The reaction of vegetation to such a rapid geodynamics and to a specific bedrock geochemi- stry (carbonates) allowed us to analyse the ecological interactions between geomorphic and sedimentary pro- cesses, the soil evolution and the factors determining it (climate, human influen- ce, etc.). Moreover, this interdisciplinary research intends to promote the con- servation and the enhance- ment of this area located between two wide protec- ted areas, the Stelvio National Park and the Swiss National Park, showing at present a scant human presence. 22.. SSTTUUDDYY AARREEAA The Valle del Gallo is situated on the Northeast extremity of the Lombardy (Northern Italy), in the Stelvio National Park, (Fig. 1). The studied area extends from Passo di Fraele (1952 m) to Ponte di Pra Grata (1864 m) (Fig. 1), and is entirely included in the carbonatic succes- sion of the Norian age rela- ted to the Quattervals Nappe, except for some klippen belonging to the Scarl-Umbrail System for- ming some mountain tops (Pozzi & Giorcelli, 1960; Bonsignore et al., 1969; Berra, 1994). As showed by the analysis of the data (Santilli, 2000) from the meteorological station in Cancano (AEM Milano, 1948 m a.s.l., see Fig. 1), the climate of the upper Valle del Gallo corresponds to the Alpine continental one (Fig. 2). The average annual precipitation are scarce (822 mm), but, due to the altitude, the potential evapotranspiration is rather limited (428 mm). The soil water balance shows a small deficit in July. The carbonatic bedrock, the diffuse cover of coarse detrital sediments and the soils features redu- ce the available water capacity; consequently some ari- dity problems can occur during dry years. Due to the high altitude of the surrounding moun- tains (up to 3180 m), wide portions of the slopes are beyond the tree limit and are without vegetative cover. Hence the climate is favourable for a strong weathering which can produce large quantities of debris that, accu- mulating on the slopes, are susceptible of moving due to the frequent heavy summer storms. As regards vegetation, the studied area is included in the upper Subalpine Belt and is dominated by mono- specific Pinus montana forests. Pinus montana in the Fig. 1 - Location, hydrography and orography of the studied area. The Valle del Gallo covers the area northward the Passo di Fraele and belongs to the hydrographic basin of the Black Sea (through the Danube). The box shows the area represented in Fig. 5. Ubicazione, idrografia e orografia dell’area studiata. La Valle del Gallo si estende a Nord del Passo di Fraele e appartiene al bacino idrografico del Mar Nero (attraverso il Danubio). Il riquadro mostra l’area rappresentata in Fig. 5. broadest sense of the word (Pinus mugo aggregate, according to Minghetti, 1997; Pinus of the mugo group according to Pignatti, 1982; see also: Hess et al., 1967; Gaussen et al., 1993) includes two main species: Pinus mugo Turra 1764, having a dwarf habitus, which prefers carbonate substrata and grows in the Eastern Alps and in the Carpathian, and Pinus uncinata DC. in Lam & DC. 1805, having an arborescent habitus, which prefers sili- ceous substrata and grows in the Western Alps and in the Pyrenees. Moreover they are different for several morphological features such as the cone shape or the needle anatomy. Nevertheless, they frequently originate intermediate or hybrid forms hard to be taxonomically located. Mountain pines of the Valle del Gallo have a prevalent arborescent habitus (see Fig. 3), but even if this feature seems to be proper to Pinus uncinata DC., its geographical and ecological location is similar to that of Pinus mugo Turra, thus in the present study the generic definition of “mountain pine” is preferred. The valley was subject to an intense anthropic 189Late Holocene environmental evolution ... exploitation for the presence of rich iron deposits that surface on its slopes. The mining activity, documented from 1272 to 1875, helped the development of a prospe- rous industry, that brought about the building of roads, structures and furnaces for the mineral fusion (Agostoni,1923; Leoni, 1953a-b; Zazzi, 1982). The fur- naces were fed with coal produced in the several char- coal pits which are still disseminated all over the valley; the wood was obtained by felling the mountain pines and removing their stumps. The intense deforestation has probably had a great influence on the recent evolu- tion of the soil and vegetation. The bibliography available on the natural history of the Valle del Gallo is rather limited. General geomorpho- logical information was provided by Giorcelli (in Pozzi & Giorcelli, 1960) and by Saibene (1973). These authors pointed out a series of erosive cycles of the Neogene- Fig. 2 - Ombrothermic diagram by Walter & Lieth and soil water balance according to Thornthwaite on the basis of the data of the meteorological station in Cancano (AEM Milano, see Fig. 1). The temperature and precipitation course shows the absen- ce of an aridity period (P>2T) and a continental-like pluviome- tric system; the soil water balance shows a little deficit in July. The AWC calculated (35 mm) refers to a skeletal soil represen- tative of the fan. Diagramma ombrotermico di Walter & Lieth e del bilancio idrico del suolo secondo Thornthwaite in base ai dati della stazione meteorologica di Cancano (AEM Milano, cfr. Fig. 1): gli anda- menti delle temperature e delle precipitazioni indicano l’assen- za di un periodo di aridità (P>2T) ed un regime pluviometrico di tipo continentale; il bilancio idrico del suolo mostra un piccolo deficit nel mese di luglio. L’AWC calcolata (35 mm) si riferisce ad un suolo scheletrico rappresentativo dei conoidi. Fig. 3 - Remarkable scar on the stem of a mountain pine hit by a debris flow in 1959. The development of compression wood contributes to the trunk straightening, following its inclination. The box represents a stem section showing compression wood (darker in the lower part) and a corrasion scar (in the upper part); both of them date back to 1986. (photos by Santilli). Grossa cicatrice sul fusto di un pino montano investito da una colata di detrito risalente al 1959. Lo sviluppo di legno di com- pressione coadiuva il raddrizzamento del tronco successivo alla sua inclinazione. Il riquadro raffigura una sezione di fusto mostrante legno di compressione (più scuro in basso) e una cicatrice d’abrasione (in alto); entrambi risalgono al 1986 (foto Santilli). 190 Quaternary mainly caused by glaciations. Glacial ero- sion on the main valleys promoted a regressive erosion and fluvial captures which modified the ancient hydro- graphy. Useful information on vegetation is available in Furrer (1914), Furrer & Longa (1915), Braun Blanquet et al. (1954), Giacomini & Pignatti (1955), Pirola & Di Benedetto (1959), Giacomini (1960), Pirola (1973), Pedrotti et al. (1974), Minghetti (1996). 33.. MMAATTEERRIIAALLSS AANNDD MMEETTHHOODDSS The environmental setting of the upper Valle del Gallo was deduced from the integration of a detailed geomorphological survey with photointerpretation (pho- tograms no. 11-12-13, stripe 55, flight TEM 1 Regione Lombardia 1981, and no. 2571-2572, flight Alifoto 1975), and with stratigraphic, dendrochronological, pedological and vegetational studies. A general resear- ch was carried out on the whole valley sector and a more detailed study on the Valle della Casina fan, one of the lateral valleys. Moreover some historic informa- tion was provided by some photographs dating back to the end of the 19th century, and by a map of the land register of 1844 “Mappa del Comune Censuario di Valle di Dentro”. Seventeen stratigraphic sections were described. Some buried layers rich in organic matter and a charred wood were dated through radiocarbon. The definition of stratigraphic units is the one used by the International Stratigraphic Code (Salvador, 1994). Some debris flows of the Valle della Casina fan were dated using dendrogeomorphological techniques (e.g. see: Alestalo, 1971; Shroder, 1980; Heikkinen, 1994; Strunk, 1995) applied to mountain pines. More than two hundred increment cores were extracted from damaged Pinus montana, along the Valle della Casina fan channels and some cross-sections of stems and branches were made. The trees showed corrasion scars and/or compression wood on slanting stems, both cau- sed by the impact of debris carried by water on the stems (Fig. 3). The compression wood is produced by conifers in order to bring the stem back to its vertical position whenever its orientation is altered. The scars and the year of appearance of the compression wood were dated. Owing to the fact that the trees were usually heavily damaged, the ring recognition and counting on many samples were often difficult. Finally only 53 trees were useful for dating purposes. Some historical data together with daily precipitation recorded at the meteo- rological station in Cancano (after 1961) confirmed some episodes which had been dated, even if it is not so easy to correlate the occurrence of the debris flows with precipitation (e.g. see: Govi et al., 1985; Polloni et al., 1991; Govi et al., 1994; Flageollet et al., 1999; Van Asch et al. 1999). Thirty-two pedological profiles have been dug. For each horizon the color, the roots, the skeleton, the struc- ture, the texture, the boundaries and the effervescence to HCl have been considered (according to Sanesi, 1977). The analysis carried out in laboratory, on the fine earth, sifted at 2 mm, determined the organic carbon (oxidation with bichromate potassium), the pH (soil/water ratio 1:2.5), the total carbonates (gas-volu- metric method) and the apparent texture (by smoothing with an Esenwein’s pipette) (according to the Osservatorio Nazionale Pedologico, 1994). Soils have been classified according to the World Reference Base for Soil Resources taxonomic system (ISSS-ISRIC- FAO, 1998). Vegetation has been studied by using the phytoso- ciological method of the Zurich-Montpellier school (Braun-Blanquet & Jenny, 1926; Braun-Blanquet, 1964). Thirty-eight relevés have been processed by using the Braun-Blanquet cover-abundance scale modified by Pignatti (1952), in areas considered homogeneous from a geomorphological point of view. UPGMA has been used as clustering method to carry out the cluster analy- sis. A first analysis has been performed with the chord distance index using the cover values of the species transformed into numerical values (Van der Maarel, 1979). The following one has been carried out by using M. Santilli et alii Fig. 4 - Panoramic photo of the upper Valle del Gallo (see Fig. 5): Val Paolaccia fan (1), Valle della Casina fan (2), Val Mora fan (3), “Valle del Pizzo Aguzzo” fan (4; actually, the valley does not have any place-name), inactive fan (5a-5c) and active fan (5b) of Val Bruna, moraine (6) (photo by Santilli). Panoramica dell’alta Valle del Gallo (cfr. Fig. 5): conoide della Val Paolaccia (1), conoide della Valle della Casina (2), conoide della Val Mora (3), conoide della “Valle del Pizzo Aguzzo” (4; in realtà la valle è priva di toponimo), conoide inattivo (5a-5c) e conoide attivo (5b) della Val Bruna, morena (6) (foto Santilli). 191 the Jaccard coefficient and the data relevant to the spe- cies presence/absence. Sporadic species have been excluded from the clustering processing. The floristic nomenclature is the one used by Pignatti (1982). 44.. RREESSUULLTTSS 44..11.. GGeeoommoorrpphhoollooggiicc ssuurrvveeyy Debris flow fans with a complex structure most of which are coalescent and come from both the valley sides occupy the upper Valle del Gallo. (Fig. 4-5). Creeks, often active only during heavy rains, sink on the eastern slope of the fans near their apexes. On the con- trary, on the western slope they partially flow on their respective fan surface before sinking. This mirrors the northward/northward-eastward dipping of the strata (Saibene, 1973). At the valley head the Val Paolaccia fan comple- tely occupies the valley bottom (1 in Fig. 4-5) originating the Passo di Fraele. The Valle della Casina (2) and the Val Mora fans (3) extend on the eastern valley side Late Holocene environmental evolution ... Fig. 5 - Map of the fans and of the Quaternary deposits of the upper Valle del Gallo (see Fig. 4) (from the Regio- nal Technical Map 1:10.000 sections N° D1b3 “Lago di Livigno Sud” and N° D1c3 “S. Giaco- mo di Fraele”, realisation 1980- 83; coordinates are in the Gauss- Boaga system). Carta dei conoidi e dei depositi quaternari dell’al- ta Valle del Gallo (cfr. Fig. 4) (dalla Carta Tecnica R e g i o n a l e 1:10.000, sezioni N° D1b3 “Lago di Livigno Sud” e N° D1c3 “S. Gia- como di Fraele”, r e a l i z z a z i o n e 1980-83; le coor- dinate sono nel sistema Gauss- Boaga). 192 while the “Valle del Pizzo Aguzzo” (4) and the Val Bruna ones occupy the western slope (5). The latter is compo- sed of a central active portion (5b), a southern inactive part (5a) and a northern one (5c) located westwards of a hillock (6). According to Giorcelli (in Pozzi & Giorcelli, 1960) this hillock is a moraine deposited during the Gschnitz stadium (early Late-glacial). The Val Mora fan and the southern part of the Val Bruna one (5a) are now inactive surfaces bordered by scarps, up to 50-m high, facing the Val Bruna, Val Mora and Valle del Gallo the respective rivers of which flow at lower altitudes. These surfaces are considered (according to Martina, 1954) as the remains of an ancient flat valley bottom successively dissected by Acqua del Gallo, the river of the main val- ley, and its tributaries; other edges of the same valley bottom are represented by some small parts of the Valle della Casina and “Valle del Pizzo Aguzzo” fans. Near the Val Paolaccia fan, the river Acqua del Gallo carves silty deposits (see par. 4.2.3.). The border of terraces formed by the crack has a winding shape which is more evident down the valley and represents an area where erosion gives birth to small incisions separated by crests (see Fig. 8). The Valle della Casina fan (Figs. 4-5-6) extends nearly for 1 km between an altitude of 2020 and 1885 m a.s.l. There is a main channel (A in Fig. 6) near the northern margin; it is normally dry and marked by terraces separating other channels and inter- channel zones with Pinus montana fore- sts. A part of the channel is edged by a high debris terrace (D). Downhill the Val Mora road (1935 m) the channel still car- ves the fan until it forms a ten-meter deep scarp before flowing into Acqua del Gallo. Two secondary channels (B) and (C) originate from the main channel (A). Channels (A) and (B) border a zone with a discontinuous vegeta- tion and dwarf and damaged trees (E). Nearby there is another zone covered with a taller and thicker forest (F). Channel (G) origina- tes from the secon- dary channel (C). Finally, at the fan-toe a few meter above the creek Acqua del Gallo a terrace (H) exists. 44..22.. QQuuaatteerrnnaarryy ssttrraattiiggrraapphhyy The quaternary deposits cropping out on the valley bottom of the upper Valle del Gallo have been assigned to three lithostratigraphic units, the names of which derive from specific localities: the Acqua del Gallo Unit, the Valle della Casina Unit and the Val Paolaccia Unit (Fig. 5). 44..22..11.. AAccqquuaa ddeell GGaalllloo UUnniitt DESCRIPTION AND INTERPRETATION. The Acqua del Gallo Unit is a carbonatic clay sedimentary succession with a coarsening-upward sequence (Fig. 7). The lower part is formed by massive grey clays; the middle part by horizontally laminated clays alternating between levels of silty clays and fine sands in fining-upwards laminas. Deposits are interpreted as decanted laminites and microturbidites deposited in a lacustrine environment. The uppermost part of the succession alternates between graded and well-sorted sands, silt and silty clays. The succession is clast free. Plant remains and macrofossils were not observed. THICKNESS. The maximum visible thickness is 8 m (section 3; Figs. 5-6-7); the estimated thickness is at least 35 m, between 1885 and 1920. NAME. Its name comes from the creek that carved it and exposed the succession. LATERAL EXTENT AND RELATIONS WITH THE OTHER M. Santilli et alii Fig. 6 - Morphological sketch map of the Valle della Casina fan. The box shows the area represented in Fig. 11. Schema morfologico del conoide della Valle della Casina. Il riquadro mostra l’area rappresentata in Fig. 11. STRATIGRAPHIC UNITS. Boreholes carried out by AEM Milano at the Passo di Fraele (Giorcelli in Pozzi & Giorcelli, 1960) and the exposed sections along the river bed of Acqua del Gallo allow us to identify these lacu- strine deposits for more than 2 km from the inactive fan of Val Bruna (5a in Figs. 4-5) to Passo di Fraele. The uppermost part of the Acqua del Gallo Unit is interdigita- ted with alluvial deposits included in the Valle della Casina Unit. TYPE SECTION. Section 3 (Fig. 7; Appendix I); coor- dinates: 46° 34' 00'' N, 10° 14' 40'' E. AGE. No direct dating is available for this unit. Considering its stratigraphic position, it is older than 4710 ± 90 14C yr B.P. (Tab. 1; see par. 4.2.2.). PREVIOUS STUDIES. Giorcelli (in Pozzi & Giorcelli, 1960) mentions a paleolake originated by the erosion processes of the last glacial cycle that caused the for- mation of a watershed in the upper Valle del Gallo, and by the obstruction of the Val Paolaccia fan at the Passo di Fraele. 44..22..22.. VVaallllee ddeellllaa CCaassiinnaa UUnniitt DESCRIPTION AND INTERPRETATION. This unit (Fig. 5) is made up of several interdigitated bodies of gravelly deposits expanding on the valley bottom at the base of tributary valleys. Each element is composed of lenticular successions, scantily stratified, of clast-supported gravel beds with sandy matrix, occasionally separated by well- sorted sandy-silty levels. Both fine fraction and clasts are carbonatic but locally are siliceous erratic, meta- morphic clasts and iron ores. Clasts have a subangular or subrounded shape, are chaotic or have a weak strati- fication, with sands and a high primary porosity. Buried soils are common; brownish silty horizons rich in coal are sporadically embedded. The entire succession is markedly inclined towards the valley bottom and distinc- tly finingdownward. It can be considered as a series of wedge-shaped and coalescent debris flow fans. The uppermost part of section 3 (Acqua del Gallo Unit) shows the progradation of a fan covering gravelly and sandy alluvial deposits. The latter differ from the debris flow deposits because they are organized in channel bars, are better sorted, graded and partially stratified. They are included in the Valle della Casina Unit. NAME. The present study analyses in detail the fan of Valle della Casina. BOUNDARIES AND RELATIONS WITH THE OTHER STRATI- 193Late Holocene environmental evolution ... Table 1 - Radiocarbon dating. Date 14C. Fig. 7 - Section 3 of the Acqua del Gallo Unit (see Appendix I) Sezione 3 dell’Unità dell’Acqua del Gallo (cfr. Appendice I). GRAPHIC UNITS. The exposed upper surfaces are charac- terized by an active aggradation or by the development of Leptosols (locally Cambisols) (see par. 4.4.). The basal part of the unit is interdigitated with lacustrine deposits of the Acqua del Gallo Unit. Some fans are interrupted by river downcuttings, between which the Val Bruna Subunit (see further on) is embanked. A depressed sector in the southern part is covered by the Val Paolaccia Unit. The contact surface is marked by a buried Leptosol, locally eroded (see Fig. 10 and par. 4.2.3.). AGE. A thin layer rich in coal at the “Valle del Pizzo Aguzzo” fan-toe (see Fig. 6) was dated 4710 ± 90 14C yr B.P. (Tab. 1). A buried charred wood embedded in silty, reddish horizons rich in coal and partly pedogenized between gravelly layers in the upper part of the Valle della Casina fan (section 5 in Fig. 6) was dated 1720 ± 50 14C yr B.P. (Tab. 1). Some recent surfaces (since the end of the 19th century) were dated through dendrogeo- morphology. PREVIOUS STUDIES. These fans were mentioned by Martina (1954), Borgonovo (1955) and Giorcelli (in Pozzi & Giorcelli, 1960). SELECTED SECTIONS. No. 5 (coordinates: 46° 34' 10'' N, 10° 15' 00'' E; Appendix I), 6 (46° 34' 00'' N, 10° 15' 10'' E) and 19 (46° 34' 10'' N, 10° 14' 50'' E) (Fig. 6). SUBUNITS. The deposits embanked in the Valle della Casina Unit and separated from the latter by an erosional surface were assigned to the Val Bruna Subunit. The latter includes the active part of the Val Bruna fan (from which the name derives - 5b in Fig. 5) and the northern sector of the “Valle del Pizzo Aguzzo” fan (Fig. 5). The terrace (H) (Fig. 6; see Fig. 13) of the Valle della Casina fan is considered as a part of the “Valle del Pizzo Aguzzo” fan separated by the erosion activity of the creek Acqua del Gallo. Sections 40 and 46 (Fig. 6; see Fig. 13), coordinates 46° 34' 10'' N, 10° 14' 40'' E, are representative of this subunit. The buried soil included in section 46 was dated 3910 ± 60 14C yr B.P. (Tab. 1). 44..22..33.. VVaall PPaaoollaacccciiaa UUnniitt D E S C R I P T I O N A N D I N T E R P R E T A T I O N . The Val Paolaccia Unit (Fig. 8) is formed by a succession of cen- timetric layers made up of an undulating stratification of massive and well-sorted carbonatic silts and fine sands up to more than 4-m thick and totally clast-free. Coal fragments are locally common, dispersed, concentrated in pockets or related to pedoturbations. Pedogenized horizons and pedoturbations can only be found in the uppermost part (see section 2 in Fig. 9 and Appendix I). The structure of this succession suggests that it was for- med in a terrestrial environment. The fine grain size, the good selection and the massive structure indicate that each layer resulted from the accumulation of single sheetfloods. These processes occur when dilute mixtu- res of water and sediments flow as traction currents spreading on the fan surface (Bull, 1972; Nichols, 1999). They cause a thin burial which does not kill the vegeta- tion (see par. 4.5.3.), which promptly reacts by develo- ping new roots and emerging stems. This also explains the common pedoturbation associated with the pedoge- nized horizons observed in the upper part of the succes- sion. NAME. These sheetflood deposits come from the Val Paolaccia (Figs. 5- 8). 194 M. Santilli et alii Fig. 8 - Val Paolaccia Unit: loca- tion, extent, erosion morpholo- gies (in the box) and location of the stratigraphic sections (num- bers). Unità della Val Paolaccia: loca- lizzazione, estensione, morfolo- gie d’erosione (nel riquadro) e ubicazione delle sezioni strati- grafiche (numeri). 195Late Holocene environmental evolution ... LATERAL EXTENT, THICKNESS AND RELATIONS WITH THE OTHER STRATIGRAPHIC UNITS. The unit is interdigitated with the gravel deposits forming the Val Paolaccia fan and covers with an onlap contact the depressed sectors around it. These fine deposits can also be found on the fan surface where they locally bury the soil and vegetation with a layer up to 10-15-cm thick. Some thicker succes- sions have been observed in wide natural downcuttings in the north-west sector of the unit. Sections 39 and 49 show the contact with the Valle della Casina Unit marked by a buried Leptosol (locally eroded) (Figs. 8-9-10). SELECTED SECTIONS. No. 39, 49, 2 (coordinates: 46° 33' 50'' N, 10° 14' 50'' E; Figs. 8-9-10; Appendix I). AGE. The tops of horizon A of the soil lying on the upper surface of the Valle della Casina Unit and buried under the Val Paolaccia Unit provided two dates (Figs. 6-9-10, Tab. 1). We think that the difference between these dates, respectively 1270 ± 70 14C yr B.P. (section 39) and 2570 ± 70 14C yr B.P. (section 49) depends on the different time of burial, which is related to their altitu- dinal position: the soil at a lower altitude was buried ear- lier. Therefore, the difference observed in the ages pro- vides a rough estimation of the average sedimentation rate (0.6 mm/yr). 44..22..44.. DDeeppoossiittss ooff uunnsseettttlleedd ssttrraattiiggrraapphhiicc ppoossiittiioonn Some blocks of monogenic clast-supported con- glomerates, locally passing to medium sandstone, were found along the river bed of Acqua del Gallo. Clasts are carbonatic and the red or grey matrix is scarce. The pro- venance of this material is unknown. Surveys carried out by the AEM Milano at the Passo di Fraele detected conglomerate layers embedded in the gravel. Moreover a conglomerate layer a few centimetre thick is embed- ded in the sequence of clay, gravel and sands along the escarpment of the Val Bruna fan. Fig. 9 - Stratigraphic sections of the Val Paolaccia Unit coming into contact with the Valle della Casina Unit (see Appendix I). Sezioni stratigrafiche dell’Unità della Val Paolaccia al contatto con l’Unità della Valle della Casina (cfr. Appendice I). 44..33.. DDeennddrroocchhrroonnoollooggyy ooff ddeebbrriiss fflloowwss The dendrogeomorphological study concerns the most recent history of the Valle della Casina fan. The integration of the historical, meteorological and dendro- chronological data allowed us to reconstruct the fol- lowing chronological sequence of debris flows in the Valle della Casina (Fig. 11) (Santilli & Pelfini, 2002). The oldest dated debris flow occurred at the end of the 19th century: five trees in area (F) showed com- pression wood events dating back to 1888. Two close dates obtained by analysing eleven trees located in channel (B), area (F) and terrace (D) are 1927-1929. This could be considered as a single episode. Between 1936 and 1941 four damaged trees in areas (E) and (F) were sampled. Two scars dating back respectively to 1936 and 1941, point out that probably different debris flows were responsible for them. A tree located in the secondary channel (B) was damaged in 1953. This debris flow can be connected to a flood, which occurred after heavy rain between 12th and 13th August (Borgonovo, 1955). Clear scars on three trees located in the secon- dary channels (B) and (C) witness an event that took place in 1959 (see Fig. 3). The damage on a tree located in area (F) jutting out in the main channel (A) witness a debris flow in 1964 (see sample (c) in Fig. 5). Heavy rain occurred on June 2nd (34.2 mm) and October 8th (42.8 mm). A flood which occurred in 1978 left traces everywhere; twenty-one damaged trees, in the main channel (A), in the secondary channels (B) and (G), and on the terrace (D), were sampled. This event is witnes- sed by the aerial photographs: the secondary channel (G) does not appear in 1975 whereas it is present in 1981 photos. Heavy rain was recorded on May 21st-22nd (57.3 mm), June 24th (58.2 mm) and July 4th-6th (84.7 mm). Nine damaged trees located in the main channel (A), in the margin of area (F) and in the upper part of the terrace (D) witness an event occurred in 1986. Intense precipitation took place on August 19th (43,8 mm). Finally a debris flow in 1992 is documented by five trees located in the main channel (A) and on terrace (D). This event coincides with a general flood of the whole upper Valtellina, that caused a lot of damage especially near Bormio, where 74 mm of rain fell in 2.5 hours on July 22nd (Mortara et al. 1994); the station in Cancano recorded 40.8 mm on July 23rd, but heavy rain also fell on June 19th-20th (67.2 mm) and on October 6th (39.4 mm). 44..44.. SSooiill ttyyppeess Four soil reference groups have been identified: Leptosols, Cambisols, Fluvisols and Regosols. LEPTOSOLS. They are shallow scarcely developed soils, with an (A)C or AC profile. They develop on sandy-matrix carbonate gravel (e.g. Valle della Casina Unit) and belong to the most represented group inclu- ding four different types of soil. 1. Calcari-Hyperskeletic Leptosols. They are made up of coarse pebbles superficial layers (AC or CA) under which fine material accumulates. The layer, which may contain a significant amount of organic matter, is very modest and discontinuous. It lacks in structure and its skeleton is mainly made up of gravel with a sandy texture matrix. Its colors are usually light (hue 2,5Y), the effervescence is quite strong and the pH is definitely basic. 2. Calcaric Leptosols. Horizon O (Oi + Oe) is 3-4- cm thick; horizon A is 10-cm thick and can assume dark colors. Its structure is scarcely developed, its texture is silt loam or sandy loam, its organic carbon content is of 29-35 g/kg. The amount of carbonates is high (676-757 g/kg), therefore its reaction is basic, too (pH 7.6-8.0). 3. Calcari-Humic Leptosols. Horizon O (Oi + Oe) is 2-4-cm thick; horizon A is usually about 10-cm thick, has dark colors (often 10YR 2/2), a weakly or moderately marked fine or medium granular structure, is rich in rock fragments, has a sandy loam texture, a very light or null effervescence, a pH of 7.4-7.9, a 51-97-g/kg organic carbon content and a variable total carbonate content (61-595 g/kg). 196 M. Santilli et alii Fig. 10 - Strati- graphic scheme of the north- western sector of the Val Paolaccia Unit. Schema stratigra- fico del settore nordoccidentale dell’Unità della Val Paolaccia. 4. Humi-Rendzic Leptosols (Calcaric). Horizon O (Oi + Oe; which is sometimes totally absent, and someti- mes includes also Oa) can be up to 4-cm thick; horizon A, which can sometimes be divided into two subhori- zons having the characters of the mollic epipedon, is usually 10-25-cm thick, has a dark color (10YR 2/1-2/2), a weakly or moderately marked fine or medium granular structure, is rich in rock fragments, has a mostly sandy loam texture, a pH varying from 7.0 to 7.9 (seldom lower than 7.0), a very variable total carbonate content (0-569 g/kg) and a 12-130-g/kg organic carbon content. A subordinate fraction of parent material is locally sili- ceous (see par. 4.2.2), what reduces the carbonate con- tent in lower horizons. CAMBISOLS. They are moderately developed soils, with an ABC profile and a cambic B horizon (Bw). Cambisols develop on carbonate gravel, with a local subordinate siliceous fraction. These soils are little represented, and only one type of them exists. Skeleti-Calcaric Cambisols. Horizon O (Oi + Oe) is 1-2-cm thick; horizon A is not more than 10-cm thick, does not have very dark colors (10YR 3/2-4/3), contains few or normal rock fragments, has a moderately marked fine or medium gra- nular structure, a sandy loam or silty loam texture, a weak or null efferve- scence, a variable total carbonate con- tent (10-559 g/kg), an organic carbon content between 33 and 75 g/kg and a pH of 6.7-7.8. Horizon Bw is about 15- cm thick, is brown (10YR 3/3-3/4), has a moderately marked medium granular structure, a loamy, sandy loam or silty loam texture, a variable effervescence, a total carbonate content between 108 and 601 g/kg, a pH of 7.8-8.0 and a 18- 36 g/kg organic carbon content. FLUVISOLS. These soils have an (A)C or AC profile, and sheetflood sediments are periodically brought to them deter- mining the stratification of the parent material and an irregular content of organic matter along the profile. Fluvisols are only located on silty depo- sits without rock fragments in the Val Paolaccia Unit. Calcaric Fluvisols. Horizon O (Oi and/or Oe) has little importance (0-2 cm) and can also be absent; topsoils (A horizon) have very variable characteristics: their thickness is between 2 and 25 cm, they have light (2,5Y 5/2) to dark (10YR 2/2) colors, a moderate to strong efferve- scence, a pH of 7.5-8.3, a very scarce decarbonatation (549-946 g/kg CaCO3, whereas the CaCO3 content in the parent material is only slightly higher) and a 4-80 g/kg organic carbon content. The pedogenesis is always weak, and the dynamic characteristics of the depo- sits give rise to buried horizons, which can be distinguished by a weak structu- re and by a little accumulation of orga- nic matter. REGOSOLS. Soils with a limited develop- ment not belonging to other soil groups. Anthropic Regosols. These soils deve- lop on little level areas for the charcoal production which are very widespread in this zone. The topsoil is made up of a black layer of pure charcoal, about 20- cm thick, that can be divided into two levels: the upper one differs from the 197Late Holocene environmental evolution ... Fig. 11 - Dating of debris flows: event-response yearly maps pointing out the position of the sampled trees used for dating purposes; the black circles indicate the specimens in which event years (corrasion scars or first developing compression wood) relevant to the same episode were identified. The box is showed in fig. 6. Datazione dei debris flows: mappe annuali evento-risposta mostranti la posizione degli alberi campionati utilizzati per le datazioni; i cerchi neri indicano gli individui in cui sono stati identificati anni caratteristici relativi ad uno stesso episodio (cicatrici d’abrasione o inizio dello sviluppo di legno di compressione). L’area è mostrata in fig. 6. 198 M. Santilli et alii lower one in a more acid reaction (respectively pH 5.7 and pH 7.1) and in a weak structure granted by the roots. Many Leptosols and Cambisols show some traces of surface erosion causing a partial lack in vegetation and emerging roots on a remarkably wide surface. 44..55.. PPllaanntt ccoommmmuunniittiieess Three main groups of relevés have been identified through the chord distance analysis: pioneer lithophilous communities, mountain pine-dominated woods and scrubs, and grasslands. A further distinction has been made through the presence/absence analysis. 44..55..11.. PPiioonneeeerr lliitthhoopphhiilloouuss ccoommmmuunniittiieess (Tab. 2) A sparse herbaceous layer, locally associated with a thin layer of young/small pines characterizes these communities. The cover values never exceed 20-30% and are sometimes lower than 5%, according to the age and the stability of the surface. The average number of species per relevé is 18. The moss layer has a negligi- ble cover value while lichens lack. These communities are characterized by the constant presence of Hieracium staticifolium, as well as of Pinus montana and of many lithophilous and basiphilous species such as Trisetum distichophyllum, Saxifraga caesia and Athamanta cretensis. Other species are sporadically present and are sometimes linked to peculiar local para- meters such as a higher humidity (e.g. Saxifraga aizoi- des). These communities belong to the association Athamanto-Trisetetum distichophylli (Jenny-Lips 1930) Lippert 1966 (class Thlaspietea rotundifolii Br.-Bl. 1926 et al. 1948, order Thlaspietalia rotundifolii Br.-Bl. in Br.- Bl. & Jenny 1926 em Seib 1977, alliance Petasition paradoxi Zollitsch 1966). The importance of Pinus mon- tana in this pioneer vegetation, which is instead usually found only in the most advanced stages of the associa- tion (Pirola & Di Benedetto, 1959), must be stressed. 44..55..22.. MMoouunnttaaiinn ppiinnee wwooooddss aanndd ssccrruubbss (Tab. 3) Pinus montana woods and scrubs dominate the vegetation of the upper Valle del Gallo. The vegetative cover is nearly complete, even if in pioneer or degraded stages it can reach low values (<50%). The arboreal layer, where present, has a mean cover value of 60% and is 4 to 8-m high, while the shrubby layer (locally including Juniperus nana) has a mean cover value of 35% and is 0.3 to 3-m high. The herbaceous layer is well developed and reaches a cover value of about 70%. Both mosses and lichens reach a cover value of 15-20%. These communities share in general xerophitic and heliophilous characteristics and seem quite homo- geneous. Nevertheless, some different aspects can be observed. The syntaxonomic treatment of pinewoods is difficult because of the uncertain taxonomy of the pine itself. According to Minghetti (1996), all the sampled communities should be included in the class Erico- Pinetea Horvat 1959, order Erico-Pinetalia Horvat 1959, alliance Erico-Pinion mugo Leibungut 1948. CA R E X H U M I L I S A N D SE S L E R I A V A R I A-D O M I N A T E D PINEWOODS. These groupings are joined together by some exclusive or preferred species like Polygala cha- maebuxus, Calamagrostis varia and Daphne striata, and by high cover values of Erica carnea, Carex humilis and Sesleria varia. Two main situations can be identified. 1. Pinewoods on stabilized substrata. They are characterized by a wider cover of arboreal layer, by some species indicating that the soil has started to aci- dify (Moneses uniflora and Hieracium tenuiflorum) and by an accumulation of organic matter. According to the tree layer and Erica carnea cover, the presence and abundance of Vaccinium vitis-idaea and the average number of species, four different degrees of evolution can be identified: - Low degree of evolution. Young pinewoods or scrubs, having a scanty arboreal and Erica carnea cover. The average number of species is 32. - Intermediate degree of evolution. A thicker tree layer, a wider heather cover and a greater average number of species (35) characterize it. A group of differential Table 2 - Synoptic table of the pioneer lithophilous communi- ties. This table (see also tab. 3-4) shows the percentage fre- quencies of the most widespread species in the relevés, except in those represented by one single relevé, where the cover values are directly reported. Tabella sinottica delle comunità pioniere litofile. Questo tipo di tabella (cfr. anche tab. 3-4) mostra le frequenze percentuali delle specie più diffuse nei rilievi, eccetto nelle tipologie con un solo rilievo, dove sono direttamente riportati i valori di copertura. species, some of which are linked to grasslands, is present; the main species distinguishing this com- munity from the previous one is Vaccinium vitis- idaea. – Cowberry-dominated. It is characterized by the dominance of Vaccinium vitis-idaea and by a lower average number of species (26). 2. Regressive stages due to the soil erosion: the marked soil erosion causes a reduction in the arboreal cover to the advantage of the shrubby one. Near the typical species of pinewoods grow more heliophilous species linked to thinner communities like Carlina acaulis, Senecio abrotanifolius and Thymus polytrichus. The average number of species rises to 40. All these communities are similar to the asso- ciation Erico carneae-Pinetum prostratae Zöttl 1951, with a particular reference to the subassociation cari- cetosum humilis Br.-Bl. 1939 RHODODENDRON HIRSUTUM-DOMINATED PINEWOOD. It is really different for the dominant role of Rhododendron hirsutum in the shrubby layer, for the scanty cover of Vaccinium vitis-idaea and for the low floristic richness (17 species) mainly due to the disappearance of the most thermophilous species. The herbaceous layer is sparse and dominated by heather only. This kind of vegetation can only be found on slopes facing northward and can be ascri- bed to the association Rhododendro hirsuti-Pinetum prostratae Zöttl 1951. PINEWOODS GROWING ON SCREES. The presence of lithophilous species make these coenoses similar to the pioneer herbaceous ones growing on early developed soils. Two kinds of pinewoods can grow on screes: one is characterized by scrubby pines growing on young surfaces, and the other one repre- sents a former pinewood buried by low energy debris flows. These communities have an uncertain syntaxonomic treatment and can be considered as a transition between Athamanto-Trisetetum and pri- mary stages of Erico-Pinetum. 44..55..33..GGrraassssllaannddss (Tab. 4) Grasslands are very rich in species (the avera- ge number of species per relevé is 52). The herba- ceous layer is almost continuous but often not very thick, probably because of the low cover value of grasses (mainly Festuca nigrescens and Sesleria varia) compared to the rosette-shaped plants (Plantago alpina, Cirsium acaule and Carlina acaulis). Grasslands include the most acidophilous and the least xerophilous communities in the studied area and can be all regarded as pastures. Although all the grasslands share a high cover value of Sesleria varia, two main communities, growing on different substrata, can be distinguished; some tran- sition stages can also be observed. POA ALPINA AND FESTUCA NIGRESCENS GRAS- SLANDS. These communities grow on fan gravelly deposits and are the richest in species (up to 68 per relevé). Grasses and sedges (Sesleria varia, Poa alpina, Festuca nigrescens, Carex sempervirens and 199Late Holocene environmental evolution ... Table 3 - Synoptic table of the mountain pine woods and scrubs. The synoptic tables point out only the main communities and not those belonging to an inferior rank which are instead relevant as concerns the dynamic aspects. Tabella sinottica dei boschi e degli arbusteti a pino montano. Le tabelle sinottiche evidenziano solo le comunità principali e non quelle di rango inferiore, che sono invece importanti per gli aspetti dinamici. 200 M. Santilli et alii Table 4 - Synoptic table of the grasslands. Even though Androsace chamaejasmae is widespread it is not present in the table because of its premature disappearance during the vegetative season. Tabella sinottica dei pascoli. Nonostante la diffusa presenza, Androsace chamaejasmae non compare nella tabella per via della sua precoce scomparsa durante la stagione vegetativa. Elyna myosuroides) have very high cover values. Some species (e.g. Luzula multiflora, Elyna myosuroides and Avenella flexuosa) show a soil acidification and/or an accumulation of organic matter. These coenoses can be included in the class Seslerietea albicantis Oberd. 1978 corr. Oberd. 1990 (Mucina et al., 1993) and probably be referred to Seslerieto-Semperviretum Br.-Bl. 1926 (alliance Seslerion coeruleae Br.-Bl. in Br.-Bl. & Jenny 1926). However, species belonging to different syntaxa (in particular the acidophilous taxa) can also be present thus making the syntaxonomic and synecological treat- ments difficult. CIRSIUM ACAULE AND PLANTAGO ALPINA GRASSLANDS. These communities grow on the silty deposits in the Val Paolaccia Unit, have a lower floristic richness (30-40 species per relevé) and a less continuous structure, where grasses and sedges play a less important role. The dominant or significant species are Cirsium acaule, Carex flacca and Plantago alpina. The syntaxonomic classification is very difficult in this case, too. Carex flacca regressive stage. Where the erosion prevails (see Fig. 8) grassland species are replaced by other species as Tussilago farfara and Carex flacca, which become dominant. PINE SETTLED OVER GRASSLANDS. Even if these com- munities share many species with the grasslands of silty deposits, the sparse tree layer allows the pine wood species as Erica carnea to grow and causes the majo- rity of the heliophilous grassland species to disappear. 55.. EENNVVIIRROONNMMEENNTTAALL EEVVOOLLUUTTIIOONN OOFF TTHHEE UUPPPPEERR VVAALLLLEE DDEELL GGAALLLLOO 55..11.. HHoolloocceennee eevvoolluuttiioonn ooff tthhee vvaalllleeyy bboottttoomm The oldest sediments cropping out on the valley bottom are lacustrine deposits belonging to the Acqua del Gallo Unit. The paleolake was buried by a coarse- ning-upward succession of clays, microturbidites, allu- vial and deltaic deposits transported by the fans progra- ding from the tributary valleys (Fig. 12). The basin had already dried up before the middle Holocene (e.g. befo- re 4710 ± 90 14C yr B.P.). This phase caused the forma- tion of an ancient flat valley bottom (belonging to the Valle della Casina Unit) now representing inactive por- tions of some fans (Fig. 5; see par. 4.1); its southern part dates back at least to the Iron Age (2570 ± 70 14C yr B.P.), but it is probably much older (e.g. before 3910 ± 60 14C yr B.P.). Later, the northern and southern sectors of the upper Valle del Gallo had a different evolution. After eroding the northward lacustrine threshold, creek Acqua del Gallo started an intense regressive ero- sion process which at first affected the northern sector of the valley (Fig. 12). This caused the formation of deep gorges (up to 50 m) in the fans of Val Mora and Valle della Casina (and locally in the bedrock), and the dismantling of wide portions of the fans of Val Bruna and “Valle del Pizzo Aguzzo”, what isolated some still existing parts of the original valley bottom (Fig. 5; see par. 4.1). Yet a series of important flooding phases tem- porary interrupted the erosion process at least since the Copper Age (3910 ± 60 14C yr B.P.) as testified by the Val Bruna Subunit which is embanked in the valley bot- tom (Figs. 5-12-13). The subunit is intercalated by buried soils documenting that it formed in successive stages, separated by significant phases of stability and some changes in the erosion direction of creek Acqua del Gallo (see terrace (H) in Fig. 13). The southern sector of the studied area (Fig. 12) was not dissected by erosion until recent years. For this reason, a thick succession of fine deposits originated by sheetfloods (Val Paolaccia Unit) accumulated among the coalescent fans of Valle della Casina, “Valle del 201Late Holocene environmental evolution ... Fig. 12 - Stratigraphic relations of the upper Valle del Gallo. An ideal profile was used to show the different units at the same time. Schema dei rapporti stratigrafici dell’alta Valle del Gallo. È stato impiegato un profilo ideale per mostrare contemporaneamente le diver- se unità. Pizzo Aguzzo” and Val Paolaccia. Depositional events, which were originally very frequent, prevented the set- tlement of vegetation (see section 2 in Fig. 9 and Appendix I; see par. 4.2.3). Meanwhile, the regressive erosion extended southward along the river bed of Acqua del Gallo isola- ting distal portions of some fans (e.g. under the Val Mora road for the Valle della Casina fan). In the Middle Ages (1270 ± 70 14C yr B.P.) it reached the Val Paolaccia Unit, favouring the drainage and the reduction of the sedimentation rate. Therefore Fluvisols started to develop together with a kind of vegetation suitable to adapt to frequent burials (Cirsium acaule and Plantago alpina grasslands) the features of which show the persi- stence of vegetation during aggradation. Only some parts of the fan surfaces were affected by these changes in the sedimentary activity and in the vegetation. Other portions of the fans were still under an active aggradation. The formation of the middle part of the Valle della Casina fan went on at a high rate, at least till the late Roman Age (1720 ± 50 14C yr B.P.; Fig. 14, Tab. 1), when a phase of morphologic stability cau- sed the formation of a soil (buried soil in section 5, Figs. 6-14). Since the Middle Ages, debris flows were instead mostly localized close to the fan apex (see buried soils in section 6, Figs. 6-14; see par. 5.2) and along the main channel (A in Fig. 6), its ramification (B, C, G) and the adjoining sectors (E, F, D), while its most distal part was isolated by the downcutting formed by the main channel, under the Val Mora road. Therefore, on the whole, since the Middle Ages, the regressive erosion of the valley bottom is responsi- ble for the general stabilization of many surfaces, since debris flows which were locally canalised can no more spread on the entire fan surface. 55..22.. RReecceenntt eevvoolluuttiioonn ooff tthhee VVaallllee ddeellllaa CCaassiinnaa ffaann The dendrochronological reconstruction of the debris flow sequence in the Valle della Casina (Fig. 11) begins at the end of the 19th century because of the young age of the trees. The presence of young forests (100-200 years old) is probably due to the drastic defo- restation which occurred over past centuries. The most ancient dated event happened in 1888 and involved area (F) besides the channels and area (E) (Fig. 15), where it covered a part of the fan surface currently exposed downhill. Later debris flows (in 1927-29 and in 1936-41) interested this area only marginally (Fig. 15). The most recent episodes were mostly confined to the main channel (A), where they affected the margin of area (F), and to the secondary channels (B) and (C). The 1978 debris flow represents the most intense episo- de during which a new channel developed (G in Fig. 6). The event, which occurred in 1986, formed the upper part of the terrace (D) (see Fig. 14). The main channel (A) of the Valle della Casina fan constitutes the course of normal drainage, with a maxi- mum capacity able to contain the smaller flows (e.g. 1964 and 1992), the traces of which (damaged trees and terraces) are only visible along the margins. Below Val Mora road, scarps border the main channel so in the lower part of the fan the flows do not have any direct effect outside the river bed and can reach the river Acqua del Gallo. When the flows exceed the maximum flow rate in the main channel, the secondary channels 202 M. Santilli et alii Fig. 13 - Hypothetical succession of events that originated the Val Bruna Subunit and the terrace (H): 1) ancient post-lacustri- ne valley bottom; 2) first downcutting phase by the river Acqua del Gallo; 3) dismantlement of a part of the “Valle del Pizzo Aguzzo”fan; 4) formation of a new portion of the “Valle del Pizzo Aguzzo” fan; 5) second downcutting phase by the river Acqua del Gallo; 6) new flooding on the “Valle del Pizzo Aguzzo” fan; 7) present downcutting by the river Acqua del Gallo and separation of the terrace (H). Ipotetica successione di eventi che avrebbe dato origine alla Sottounità della Val Bruna e al terrazzo (H): 1) antico fondoval- le post-lacustre; 2) prima fase di incisione da parte dell’Acqua del Gallo; 3) smantellamento di parte del conoide della “Valle del Pizzo Aguzzo”; 4) costruzione di una nuova porzione del conoide della “Valle del Pizzo Aguzzo”; 5) seconda fase di inci- sione da parte dell’Acqua del Gallo; 6) nuova fase di alluviona- mento del conoide della “Valle del Pizzo Aguzzo”; 7) incisione attuale dell’Acqua del Gallo e separazione del terrazzo (H). (B) and (C) are involved. Trees slacken these lateral flows that can spread to larger surfaces, hence they do not reach the river Acqua del Gallo (e.g. 1927-29, 1953, 1959 and 1978). Exceptional flows can sometimes form new courses when they exceed also the maximum flow rate of the secondary channels. In 1978 channel (G) ori- ginated in this way. The subsequent restoration of the main channel banks made the lateral channels inactive again. 55..33.. RReellaattiioonnss bbeettwweeeenn ssooiill aanndd vveeggeettaattiioonn aanndd ddyynnaammiicc ooff tthhee eeccoollooggiiccaall ssuucccceessssiioonn Soils and vegetation of the upper Valle del Gallo are, in general, scarcely developed. This is probably due to five principal causes: the relatively recent age of those areas subjected to pedogenesis; the periodic burial affecting some areas; the climatic restriction due to low temperature and scanty rainfall; the effects of human deforestation during the last millennium; the extent of areas subjected to an intense soil erosion. The relations between plant communities and soils allow us to make some considerations. Pioneer litophi- lous communities grow only on Calcari-Hyperskeletic Leptosols. Pinewoods do not show soil more developed than Leptosols, even at their most advanced stages. The stage characterized by the presence of cowberry on the Carex humilis and Sesleria varia-dominated pinewoods occurs on Humi-Rendzic Leptosols (Calcaric); these soils locally grow under less developed vegetation together with Calcaric and Calcari-Humic Leptosols. The presence of a Cambisol under a regres- sive stage of the Carex humilis and Sesleria varia-domi- nated pinewood suggests that pinewoods growing on Skeleti-Calcaric Cambisols are the most developed situation found on the fans (a higher evolution degree concerning the woody vegetation can be found only locally on the slopes, out of the fans). Grasslands show a close relation between plant communities and soil typologies. For example Cirsium acaule and Plantago alpina grasslands only occur on Calcaric Fluvisols while Poa alpina and Festuca nigre- scens grasslands develop only on Leptosols or locally on Cambisols with a moderate degree of decarbonata- tion as suggested by the presence of some acidophilous species (see par. 4.5.3.). It is now possible to outline the dynamic succes- sion of soil and vegetation. Surfaces formed by different sedimentation and erosion processes are progressively colonized by vegetation and are subjected to pedogene- sis as they become sufficiently stable. The ecological succession occurs on two kinds of substrate, both carbo- natic, different in the grain size and originated by two sedimentary processes: gravel with a sandy matrix deri- ving from debris flows (Valle della Casina Unit); silts and fine sands without skeleton deriving from sheetfloods (Val Paolaccia Unit). A third subordinate kind of parent material is constituted by coal masses from charcoal pits. The dynamic succession on a gravelly substrate (Fig. 16) starts with plant communities collected in the Athamanto-Trisetetum grown on Calcari-Hyperskeletic Leptosols. Under geomorphological stability, a pine scrub settles and discontinuous mosaic-like horizons O and A grow up connected with pioneer vegetation. The further soil evolution involves an increase in the fine fraction and the development of a thin horizon A (Calcaric Leptosols). When the mass of organic matter and the decarbonatation become significant, soils pass to a stage of Calcari-Humic Leptosols, existing until a mollic horizon develops (Humi-Rendzic Leptosols (Calcaric)). Locally a Bw horizon sometimes evolving 203Late Holocene environmental evolution ... Fig. 14 - Block-diagram of the hydrographic left side of the main channel (A in fig. 6) of the Valle della Casina fan showing the position of sections 5 and 6 and of the buried charred wood (star) dated 1720 ± 50 14C yr B.P. (Tab. 1). Block-diagramma del lato sinistro idrografico del cana- le principale (A in Fig. 6) del conoide della Valle della Casina che mostra la posi- zione delle sezioni 5 e 6 e del tronco carbonizzato sepolto (asterisco) datato 1720 ± 50 anni 14C B.P. (Tab. 1). into a cambic horizon (Skeleti-Calcaric Cambisols), can originate. At the same time the development of vegeta- tion brings about an increase in the number of species and in the ground cover until Carex humilis and Sesleria varia-dominated pinewoods develop, where the first aci- dophilous species appear. The progressive acidification of the upper horizons causes an increase in the aci- dophilous species, the decrease in the basiphilous ones and an increase in the tree layer and in the Erica carnea cover, until the species richness decreases to the advantage of Vaccinium vitis-idaea (the cowberry-domi- nated stage). The latter is the most developed stage of vegetation found, which probably took place after a few hundred years. The Rhododendron hirsutum-dominated pinewood is mainly limited to the northward exposure. All the remaining vegetation typologies represent regressive stages or secondary vegetation. The burial caused by debris flows brings the pine-forest back to Athamanto-Trisetetum with a tree layer; this is probably susceptible to a rapid development due to the litter sup- plied by the tree cover. The surface soil erosion makes the succession regress to less developed stages becau- se of the removal of the upper horizons and the progres- sive crop out of parent material; the pine-forest is thin- ned out and the number of heliophilous species increa- ses. On the contrary grasslands are to be considered as secondary vegetation deriving from pine-forests defore- station. The ecological succession on a silty substrate (Fig. 16) is periodically interrupted by centimetric burials caused by sheetfloods. However, the soil evolution is connected to the frequency of burial episodes, depen- ding on the closeness of the downflow direction. Soils are Calcaric Fluvisols, periodically rejuvenated, without a meaningful decarbonation and an organic matter accumulation. The vegetation dynamics stops to a stage of blocked evolution (Cirsium acaule and Plantago alpi- na grasslands). The morphological attributes of the dominating species indicate that this community seems to “float” on a substrate in periodic aggradation; in fact Carex flacca is a stoloniferous plant while Plantago alpi- na and Cirsium acaule develop tap-roots up to 50-cm long. This reaction of the vegetation causes the com- mon radical pedoturbation. On the contrary, where ero- sion is intense, substitution communities prevail (Carex flacca regressive stage). Whenever grasslands disappear Pinus montana forms a tree layer again. 66.. CCOONNCCLLUUSSIIOONN During the Holocene, the upper Valle del Gallo experienced a series of intense transformations that progressively and quickly transformed the environmen- tal setting that became established after the retreat of the würmian glaciers. Complex and still unclear erosive and depositional cycles of glacial origin formed thre- sholds that originated a lake, although this is a pass zone, where outflows should therefore have been facili- tated. Environments originally poor or without vegeta- tion, supplied an ideal substrate for the weathering and denudation processes of the slopes, originating huge amounts of debris that water could easily mobilize and transport to the lacustrine basin. Its width and depth 204 M. Santilli et alii favoured a pelagic terrigenous sedimentation, than in a few thousand years progressively evolved into an allu- vial and deltaic environment characterized by shallow water and finally a terrestrial environment dominated by fans mostly formed by debris flow processes. Vegetation adapted finding a balance with the environ- mental dynamics, characterized by a high instability of the slopes causing erosion and covering processes that often determine ecological situations with a blocked or slowed down evolution. The valley setting was finally modified by human action. Because of the complexity and the interaction of the various ecological components, the study of this kind of environment necessarily demanded an interdisci- plinary approach. The several methods used span various temporal intervals and space scales and have different resolution power, but each of them contributed in an effective way to explain every single aspect of the environmental evolution of the examined territory, allowing the final integration and synergy of the different results obtained. This route forms in a more and more detailed way for the recent times: the geomorphological and stratigraphic researches, allowed us to reconstruct the main evolutionary stages of the whole territory during the Holocene and to identify some factors con- trolling the ecological successions; the study of the soils and vegetation has allowed us to clarify the relations existing between the various environmental components and the adaptations to the dynamics of the geomorpho- logical and sedimentary processes occurred during rela- tively recent times; finally, the dendrogeomorphological Fig. 15 - Scheme showing area (F) and the different age sec- tions composing it. Below this area a buried soil (section 19 in Fig. 6) and a buried stump in position of life were found (star) (Santilli & Pelfini, 2002). Schema mostrante l’area (F) e le porzioni di differente età che la costituiscono. Sotto quest’area è stato inoltre trovato un suolo sepolto (sezione 19 in Fig. 6) e un tronco sepolto in posi- zione di vita (asterisco) (Santilli & Pelfini, 2002). study has allowed us to know the precise dating and the extent of every single debris flow that, starting from the end of the 19th century, affected one of the fans. The environmental complexity of the upper Valle del Gallo must be deepened further on. In spite of the great amount of data and information collected up to now, new stratigraphic sections, especially relevant to the lacustrine succession, would be necessary since they could clarify the valley evolution starting from the Late-glacial period and the influence of the successive alternate phases of climatic deterioration and improve- ment; further surveys should concern the taxonomic position of the mountain pine, the widespread superficial erosion of pinewoods and the important role of the anth- ropic influence, which has been up to now only partially documented. We want to emphasize the importance of an inte- grated approach of different disciplines to the studies on the environment. The efforts necessary to carry out interdisciplinary studies often must be restricted to small areas; however starting from local events it is possible to make comparisons and to extrapolate results that can refer to bigger areas or be applied on a regional scale. Researches carried out up to now constitute the- refore a good starting point for the study of the peculiar environment of the Vale del Gallo, to which an elevated naturalistic value to be preserved and exploited is ack- nowledged. The research has been carried out with the co- financing of MURST-COFIN 40%-1999 - project: Movimenti di massa come indicatori di cambiamenti cli- matici dal Tardiglaciale ad oggi. National responsible Prof. M. Panizza, local responsible Prof. G. Orombelli. AACCKKNNOOWWLLEEDDGGEEMMEENNTTSS The authors wish to thank Prof. G. Orombelli of the University of Milano-Bicocca, for the critical revision of the manuscript, the Parco Nazionale dello Stelvio for authorising researches in its territory and AEM Milano for supplying the climatic data. RREEFFEERREENNCCEESS Agostoni U. (1923) - I giacimenti ferriferi dell’Alta Valtellina. In “La miniera italiana”, published on the journal “Valtellina”, issues of April 11th and 18th, 1923. Alestalo J. 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Ministero delle risorse agricole, Alimentari e Forestali, Roma, pp. 207. Pignatti S. (1952) - Introduzione allo studio fitosociologi- co della Pianura Veneta Orientale. Archivio Botanico, 28(4): pp. 265-329. Pignatti S. (1982) - Flora d’Italia. 3 vol., Edagricole, Bologna. Pedrotti F., Orsomando E., Cortini Pedrotti C. (1974) - Carta della vegetazione del Parco Nazionale dello Stelvio 1:50.000. Ediz. Amministraz. Parco Naz. Stelvio, Bormio, pp. 87. Pirola A. & Di Benedetto G. (1959) - Osservazioni sul dinamismo della vegetazione e del suolo nel mugeto-ericeto del Monte Reit (Alpi Retiche Bormiesi). Fondazione per i problemi montani del- l’arco alpino, Milano, Pubblicazione n. 33, Catania, pp. 29. Pirola A. (1973) - La vegetazione. In “Le alte valli dell’Adda e dello Spöl, un ambiente da conserva- re”, a cura dell’Assessorato economia montana e foreste. Regione Lombardia. pp. 77-94. Polloni G, Ceriani M, Crosta G, Lauzi S, Padovan N, (1991) - Rainfall and soil slipping events in Valtellina. In Bell DH (Eds.). Landslides. Proc. VI Int. Symposium on landslides. Balkema, Rotterdam: 183-188. Pozzi R. & Giorcelli A. (1960) - Memoria illustrativa della carta geologica (scala 1:35.000) della regione compresa fra Livigno ed il passo dello Stelvio (Alpi Retiche). Boll. Serv. Geol. It., vol. 81, Serie G, Pubbl. n. 131, Milano, pp. 72. Saibene C. (1973) - Lineamenti di geomorfologia. pp. 69-76 in Le alte valli dell’Adda e dello Spöl, un ambiente naturale da conservare. A cura dell’Assessorato all’Economia montana e Foreste. Regione Lombardia, pp. 109. Salvador A. ed. (1994) - International Stratigraphic Code. Second Ed., The Geological Society of America. Sanesi G. (1977) - Guida alla descrizione del suolo. Progetto Finalizzato Conservazione del Suolo, C.N.R., Firenze, pp. 157. Santilli M, (2000) - Storia ed evoluzione dei conoidi del- l’alta Valle del Gallo (Alta Valtellina-Sondrio). Studio geomorfologico, stratigrafico, dendrocrono- logico, pedologico e vegetazionale. Thesis in Natural Sciences. Dipartimento di Scienze della Terra, Università di Milano, Italy, 273 pp. Santilli M, Pelfini M (2002) - Dendrogeomorphology and dating of debris flows in the Valle del Gallo, 206 M. Santilli et alii Central Alps, Italy). Dendrochronologia 20 (3), pp. 269-284. Shroder J.F. (1980) - Dendrogeomorphology: review and new techniques of tree-ring dating. Progress in Physical Geography 4, pp. 161-188. Strunk H. (1995) - Dendrochronologische Methoden zur Ermittlungen der Murfrequenz und Beispiele ihrer Anwendung. Regensburg: S. Roderer Verlag, pp. 196. Stuiver M., Reimer P. J., Bard E., Beck J. W., Burr G. S., Hughen K. A., Kromer B., McCormac F. G., Plicht J. & Spurk M. (1998) - INTCAL98 Radiocarbon age calibration 24,000-0 cal BP. Radiocarbon 40, pp. 1041-1083. Van der Maarel E. (1979) - Transformation of cover- abundance values in phytosociology and its effects on community similarity. Vegetatio, 39 (2), pp. 97-114. Zazzi S. (1982) - Per una storia delle miniere di Val Fraele e dei forni da esse alimentati. Notiziario n. 30, dicembre 1982, Banca Popolare di Sondrio, pp. 78-83. AAPPPPEENNDDIIXX II ((DDEESSCCRRIIPPTTIIOONN OOFF TTHHEE SSTTRRAATTIIGGRRAAPPHHIICC SSEECCTTIIOONNSS)) SSeeccttiioonn 33 -- AAccqquuaa ddeell GGaalllloo UUnniitt (Fig.7). From the bot- tom upwards: 0-2,5 m: grey (2,5Y 5/1) and brown (10YR 5,5/3) clayey deposits with a flat parallel lamination, made of thin massive clay laminas, less than 1-mm to 3- mm thick, alternating with subordinate silty clay and silt levels. Very fine grey and well sorted sand laminas are widespread too; 2,5-4,5 m: deposits mostly clayey and laminated, yellow reddish and analogous to the previous ones, but showing on average thicker laminas. Locally depo- sits loose this flat parallel lamination and sedi- ments assume a “mottled” aspect due to the diffe- rent colors according to the grain size 4,5-5 m: centimetric layers with a flat parallel lamination mainly composed of brown clay laminas and subordinate intercalations of silt and very fine grey sands; 5-5,8 m: dark grey sand layer, the structure of which alternates between middle-sized and fine sands in a wavy parallel lamination, with subordinate milli- metric grey-brown silty levels. In the upper part a layer with a low-angle cross lamination can be observed, then assuming a climbing style and gra- dually reconstituting the wavy parallel lamination and then again the flat parallel one; 5,8-8 m: laminated deposits mostly clayey, brown reddi- sh, with subordinate intercalations of very fine sands 2,5Y 5/2; 8-12 m: in this layer the first gravel levels can be found. These deposits tend to have a normal gradation layer, some-centimetres to 1-2-decimetres thick. Each layer is composed at its base of variable thickness centimetric levels of fine and middle- sized gravel with loose centimetric clasts in clear contact, often of erosive type, on the underlying finer sediments; towards the top the layer structure gradually passes to grey middle-sized and fine sands, to silt and brown reddish silty clay, in a wavy subparallel lamination at intervals little evi- dent; 12-15,5 m: centimetric alternations of fine sands, midd- le-sized sands, gravelly sands and gravel with subordinate silty levels that, quantitatively, tend to reduce towards the top; 15,5-18,5 m: unselected gravelly and clast-supported sediments alternated with lens and intercalations of sands and silt sometimes containing gravel with matrix support (sometimes reddish). A rough stra- tification, locally passing to a cross stratification, can be observed; 18,5-22 m: proceeding towards the top unselected gra- vel sediments with a clastic support without sedi- mentary structures gradually become prevalent. At the top of the scarp there is a soil (Humi-Rendzic Leptosols (Calcaric)). SSeeccttiioonn 3399 -- VVaall PPaaoollaacccciiaa UUnniitt (Fig. 9) - Sequence of silt and sand deposits lacking in skeleton, organised in little differentiated sedimentary sublevels and laying upon a buried soil developed on the underlying gravel. +1-0, 0-1 cm: horizon O (Oi + Oe); 1-7 cm: horizon A, light grey 2,5Y 5/2, with a silty loam texture and a little marked fine granular structure; numerous plant remains, many thin and middle- sized roots, a clear wavy boundary, and a high effervescence. Analytical data: pH 7,8, organic C 24,8 g/kg; 7-18 cm: horizon C composed of massive grey sands, 2,5Y 5/2, with brown variegations,10YR 4/2, woody subhorizontal roots up to 1 cm in diameter; 18-37 cm: middle-sized to fine sands as above, well selected, but with a dark brown tonality and a clear wavy boundary; little coal fragments are wide- spread at the base; 37-49 cm: middle-sized grey sands with thin disconti- nuous brown lens including coal fragments; coal remains are scattered inside the level, too; 49-53 cm: yellow-ochre sandy silt; clear boundary; 53-57 cm: middle-sized to fine brown-grey sands with an evanescent lamination and a clear lightly wavy boundary; sparse coal fragments; 57-60 cm: horizon Ab, brown 10YR 3/2, with a poor ske- leton, a well marked fine granular- polyedric subangular structure, a silty loam texture, thin and middle-sized common roots, a wavy abrupt boun- dary and a high effervescence. Analytical data: pH 8,0, organic C 34,3 g/kg. This buried soil dates back to 1270 ± 70 14C yr B.P. (Tab. 1); 60-65 cm: horizon C1' composed of incoherent gravel with clasts up to 10 cm with the support of a sandy middle-sized to coarse matrix and fine gra- vel, grey brown 10YR 5/3, with very thin common roots, a gradual wavy boundary, a high efferve- scence and unaltered or only lightly altered peb- bles; 65-95 cm: rough stratified gravel alternating between clastic support and matrix support up to 10 cm big, with a coarse sandy matrix, subangular pebbles, sometimes subrounded. The horizon shows a red- dish matrix and reddish patinas on pebbles that at some levels form rather wide pellicles. Roots are absent. 95-250 cm: fine gravel with matrix support alternating with sand lens lightly altered; in this case too the matrix of the gravelly levels shows reddish patinas that harden the sediment. 207Late Holocene environmental evolution ... SSeeccttiioonn 4499 -- VVaall PPaaoollaacccciiaa UUnniitt (Fig. 9) - Sequence of sandy and silty deposits lacking in skeleton, organised in sedimentary differentiated sublevels and laying upon a buried soil developed on the underlying gravel. 0-10 cm: horizon A, dark brown, having a weakly marked fine granular structure, a sandy loam tex- ture, abundant roots and a clear wavy boundary; 10-30 cm: bioturbated massive fine sands, light grey with ochre tones, 2,5Y 5/2; 30-33 cm: grey middle-sized to fine sands; 33-40 cm: yellowish silty fine sands, 2,5Y 5/2, having a weakly marked fine polyedric subangular structure and sparse thin roots; 40-45 cm: middle-sized laminated and well selected sands, 2,5Y 4/2; 45-51 cm: sandy silt with fine sand, brown-yellowish, 2,5Y 5/3; 51-56 cm: middle-sized to fine laminated yellowish sands, 2,5Y 5/2; 56-66 cm: massive fine sands with a weakly marked lamellar structure, grey, 2,5Y 4/2, with sparse roots and a clear boundary; 66-75 cm: fine sand sandy silt, yellow-brown, 10YR 5/3, having a moderately marked middle polyedric- lamellar structure; 75-93 cm: yellowish fine sand sandy silt, 2,5Y 5/3, having a weakly marked polyedric-lamellar structu- re; 93-101 cm: grey middle-sized sands, 2,5Y 4/2; 101-123 cm: weakly sandy silt with fine sand, with colors zoned between yellow and yellow-brown, and a very weakly marked lamellar structure; 123-129 cm: middle-sized grey sands; clear boundary; 129-140 cm: yellow-brown weakly sandy silt with fine sand, a weakly marked lamellar structure and a clear boundary; 140-145 cm: horizon Ab 10YR 4/3, with a well marked middle polyedric angular structure, a clayey silty loam texture, a high effervescence, a poor skele- ton on the surface, more abundant at the base of the horizon, where it gradually passes to the underlying horizon. Analytical data: pH 8,0, orga- nic C 28,5 g/kg. This buried soil dates back to 2570 ± 70 14C yr B.P. (Tab. 1); 145-160 cm: altered gravel in a sandy matrix 10YR 5/4 altered too; patinated pebbles altered on the surfa- ce, patinas having the same color as the soil, little marked fine granular structure. SSeeccttiioonn 22 -- VVaall PPaaoollaacccciiaa UUnniitt (Fig. 9) - Sequence enti- rely composed of sandy and silty fine deposits, without skeleton, organised in sedimentary sublevels. The first 90 cm differ from the remaining part because of the pre- sence of buried horizons and organic matter. 0-10 cm: horizon AC little pedogenized composed of grey massive fine sands, 10YR 5/1,5, with very poor roots and a wavy boundary; 10-14 cm: horizon Ab composed of silty sands 10YR 4/2, having a moderately marked middle-sized to fine polyedric structure and thin sparse roots; 14-18 cm: massive grey fine sands 10YR 5/1; clear wavy boundary; 18-26 cm: horizon Ab' composed of fine silty sands 10YR 4/2, with brown variegations explainable as the result of radical bioturbations; well marked middle-sized to fine polyedric structure, with spar- se roots, sometimes woody too, with a subhorizon- tal course, an irregular boundary with pedoturba- tion of biological origin going down to the underlying levels (up to 60 cm); 26-30 cm: massive grey fine sands as above, 10YR5/1, with a light wavy boundary; 30-34 cm: horizon Ab'' composed of lightly silty fine sands 10YR 4,5/2, with a weakly marked middle- sized to fine polyedric structure and a lightly wavy boundary; 34-39 cm: lens of grey sand as above, 10YR 5/1; 39-46 cm: horizon Ab''' composed of fine silty sands 10YR 4,5/2, with a weakly marked medium-sized granular structure and a clear wavy boundary; pre- sence of partially decomposed needles; 46-70 cm: grey massive fine sands, alternating with fine silty-sandy ochre levels, subhorizontal and lightly wavy; clear boundary; 70-90 cm: ochre-yellowish massive sandy silt 2,5YR 5/3, with variegations composed of small but long empty spaces sometimes filled with decomposing dark roots; in the upper part a dark brown level rich in very little coal fragments (71-72 cm) and an ochre level with sporadic millimetric coal frag- ments (72-77 cm) can be observed; 90-400 cm: yellowish sandy silt alternating with subordi- nate centimetric levels of mainly massive grey silty fine sands, with little clear-cut lamination; locally (about 130 cm) there are laminas with a normal gradation. It is a monotone succession, totally lacking in organic elements. Pedogenized buried horizons have not been identified. Between 200 and 400 cm sparse little shells of gastropods can be found. 208 Ms. ricevuto il 23 ottobre 2001 Testo definitivo ricevuto il 10 settembre 2002 Ms. received: October 23, 2001 Final text received: September 10, 2002 M. Santilli et alii