Impaginato Panizza RREEPPOORRTT OONN ““GGLLAACCIIAALL”” FFOORRMMSS OOBBSSEERRVVEEDD OONN AAMMBBAA AARRAADDAAMM MMOOUUNNTTAAIINN ((NNOORRTTHHEERRNN EETTHHIIOOPPIIAA)) MMaarriioo PPaanniizzzzaa Dipartimento di Scienze della Terra – Università di Modena e Reggio Emilia (Italy) ABSTRACT Some geomorphologic characteristics observed on Amba Aradam Mountain (Tigray region, northern Ethiopia) at altitudes of about 2500 to 2780 m a.s.l. and latitude of about 13°30’ N, are described. These features might be ascribed to glacial modeling: cirques, roches moutonnées, glacial shoulders, moraine deposits and moraine arc and palustrine deposit due to moraine barrage. This framework is strongly in contrast with the fact that in Ethiopia the presence of glacial cirques and moraines was reported only in mountain areas at altitudes exceeding 4000 m a.s.l. An alternative assumption is that all these features might constitute a very singular case of “geo- morphologic convergence”, that is, landforms that have the same shape and appearance but different genesis. RIASSUNTO Relazione su forme “glaciali” osservate sul Monte Amba Aradam (Etiopia settentrionale). Vengono segnalate alcune caratteristiche geo- morfologiche sul Monte Amba Aradam (regione del Tigrai, nord Ethiopia) a quote fra circa 2500 e 2780 m .s.l.m. ed a una latitudine di circa 30°30’ N, che potrebbero far pensare a un modellamento glaciale: circhi, rocce montonate, spalle glaciali, depositi e arco moreni- ci, deposito palustre di sbarramento morenico. Questo quadro risulta in contrasto col fatto che in Ethiopia la presenza di circhi e more- ne glaciali sono state segnalati soltanto a quote superiori ai 4000 m s.l.m. Un'ipotesi alternativa è che ci si trovi di fronte a un caso molto singolare di “convergenza geomorfologica”, cioè di forme del rilievo che hanno la stessa forma, ma che sono il risultato di proces- si morfogenetici differenti. Keywords: Quaternary glaciation, Ethiopia, glacial morphology, geomorphological convergence Parole chiave: Glaciazione quaternaria, Etiopia, morfologia glaciale, convergenza geomorfologica. Il Quaternario Italian Journal of Quaternary Sciences 1155(2), 2002, 275-278 During the IAG International Symposium on “Climate Changes, Active Tectonics and Related Geomorphic Effects in High Mountain Belts and Plateaux”, held in Ethiopia in December 2002, and, in particular, during an excursion to Amba Aradam Mountain, I had the opportunity to observe some very interesting geomorphologic features which are here described. The area surveyed is located SW of Makalè, in the Tigray region, in northern Ethiopia (fig. 1), at an altitude of 2500 to 2780 m a.s.l. and an latitude of about 13°30’ N. It corresponds to an E-W stretching valley, whose head is formed by two small tributary valleys with a NE to SW arrangement. From the geologic standpoint, it is made up of a silicoclastic sandstone sequence of conti- nental facies with quartz conglomerate, shaly and lateri- te levels, known as “Amba Aradam Formation” (Shumburo, 1968; Beyth, 1972; Dramis, Coltorti and Pieruccini, 2002), also known as “Upper Sandstone” (Merla and Minucci, 1938; Mohr, 1962). The age of this formation is ascribable to the Cretaceous (see: Dramis, Coltorti and Pieruccini, 2002; Nyssen et al., 2002). From the structural viewpoint, the valley’s lower portion seems to be affected by an E-W oriented tectonic line which might have conditioned the original modelling. This tec- tonic feature does not seem to be linked to the main fault systems described by Arkyn et al. (1971) and by Beyth, (1972), which are NNW-SSE and NNE-SSW oriented (Dramis, Coltorti and Pieruccini, 2002). Starting from the double head of the valley, the geomorphologic features are as follows (fig. 1). - The head of the little northern valley (1a in the geomorphologic sketch of fig. 1) is modeled within a semi-circular cavity with high and steep flanks, partially affected by rock falls which form debris accumulation at the foot of the rocky slopes. Also the head of the little NE valley (1b in the geomorphological sketch of fig. 1) has a similar form, although it is not as clearly defined and looks more degraded by erosion and accumulation processes. - Immediately downstream of the confluence of the two small valleys, a debris deposit is found (fig. 2 and point 2 in the geomorphological sketch of fig. 1), made up of lithologically homogeneous elements (from the Amba Aradam Formation), though with quite a varied grain-size distribution (from blocks to sand and silt). These elements are mixed, non stratified or smoothed, with no apparent signs of friction. - On the flanks of the main valley some sandstone outcrops appear to have been modeled in the form of convex and smoothed surfaces, with traces of sub-hori- zontal grooves which subsequently were partially sectio- ned by rock shattering processes (fig. 3 and points 3 in the geomorphological sketch of fig. 1). - On the floor of the main valley a palustrine depo- sit is found (fig. 4A and point 4 in the geomorphological 276 M. Panizza Fig 1 - Geomorphologic sketch of the Amba Aradam south-western slope and location of area surveyed. Legend: 1 - head of small val- leys; 2 –debris deposit; 3 – rocks modeled in form of convex, worn smooth surfaces; 4 – palustrine deposits; 5 – debris deposits (5b: diamicton). Schizzo geomorfologico del versante a sud-ovest dell’Amba Aradam e ubicazione dell'area studiata. Legenda: 1 – testata di vallecole; 2 – deposito detritico; 3 – roccia modellata in superfici convesse e levigate, con tracce di scanellature; 4 – deposito palustre; 5 – depositi detritici (5b: diamicton). Fig. 2 - Debris deposit at the con- fluence of the two small valleys. Deposito detritico alla confluenza delle due vallecole sketch of fig. 1 ); its origin is evidently due to the dam- ming of the valley by debris accumulation. - Two detrital bodies developed on the two sides of the valley (fig. 4), one facing south (B) and the other north (C), appear to be responsible for the valley dam- ming and the afore mentioned palustrine deposit. On the whole these two deposits make up an arc-shaped form of varying clarity; the right-hand side deposit (B in fig. 4 and point 5a in the geomorphological sketch of fig. 1) has been partially terraced by anthropogenetic proces- ses. From the grain-size viewpoint, they are both made up of very heterogeneous materials – up to boulders of about ten cubic meters in volume – resulting from the Amba Aradam Formation. 277Report on “glacial” forms ... Fig. 3 - Arenaceous rocks modeled in the form of convex, worn smooth surfaces. Rocce arenacee modellate in superfici convesse e levigate Fig. 4 - Palustrine deposits (A) and debris accumulations (B and C); the former (B) have been partially modeled as terraces by anthropo- genetic processes. Deposito palustre (A) e accumuli detritici affacciantisi (B e C), dei quali il primo (B) parzialmente modellato a terrazzo da processi di antropizzione. Fig. 5 - Diamicton from deposit 5b of the geomorphological sketch of fig. 1. Diamicton del deposito 5b dello schizzo geomorfologico della fig. 1. 278 - Outcrop of the detrital body (fig. 5) (C in fig.4 and 5b in the geomorphological sketch of fig. 1): this is a dia- micton-type deposit, that is unsorted with sand and coarse particles dispersed through a mud matrix. It is not stratified and some elements show a certain degree of smoothing. The genetic picture of the area points to glacial modeling. There are very evident indications of this; in fact, from top to bottom, the six geomorphologic units previously described seem to be related respectively to: glacial cirques (points 1 in the geomorphological sketch of fig. 1), moraine deposit from glacial confluence (fig. 2 and point 2 in the geomorphological sketch of fig. 1), roches moutonnées (fig. 3 and points 3 in the geo- morphological sketch of fig. 1), moraine barrage pond (fig. 4A and point 4 in the geomorphological sketch of fig. 1 ), small moraine arc (B and C in fig. 4 and points 5a and 5b in the geomorphological sketch of fig. 1) and lateral and frontal moraine deposits (B and C in fig. 4 and points 5a and 5b in the geomorphological sketch of fig. 1). This framework seems to be completed by the presence of less inclined portions of slope, similar to the so called “glacial shoulders”. This picture, though, is in contrast with the fact that in Ethiopia the presence of glacial cirques and moraines was reported only in mountain areas at much higher elevations (Mts. Simien, 4620 m a.s.l.; Arsi, 4180 m; Bale, 4357 m) (Nyssen et al., 2002). Some Authors, such as Nilsson (1940) and Hovermann (1954), descri- bed glacial traces found at lower altitudes, but subse- quent researchers (Semmel, 1963; Potter, 1976; Hastenrath, 1977; Messerli and Rognon, 1980) refused this hypothesis with different arguments (Nyssen et al., 2002). Certainly the conformation of this E-W stretching narrow and deep valley, sheltered from the wind, could have favored the persistence of snow and ice during a cold Pleistocene period. This possibility, though, needs to be further investigated by means of particularly detai- led geomorphologic surveys, correlations with other similar traces in other parts of Africa placed at the same latitude, and a precise reconstruction of the climatic con- ditions (temperature, precipitation and wind regimes etc.) existing in the Tigray during the Pleistocene. As an alternative hypothesis, all these features might constitute a very singular case of “geomorpholo- gic convergence”, that is, landforms that have the same shape and appearance but different genesis. In this case, the area would be a very good educational exam- ple to alert the onlooker to simplistic deductions based mainly on exterior appearances, that is, on prevalently descriptive characteristics (Panizza, 1996). AACCKKNNOOWWLLEEDDGGEEMMEENNTTSS I wish to thank prof. F. Dramis (University of Roma Tre) and dr. J. Nyssen (University of Makalè) for the useful exchanges of ideas mainly in the field and for the bibliographic informations. RREEFFEERREENNCCEESS Arkin Y., Beyth M., Dow D.B., Levitte M., Temesgen H. and Hailu T., 1971. Geological map of Mekelle sheet ND 37-11 Tigre Province. Min. Mines, Energy and Water Res., Geol. Survey Ethiopia, Addis Ababa. Beyth M., 1972. Paleozoic-Mesozoic Sedimentary Basin of Mekelle Outlier. Northern Ethiopia. Amer. Ass. Petrol. Geol. Bull., 56, 12, 2426-2439. Dramis F., Coltorti M. and Pieruccini P., 2002. Geological and geomorphological framework of the excursion area. IAG International Symposium, Addis Ababa, december 2002, 1-12. Hastenrath, S., 1977. Pleistocene mountain glaciation in Ethiopia. J. Glaciol. [Cambridge], 18/79: 309-313. Hövermann, J., 1954. Über die Höhenlage der Schneegrenze in Äthiopien und ihre Schwankungen in historischer Zeit. Nachr. Akad. Wiss. Göttingen, 6: 111-137. Merla G. and Minucci E., 1938. Missione geologica nel Tigrai. Vol. 1 “La serie dei Terreni”, Rendic. Reale Accad. Italia, Centro Studi Africa Orient. Ital., 3, 362 pp. Mohr P.A., 1962. The geology of Ethiopia. Addis Ababa University Press, 268 pp. Messerli, B. and Rognon, P., 1980. The Saharan and East African uplands during the Quaternary. In: Williams M., Faure H. (eds.): 87-132. Nilsson, E., 1940. Ancient changes of climate in British East Africa and Abyssinia. A study of ancient lakes and glaciers. Geogr. Ann., 22: 1-79. Nyssen J., Poesen J., Moeyersons J., Deckers J., Mitiku H. and Lang A., 2002. Quaternary glaciations in the Ethiopian mountains. Earth Sc. Rev., in press. Panizza M., 1996. Environmental geomorphology. Elsevier, Amsterdam, 268 pp. Potter, E., 1976. Pleistocene glaciation in Ethiopia; new evidence. Journal of Glaciology, 17 (75): 148-150. Shumburo M.M., 1968. The Amba Aradam formation (formely the Upper Sandstone). Mobil Petroleum Ethiopia Inc., unpub. Semmel, A., 1963. Quartärgeologische Untersuchungen im Hochland von Nordwest-Aethiopien. Z. dtsch. geol. Gesellsch., 115 (2-3): 882. Ms. ricevuto il 20 gennaio 2003 Testo definitivo ricevuto il 4 febbraio 2003 Ms. received: Genuary 20, 2003 Final text received: February 4, 2003 M. Panizza