Geological Survey of Denmark and Greenland Bulletin 26, 2012, 17-20 17 Rock-cored drumlins on Bornholm, Denmark Peter Roll Jakobsen The surface morphology of Denmark is predominantly of glacial origin, created in depositional, deformational and ero- sional environments. In addition, postglacial marine, fresh- water and aeolian processes have formed a variety of land- forms. Overviews of the Danish landscape were published as geomorphological maps (Milthers 1948; Schou 1949; Smed 1981), and a new one is currently in preparation. On Born- holm, the morphology differs from the rest of the country because bedrock is present at or near the surface. This paper describes drumlins formed on bedrock on Bornholm, which have not previously been recognised. New geomorphological map of Denmark 1:200 000 The purpose is to create a map based on the Geographical Information Systems (GIS). To draw the boundaries of the different landform types as precisely as possible, the mor- phology was re-interpreted on the basis of topographical maps, LiDAR (light detection and ranging) data, geologi- cal maps as well as available literature. The scale of the new map is 1:200 000, but it is compiled at 1:100 000 and will be published in both a printed and a digital version, the for- mer in four map sheets (Jacobsen in press): (1) northern Jyl- land, (2) central Jylland, (3) southern Jylland and Fyn and (4) Sjælland, surrounding islands and Bornholm. So far, a preliminary version of southern Jylland has been published (Gravesen et al. 2004), and the map sheet covering Sjæl- land, the surrounding islands and Bornholm is completed. Most of the surface features have already been described (Milthers 1948; Schou 1949; Smed 1981). However, the morphological elements shown on these maps vary to some degree. The new geomorphological map of Denmark also differs significantly from older maps, and some surface ele- ments are re-interpreted. New elements include mega-scale glacial lineations and Rogen moraines, and on Lolland a val- ley formerly classified as a tunnel valley is re-interpreted as a fracture valley. Bornholm Large parts of Bornholm consist of pre-Quaternary crystal- line bedrock with a discontinuous, thin cover of Quaternary sediments and is classified as glacially scoured bedrock on the geomorphological map (Fig. 1). During the Quaternary glaciations, the bedrock was affected by overriding glaciers. It mainly shows erosional features such as fracture valleys and mega-scale glacial lineations, but also smaller features, not shown on the map, such as roches moutonnées and gla- cial striae (Grönwall & Milthers 1916; Gravesen 1996). One of the most pronounced terrain features on Born- holm is fracture valleys, which are subglacially eroded faults and fracture zones within the basement rocks. These valleys outline the fault systems from multiple deformation phases in the Sorgenfrei–Tornquist Zone (Grönwall & Milthers 1916; Graversen 2009). The main orientations of the frac- ture valleys are N–S, NNE–SSW and NE–SW. A few are oriented NW–SE. On Bornholm, the orientations of glacial striae show two general directions: NE–SW and ESE–WNW (Fig. 2; Grönwall & Milthers 1916). Mega-scale glacial lineations within the glacially scoured bedrock terrain are parallel with the measured glacial striae, and most of them are oriented NE–SW. In general, the two populations of glacial striae on Born- holm occur in two different groups of pre-Quaternary rocks. In the southern part of Bornholm, the pre-Quaternary ge- ology is dominated by sandstone, shale and unconsolidated or poorly consolidated sediments, which are softer than the granites and gneisses in the rest of the island. The southern part of Bornholm is dominated by a till plain. Mega-linea- tions in this till plain show a slightly curved ESE–WNW trend and are parallel to the dominant orientation of glacial striae in the area (Figs 1, 2). Marginal moraines are not as distinct as in other parts of Denmark. They occur as scattered sandy and gravelly hills, and outline three ice marginal stages in the central and northern parts of Bornholm (Fig. 2). A N–S-trending ridge in the south-eastern part of Bornholm, associated with a kame, is also interpreted as a marginal moraine (Fig. 1). It is a low ridge that separates a till plain with mega-lineations to the west from a smooth till plain to the east. A few small out- wash plains are found in south-western Bornholm. Raised beaches and beach ridges from the Baltic Ice Lake are found along the coast at different levels, especially towards the east and north (Fig. 1). © 2012 GEUS. Geological Survey of Denmark and Greenland Bulletin 26, 17–20. Open access: www.geus.dk/publications/bull 1818 Rock-cored drumlins In the northernmost part of Bornholm, within the glacially scoured bedrock terrain, there are ten elongated hills with typical drumlin shapes (Figs 1, 3). Drumlins are subglacial bedforms that were generated by the activity of overriding ice. Drumlins are oval-shaped hills that formed beneath an ice sheet and aligned in the direction of ice flow, and they are common and widespread in formerly glaciated regions. The formation of drumlins has been widely discussed, and they are one of the most studied glacial landforms on Earth (Clark et al. 2009; Johnson et al. 2010). On Bornholm, Li- DAR data have been invaluable in recognising these features (Fig. 3). Like other drumlins, those on Bornholm are oval, and the surface has smooth contours with gradual fall to all Mega-scale glacial lineation Till plain Drumlin Esker Kame Marginal moraine Outwash plain Valley system Glacial lake plain Raised beach plain (Baltic Ice Lake) Raised beach ridge (Baltic Ice Lake) Beach ridge Marine plain Bog Dune Aeolian plain Township Fracture valley Glacially scoured bedrock 5 km 10° 14°E 56° 57°N Sweden Germany 50 km Jylland Fyn Bornholm 55°15´N 55°15´N 14°58´E 14°58´E Denmark Sjælland Fig. 1. Geomorphological map of Bornholm. 19 sides. The length of the drumlins varies from 140 m to a little more than 600 m and their heights from 5 m to 31 m (Ta- ble 1). Their length to width ratio varies from 1.6 to 2.9 with an average of 2.3. The orientation of the hills is NE–SW, with an average of 50°. The drumlins have a core of granite and are more or less covered by till (Fig. 3) that has predominantly been deposited towards the south-west on the lee side of the drumlins. The ice movement direction, indicated by glacial striae, is from the north-east (Grönwall & Milthers 1916), and the till cover is thickest on the south-western lee side of the drumlins. Discussion In the northern part of Bornholm, the drumlins and glacial striae indicate an ice-movement direction from the north-east to the south-west and are developed on crystalline bedrock. A few glacial striations have an E–W orientation (Fig. 2). In the southern part of Bornholm, only ice-movement direc- tions from E to W are seen within the softer bedrock. These E–W erosional features were formed during a subsequent ice-stream event, which erased earlier NE–SW features in the southern part, but did not have the capacity to erode the harder crystalline bedrock. It appears that the subglacial conditions during the ice advances from the north-east were suitable for erosion of the granites on the northern part of Bornholm. As indicated by the fracture valleys, the fault pattern is in N–S, NNE–SSW and NE–SW directions in this part of Bornholm, which is probably also true for the joint pattern. This joint and fault pattern could also favour formation of the elongated hills in this NE–SW direction, and would re- sult in a preferred direction of erosional features by the ice advances from the north-east. The orientations of the erosional features range from 39° to 65° and could reflect several ice advances, although they cannot clearly be separated into two or more directions. Investigations of drumlins in Sweden (Hättestrand et al. 2004) show that rock-cored drumlins are formed by succes- sive phases of erosion. If this also holds for the drumlins of Bornholm, then they may have been formed during several ice advances or even ice ages, and only by glaciers advancing from the north-east. Conclusions On the northern part of Bornholm, ten bedrock-cored drum- lins are recognised and are included in the new digital, geo- morphological map of Denmark at a 1:200  000 scale. The LiDAR data were of great value in recognising them, al- though they are also recognisable from the contours of the topographic maps. The drumlins have a core of granite and are more or less covered with till that was predominantly de- posited towards the south-west on the lee side of the drum- lins. The lengths vary from 140 m to 612 m, and the height from 5 m to 31 m. The orientation of the drumlins is NE–SW, with an ave- rage strike of 50° formed by subglacial processes during one or several ice advances from the north-east. The orientation of the fracture valleys could very well intensify the preferred erosional orientation parallel to the drumlins, and thus be an important factor in the drumlin formation. Ice marginal stage Granite and gneiss Sandstone, shale and sediments Glacial striae Pre-Quaternary geology 50 km N Fig. 2. Map of Bornholm showing orientation of glacial striae on bedrock, the distribution of crystalline and sedimentary bedrock types and ice mar- ginal stages (after Grönwall & Milthers 1916; Hansen & Poulsen 1977). Number/ Length Width Height L:W Orientation Name (m) (m) (m) ratio (°) 1 Langebjerg 612 211 31 2.9 60 2 Kælderbakke 140 67 7 2.1 65 3 Høje Meder 177 102 25 1.7 46 4 Kajbjerg 332 160 10 2.1 48 5 Byggehøj 532 205 8 2.6 46 6 Hestenshøj 330 145 10 2.3 56 7 Blåholtshus 395 137 10 2.9 51 8 Brogård 225 98 5 2.3 47 9 Blåholtsgård 391 148 10 2.6 43 10 Hammershus 389 239 25 1.6 39 Table 1. Dimensions of the drumlins on Bornholm 2020 References Clark, C.D., Hughes, A.L.C., Greenwood, S.L., Spagnolo, M. & Ng, F.S.L. 2009: Size and shape characteristics of drumlins, derived from a large sample, and associated scaling laws. Quaternary Science Reviews 28, 677–692. Gravesen, P. 1996: Geologisk set – Bornholm. En beskrivelse af områder af national geologisk interesse, 208 pp. København: Miljøministeriet, Skov- og Naturstyrelsen og Danmarks og Grønlands Geologiske Un- dersøgelse. Gravesen P., Jakobsen, P.R., Binderup, M. & Rasmussen, E.S. 2004: Ge- ologisk set – Det sydlige Jylland. En beskrivelse af områder af national geologisk interesse, 188 pp. København: Miljøministeriet, Skov- og Naturstyrelsen og Danmarks og Grønlands Geologiske Undersøgelse. Graversen, O. 2009: Structural analysis of superimposed fault systems of the Bornholm horst block, Tornquist Zone, Denmark. Bulletin of the Geological Society of Denmark 57, 25–49. Grönwall, K.A. & Milthers, V. 1916: Beskrivelse til geologisk Kort over Danmark i Maalestok 1:100  000, Kortbladet Bornholm. Danmarks Geologiske Undersøgelse I. Række 13, 281 pp. + atlas. Hansen, M. & Poulsen, V. (eds) 1977: Geologi på Bornholm, 96 pp. Varv ekskursionsfører 1. København: Tidsskriftet Varv. Hättestrand, C., Götz, S., Näslund, J.O., Fabel, D. & Stroeven, A.P. 2004: Drumlin formation time: evidence from northern and central Sweden. Geografiska Annaler 86A, 155–167. Jakobsen, P.R. in press: Geomorphological map of Denmark, 1:200 000, 4 map sheets. Copenhagen: Geological Survey of Denmark and Green- land. Johnson, M.D., Schomacker, A., Benediktsson, Í.Ö., Geiger, A.J., Fergu- son, A. & Ingólfsson, Ó. 2010: Active drumlin field revealed at the mar- gin of Múlajökull, Iceland: a surge-type glacier. Geology 38, 943–946. Milthers, V. 1948: Det danske Istidslandskabs Terrænformer og deres Opstaaen. Danmarks Geologiske Undersøgelse III. Række 28, 233 pp. Schou, A. 1949: Atlas of Denmark, I, Landscapes, 160 pp. + atlas. Copen- hagen: Det Kongelige Danske Geografiske Selskab. Smed, P. 1981: Landskabskort over Danmark, sheet 4, Sjælland, Lolland, Falster, Bornholm. Brenderup: Geografforlaget. 1 2 3 4 5 6 7 8 9 10 A B Granite Clay till Meltwater sand Outwash sand Peat Aeolian sand Beach deposits from the Baltic Ice Lake 500 m 500 m N N Fig. 3. A: LiDAR map of the northernmost part of Bornholm. The elevation varies from sea level to 118 m, and the highest parts are shown in white. The drumlins are indicated with red lines. The numbers refer to Table 1. B: Geological map of the same area. Author’s address Geological Survey of Denmark and Greenland, Øster Voldgade 10, DK-1350 Copenhagen K, Denmark. E-mail: prj@geus.dk