Drumlins are relatively small, elongate hills aligned with ice-flow direction (Menzies 1979). The name drumlin originates from the Irish word druim, which means rounded hill. They are among the most widespread glacial landforms. Drumlins are often likened to half-buried eggs or inverted spoons, with one slope blunter than the other. They belong to the group of ice-flow-parallel glacial landforms that includes glacial striae, flutes, crag-and-tails and mega-scale glacial lineations. Drumlins are very common in the landscape of central and northern Scandinavia (Hättestrand 1998), but their identification in the Baltic Sea and Gulf of Bothnia has been hindered by a lack of seafloor data. Here we show a drumlin field from the southern Gulf of Bothnia.
Description
The drumlin field shown in Figure 1 stretches for >85 km and probably continues northwards beyond the coverage of the high-resolution bathymetric data available for this study. The outlines of the drumlins are generally easy to distinguish (Fig. 1c), although their surfaces are frequently scarred by iceberg ploughmarks and pits (Fig. 1e, f). The drumlins are aligned NNW–SSE (169°) and their morphology changes progressively along the mapped section. The northern part is characterized by shorter, wider drumlins, whereas in the south more elongate drumlins predominate. Drumlins often merge together to form larger complexes, where several drumlins are seemingly streamlined into the same underlying body of sediment. A sample of 342 drumlins was digitized from throughout the area. Of these, sizes range from 214 to 2843 m in length and from 62 to 1205 m in width. The average length and width are 1032 and 296 m, respectively. Within this sample, drumlin peak height from the surrounding seafloor is on average c. 6 m and ranges from a few metres to 30 m. There is no general geometry that can be used to describe the drumlins in the mapped area. Some have a blunter and wider side facing NNW, i.e. up-ice, while others are the opposite with a blunter, wider lee side (Fig. 1g). The seafloor base-level rises from c. 83 m deep in the north to 52 m in the south; the drumlin field is therefore located on a slight uphill slope with respect to the ice-flow direction. The drumlin field appears to terminate in the south at about 61°18′ N where it is cross-cut by seafloor features interpreted to be meltwater channels, and by a second set of glacial lineations oriented NNE–SSW (Fig. 1d).
Multibeam bathymetry from drumlin field recording past ice-flow from NNW to SSE in the southern Gulf of Bothnia. Acquisition system Kongsberg EM 2040, frequency 200–400 kHz in the south and Reson 7125 SV, frequency 200/400 kHz in the north. Grid-cell size 5 m. (a) Overview of study area. Regional bathymetry is from the Baltic Sea Hydrographic Commission, 2013 (Baltic Sea Bathymetry Database v. 0.9.1). (b) Location of study area (red box; map from GEBCO_08). (c) Close-up of northern area with well-defined drumlins. (d) Cross-cutting of drumlin field by elongate glacial lineations indicating a later ice-flow event towards the SSW. Both sets of drumlins (c, d) are dissected by a large meltwater channel network flowing WSW. (e, f) Icebergs have ploughed and pitted the tops of drumlins. (g) Bathymetric profile along drumlin. VE×20.
Regional marine geological mapping by the Geological Survey of Sweden (SGU) shows that the surface sediments in the drumlin area consist of till (regionally 5–10 m thick) and glacial/postglacial clays. It is not, however, presently possible to determine the internal composition of the drumlins.
Interpretation
The morphometry of the landforms from the Gulf of Bothnia described here is entirely consistent with drumlins reported from various palaeo-ice-sheet terrains elsewhere (e.g. Hättestrand et al. 2004; Clark et al. 2009). ‘Reverse’ forms, that is, those with steeper lee than stoss sides, are found to be prevalent here (cf. Spagnolo et al. 2011). The available surficial sediment and sediment thickness interpretations by the SGU, the lack of bedrock outcropping within this 85 km stretch, the ‘texture’ of the imaged drumlins (as opposed to the glacifluvial deposits in this area) and the deep incision and pitting by meltwater channels and icebergs ploughmarks all suggest that these drumlins comprise glacial tills. Seismic line interpretations by SGU, however, imply that many of these features have bedrock cores which probably acted as foci for sediment deposition.
Despite almost two centuries of study, there is a lack of consensus on the formation mechanism, relevant timescales and glacidynamic context of drumlins (Clark 2010). Observations from contemporary ice masses reveal drumlin formation both directly behind the ice margin (Johnson et al. 2010) and up-ice, at the onset of ice streaming (King et al. 2007). Their ubiquitous nature across Pleistocene glaciated terrains argues against formation under exclusively fast-flow regimes. However, given the progressive downstream elongation of the drumlins and their position and orientation with respect to the wider basin and the regional palaeo-glaciology, it is reasonable to suggest that this drumlin field represents acceleration of ice flow towards the Baltic Ice Stream. This is unlikely to be the onset zone of a full-glacial Baltic Ice Stream, since channelized meltwater landforms and very small, draping moraines are associated with the drumlin assemblage, but rather a deglacial phase with a more limited extent. Cross-cutting of the southern part of the drumlin field by extremely elongate lineations probably indicates a reorganization of ice flow during the retreat of the ice stream, and that it remained an active fast-flow conduit through deglaciation.
Until now, the dynamics of ice in the Bothnian and Baltic basins were largely a matter of conjecture, and the Baltic Ice Stream a hypothetical feature of Fennoscandian Ice Sheet reconstructions based on peripheral ice-flow signatures and the requirement to deliver ice far into southerly latitudes from a northern ice divide (e.g. Lundqvist 2004). The glacial landforms observed here begin to confirm the Baltic Ice Stream and reveal its geometry and dynamics.
- © 2016 The Author(s). Published by The Geological Society of London. All rights reserved
