Origin of glacial curvilineations by subglacial meltwater erosion: Evidence from the Stargard drumlin field, Poland

Abstract

Deciphering the origin of subglacial landforms is crucial for understanding processes operating under ice sheets. Due to the inaccessibility of the ice/bed interface limiting real-time observations, the origin of these landforms is typically interpreted from the morphological and sedimentological record exposed after ice retreat, which makes the conclusions conjectural. Here, we focus on a recently discovered new type of subglacial landform called glacial curvilineations (GCLs), whose formation generates some controversy. We use LiDAR imagery and geological data to constrain the origin of GCLs found in a tunnel valley in the Stargard drumlin field, Poland. The tunnel valley is about 3800 m long, 520 m wide, and it occupies about 2 km 2 . A total of 61 individual GCLs have been mapped there; they are up to 550 m long and typically 0.5-1.5 m high, but some reach a height of up to 5 m. The GCLs are composed of mostly massive till represented by either a single or several diamicton units occasionally separated by thin layers of meltwater sand. Till macrofabrics in the GCLs and in the nearby till plain are distinctly consistent, and have high and uniform clustering strengths. At all locations, the dominant till fabric direction broadly corresponds with the ice movement direction given by the drumlin orientations, but it is oblique to the orientation of the tunnel valley and independent of the orientations of the individual GCLs. The lack of correspondence between the orientation of the GCLs and the till fabrics in them suggests that the GCLs consist of an antecedent till deposited during the drumlinizing ice advance and subsequently carved by subglacial meltwater erosion in the tunnel valley. These results substantiate the hypothesis that GCLs are erosional features postdating the material constituting them, and emphasize the role of subglacial meltwater flows as a landforming agent.