Streamlined subglacial bedforms on the Närke plain, south-central Sweden – Areal distribution, morphometrics, internal architecture and formation

• Drumlins geomorphic differences are tied to different architectural compositions.
• Drumlins over crystalline basement areas are of the rock-cored type with sediment emplacement around a seeding rock core.
• Drumlins over Palaeozoic sedimentary rock strata with deep Quaternary drift on top are of soft-cored type.
• The latter are formed from compressional constructive deformation, accompanied by excavational deformation.
• Drumlin formation is better explained by a set of processes where equifinality is playing a significant role.

A flow set of close to 1000 drumlins has been mapped by means of LiDAR-derived digital elevation models and investigated by trenching. The area is situated on the SW part of the Närke plain and its surrounding uplands in south-central Sweden, which was deglaciated in the early Preboreal in a glacioaquatic setting. We find that there is considerable morphological difference in drumlin distribution patterns over crystalline basement areas compared to streamlined terrain over Palaeozoic sedimentary rock basement. The former area is characterized by thin Quaternary drift and the drumlins are all of the rock-cored type, built due to active deposition of sediment around obstacles to glacier flow. The latter area is characterized by deep Quaternary drift and the drumlins are more elongate and also larger in all dimensions, as compared to rock-cored drumlins. Irrespective of these geomorphological differences on local landscape scale we find that drumlin morphometric values remain part of a morphological continuum at the regional scale. Based on the internal sediment architecture as revealed in two cross-drumlin sections we find that the soft-cored drumlins were formed by compressional constructive deformation, along with excavational deformation along the flanks of the emerging drumlins, which shaped the separating troughs. Intermediate-type drumlins are those that demonstrate a coupling between underlying Palaeozoic sediment strata in areas of shallow drift sheet. These are the result of differing rheological response between incorporated sedimentary rock and a deforming bed below the ice-bed interface. An overall conclusion is that we find geomorphic and architectural compositional differences between the drumlins and the flowset they form. We can closely relate these differences to contextual geological variations with respect to basement type and drift depth. We argue that drumlin formation is better explained not by one single ‘unifying’ process but rather a set of processes (erosion/deformation/deposition) by which the subglacial system finds an efficiency equilibrium whereby an obstacle is shaped so that it enhances flow with a minimum of drag, i.e. the typical streamlined form is the result of a positive feedback cycle that tends towards efficiency. This approach argues that equifinality is playing a significant role in drumlin formation with a plurality of initial conditions and processes which, when combined with a uniformity of specific overriding processes and mechanical properties, results in a diversity of morphology and core type at the meso-scale and a continuum of landforms at the macro-scale.