Size distribution of two cross-cutting drumlin systems in northern Sweden:: a measure of selective erosion and formation time length

Nearly 9000 individual lineations (drumlins and flutes) from two ice-flow systems have been mapped over a 22 500 km(2) area of northern Sweden by interpretation of aerial photographs. The lineations were classified according to their size and orientation to allow a quantitative analysis of their distribution. The two glacial lineation systems, separated in time by at least 100 000 years and two individual overriding ice sheets, cross-cut at a high angle. The older system occurs over the entire study area and is interpreted to have formed by prolonged sheet flow during Marine Isotope Stage 5d, while the youngest system is spatially restricted, marking the presence of a laterally constrained zone of faster ice flow, developed near the end of the last deglaciation. The effect of the younger ice flow was erosion and elimination of smaller lineations in the older system, while the larger drumlinoid forms survived more or less intact, thus creating an apparent increase in average drumlin size of the older lineation system. One possible explanation for the unaltered shape of these larger drumlins is that the glacier bed of the younger ice sheet only reached the pressure melting point in the lower parts of the terrain, thus eroding the smaller forms, while the larger forms protruded into colder parts of the ice mass and were protected from erosion and deformation. There was no evidence of the deformation of older drumlins into barchanoid shapes or the development of lee-side tail features on the distal side of pre-existing lineations, as has previously been described for crossing drumlin systems. The lack of any such deformational features suggests that the younger lineation system was formed by net erosion, rather than deformation or deposition.