Drumlins carved by deforming till streams below the Laurentide ice sheet

The Peterborough drumlin field (5000 km 2 ) is the largest in central Canada and was formed during readvance of the Laurentide ice sheet shortly after 13,000 B.P. Morphometric analysis of 998 drumlins, combined with study of 6921 borehole logs and downhole geophysical data, has established the distribution of drumlin types and their stratigraphy along a 70 km flow line through the drumlin field. In the north, drumlins are spindle-shaped, composed of till resting directly on bedrock, and were formed by deformation of till into low-pressure zones in the lee of bedrock scarps. Eskers record subglacial drainage by channeled meltwaters flowing on bedrock. To the south, drift thickness increases and eskers are absent; drumlins are larger and more ovoid and are composed of overridden proglacial and glaciolacustrine sediments overlain erosively by deformation till. Till is thin over drumlin crests and thick in interdrumlin swales and records subglacial cannibalization and mixing of overridden sediments; the drumlin form is the product of erosion by streams of deforming till flowing in swales separating cores composed of older sediments. Systematic down-field changes in drumlin shape and size can be related to decreased duration of subglacial deformation toward the limit of the readvance and increased thickness of preexisting sediments available for dissection by subglacial till streams. Absence of eskers in the central and southern parts of the drumlin field suggests that the ice base was drained by mass flow of the deforming substrate. The substantial subsurface data available from the Peterborough area indicate that drumlins are predominantly erosional in origin and are the geomorphological expression of a deforming subglacial bed below the margin of the Laurentide ice sheet.