Origin of drumlins on the floor of Lake Ontario and in upper New York State

Upper New York State, USA contains one of the largest drumlin fields (12,000 km2) on the North America continent consisting of some 10,000 drumlins located between Lake Ontario in the north and the Finger Lakes; drumlin-like bedforms have also recently been discovered in deep water (∼ 200 m) on the nearby floor of the Rochester Basin in Lake Ontario. The New York State drumlin field was produced under the Laurentide Ice Sheet 18,000 years ago when it extended southwards to terminate against the Allegheny Plateau. Outcrops of drumlin cores exposed by erosion along the Lake Ontario shoreline provide windows into the stratigraphy and sedimentology of the northern part of the field. Sedimentological description of 10 coastal drumlin cores, combined with quantitative morphometric analysis of 1487 drumlins, shows that they are the product of subglacial erosion of till sheets separated by glaciolacustrine deposits. A two-phase subglacial model for drumlin formation is suggested consisting of initial aggradation of extensive till deposits during ice advance followed by erosion and streamlining possibly accompanying stillstand of the ice margin and building of moraines. Systematic changes in drumlin morphometry and spacing are evident, ranging from small numbers (< 15% of total bedforms) of widely separated, spindle-shaped forms (Length/Width ratios between 6 and 18) to more numerous (< 40%) broader forms (L/W ratios between 3 and 6) to most numerous (> 60%) closely spaced, ovoid drumlins (L/W ratios between 1 and 3). Drumlin shape appears to be systematically related to sediment thickness available for erosional streamlining; elongate spindle forms occur on the crests and lee sides of bedrock escarpments where sediment cover is thin. Larger ovoid drumlins occur in topographic depressions where sediment thickness is greater. Offshore streamlined forms on the floor of the deep water Rochester Basin in Lake Ontario have long axes at right angles to the trend of onshore drumlins. These are composed of rock (rock drumlins) and have an orientation structurally controlled by a newly identified inlier of Proterozoic Grenville Province metasediments.