ABSTRACT: Interaction of two glacial lobes in the formation of drumlins along the north shore of Lake Ontario

CGRG Bibliography of Canadian Geomorphology
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Abstract

The last advance of the Ontario ice lobe that occupied the Lake Ontario basin during the late Wisconsinan produced a drumlin field along the north shore containing approximately 600 drumlins over an area of 1500 km2. Drumlins in the vicinity of the shoreline are elongated, spindle shaped and low. Farther inland, drumlins are higher and wider displaying the characteristic inverted spoon shape. The drumlins are uniformly scattered across the field but some cluster patterns are evident. Drumlins along the lakeshore trend 318 to 346 degrees (SE-NW), but inland east of Whitby some of them are aligned NE-SW. Nine drumlin bluff exposures located along the north shore between Pickering to Bowmanville reveal variable stratigraphic sequences consisting of up to four tills separated by glacio-fluvial and glacio- lacustrine sediments. Drumlin cores consist mainly of massive, clast rich lodgement and deformed- lodgement tills. Other notable features include variable clast fabrics trending 280 degrees in the western part of the field to 44 degrees in the east. Drag folds, overturned folds and squeeze flow structures trend NW. Shear planes are numerous and are found to be steeply rising towards the SE and also gently rising NW. Selective erosion and deposition by meltwater via subglacial tunnels occurred along the flanks of some drumlins. The drumlins are products of deposition, deformation and erosion by probably two ice lobes. Orientation of drumlins,associated structures, till fabric data and the purple/red garnet ratios (0.64-0.78) suggest that the Simcoe/Kawartha ice lobe flowing NE-SW overrode the eastern half of the Oak Ridges moraine and terminated in the Ontario basin in the eastern part of the drumlin field. The Ontario lobe flowing dominantly E-W and splaying towards NW in our area took over later on. This lobal interaction is responsible for the complex stratigraphy, heavy mineral results and clast fabrics witnessed in the eastern portion of the field. The final shaping of the drumlins was by the Ontario lobe. The stratigraphy of our drumlins in the bluff sections correlates very well with the till units of the Bowmanville bluffs to the east and ties in to some degree with the Scarborough bluffs' till units to the west.

Bibliography of Canadian Geomorphology