Surficial geology of Cockburn Island, Ontario

Cockburn Island landscape is somewhat unique in the northern Lake Huron-Georgian Bay basin, in that, it's overall topography and relief is dependant primarily on glacial sediments that can exceed thicknesses of 85 m (275 feet), rather than the bedrock surface. Cockburn Island is underlain by gently dipping carbonate and clastic rocks of Middle Silurian age that outcrop in a belt that rings the northern half of the island. The crest of the Niagara Escarpment crosses the northern part of the Island. In places the bedrock surface has been affected by karst processes particularly in areas above the level of the Nipissing transgression where the land surface has been exposed for approximately 11,500 years. Karst features and alvars appear to be best developed in rocks of the Amabel Formation along the crest of the Niagara Escarpment. Evidence of the direction of glacier flow that affected Cockburn Island is primary from the orientation of streamlined forms, in particular drumlins. Three distinct sets of drumlins have been recognized. The drumlin orientation does not necessarily reflect flow during two or multiple glacial advances. The variation in orientation of the long axis of the drumlins, the south-southwest flow around the eastern shore of the island, southern flow in the central part of the island and the south-eastward flow the western shore, may reflect ice flowing around the island along the inter-island channels at a greater speed than that flowing over the higher areas of the island. Drumlins are commonly associated with till. Till is widespread across the island and occurs commonly as poorlydrained till plains, littered with boulder (lags). In addition, to till and its associated landforms, other surface landforms and sediments include a large hill of sand and gravel covered with till (McCaigs Hill), and two long ridges of gravel of probable ice-contact origin. Emphasis in this report has been placed on the Post-glacial shoreline features of Glacial Lake Algonquin and subsequent glacial and post-glacial lakes that greatly affected the landscape of Cockburn Island. The record of ancestral lake levels in the Lake Huron basin on Cockburn Island appears complete including the highest level of glacial Lake Algonquin (Main) through a series of falling glacial lake and post-glacial lake levels. These ancestral lake levels have created a spectacular record of abandoned shore bluffs, beach ridges and bars. Extensive areas of surface sand and gravel deposited in ice marginal or subglacial settings and the karst terrain along the crest of the Niagara Escarpment are the main areas of groundwater recharge on Cockburn Island.