Rock drumlins and megaflutes of the Niagara Escarpment, Ontario, Canada: a hard bed landform assemblage cut by the Saginaw–Huron Ice Stream

The late Wisconsin (c. 18,000 ybp) Saginaw–Huron Ice Stream (SHIS) of the Laurentide Ice Sheet was as much as 800 km long and 250 km wide in the Great Lakes basin of mid-continent North America. In its onset zone in the upper Lake Huron basin of Ontario, Canada it flowed south from the high standing Canadian Shield to move over gently dipping Ordovician and Silurian dolostones devoid of glacial sediment cover. The onset zone is recorded by a distinct ‘hard bed landform assemblage’ across ∼3000 km2 of north-facing escarpments (Niagara, Fossil Hill, and Kagawong) and dip slope pavements on the Bruce Peninsula and Manitoulin Island, and the adjacent floor of Lake Huron. A wide range of glacially-streamlined rock landforms were carved into dolostones below fast flowing ice. The largest are 30 km-wide bullet-shaped escarpment ‘promontories’ that face upglacier. Superposed on these are swarms of rock drumlins up to 5 km wide with downglacier lengths of as much as 10 km. Promontories and drumlins record streaming of dirty basal ice around escarpment highs and create a distinct ‘zig-zag’ planform to the escarpments. In turn, down-dip, trailing-edge dolostone pavements are corrugated by kilometre-long megagrooves and megaflutes cut by basal ice flowing around resistant high-standing bioherm mounds on dolostone bedding planes. The geomorphology of the Niagara Escarpment does not primarily reflect a lengthy history of preglacial Cenozoic fluvial erosion as classically argued, but geologically-brief episodes of accelerated abrasion and quarrying below ice streams within successive Pleistocene ice sheets.

► Onset zone of Saginaw–Huron Ice Stream (SHIS) located in Lake Huron basin (c. 18K ybp).
► SHIS flowed south over Ordovician and Silurian dolostones free of sediment cover.
► Glacially-streamlined rock landforms were carved into dolostones by SHIS.
► Their geomorphology records geologically-brief episodes of accelerated erosion.