Sentry Island, Nunavut — a long, low esker extending into Hudson Bay. Eskers form from sediment deposited in subglacial meltwater tunnels. Photo: Wikimedia Commons / CC BY 2.0.
The Laurentide Ice Sheet
At its greatest extent, roughly 20,000 years ago, the Laurentide Ice Sheet covered essentially all of Canada east of the Cordillera and extended south into the present-day United States. Ice thickness exceeded 3 kilometres over Hudson Bay, and the combined weight of ice was sufficient to depress the crust by hundreds of metres — a depression whose slow rebound (isostatic rebound) continues today, most visibly in the tilting of water levels in the Great Lakes basin and the ongoing emergence of land around Hudson Bay.
As temperatures rose at the end of the last glacial maximum, the ice sheet retreated not as a single continuous front but as a series of lobes that thinned, stagnated, and eventually melted. Each lobe left a distinct set of landforms that geologists use to reconstruct its path and rate of retreat.
Drumlins
Drumlins are smooth, elongated hills of glacial till — a mixture of unsorted clay, silt, sand, gravel, and boulders — moulded by moving ice. Their long axes are aligned parallel to the direction of ice flow, with the blunt (stoss) end pointing into the direction from which the ice came and the tapered (lee) end pointing downflow.
Drumlins rarely occur in isolation. They form in fields of dozens to thousands of individual hills, reflecting the widespread plastering of subglacial till by a broad, active ice sheet. Ontario's Peterborough drumlin field contains several thousand drumlins and is one of the largest such fields in the world. Drumlin fields are also widespread in central Nova Scotia, New Brunswick, and parts of Manitoba.
Because drumlins consist of densely compacted till with relatively poor drainage, drumlin terrain often alternates between well-drained hilltops and wet inter-drumlin hollows. Early settlers in Ontario frequently chose drumlin crests for farmsteads, leaving the swales as pasture or woodlot.
Eskers
An esker is a sinuous ridge of sand and gravel deposited by meltwater flowing in tunnels beneath or within the ice. When the ice melted, the sediment that had filled the subglacial tunnel was let down onto the landscape as a ridge — sometimes extending for tens or hundreds of kilometres with only minor interruptions.
Eskers record the drainage network that existed beneath the ice sheet at the time of deposition. They tend to run roughly perpendicular to the former ice margin, following what were the subglacial drainage channels toward the retreating edge. In the Canadian Shield, eskers are conspicuous in the landscape: narrow gravel ridges winding through boreal forest and muskeg, typically rising 10 to 30 metres above the surrounding terrain.
The Munro Esker in Ontario runs for approximately 150 kilometres northeast of Cochrane and is one of the longest continuous eskers in North America. Esker gravel is a commercially significant aggregate source throughout northern Ontario and northern Quebec, used in road construction in areas where local aggregate is otherwise unavailable.
Moraines
A moraine is any accumulation of glacially transported sediment. The term covers several distinct landform types depending on where along the glacier the sediment was deposited:
End moraines
End moraines mark former positions of the ice margin — places where the ice front stood still long enough for till and outwash to accumulate as a ridge. Ontario's Oak Ridges Moraine is the most studied example in Canada: a broad, hilly ridge of sand, gravel, and till stretching 160 kilometres east of Toronto, formed between two ice lobes during a period of stillstand roughly 12,000 years ago. The moraine's porous sediments act as a major groundwater recharge area, feeding streams that drain both south to Lake Ontario and north to Lake Simcoe and Georgian Bay.
Ground moraine
Ground moraine is the sheet of till deposited beneath a retreating glacier. It typically produces a gently rolling, low-relief plain with poorly developed drainage — the characteristic landscape of large portions of southern Manitoba, Saskatchewan, and the Clay Belt of Ontario and Quebec. The Clay Belt's flat, swampy terrain reflects not only ground moraine deposition but the subsequent flooding by Glacial Lake Ojibway, which left thick deposits of fine-grained lacustrine sediment over much of northern Ontario and Quebec.
Kettles and kettle lakes
Kettles are depressions formed by the melting of buried ice blocks. As the main ice sheet retreated, it left behind stagnant ice masses — chunks of glacial ice buried in outwash sediment or till. When these blocks eventually melted, the overlying sediment collapsed, forming circular to irregular depressions. Many kettles filled with water to become lakes; others remained as wetlands.
Kettle lakes are numerous across the prairies of Alberta, Saskatchewan, and Manitoba, where they form the "prairie potholes" — shallow, often seasonal wetlands of great ecological importance as waterfowl nesting habitat. The Prairie Pothole Region is one of the most significant wetland complexes in North America and owes its existence directly to the differential melting pattern of the Laurentide Ice Sheet's southern lobes.
Glacial lakes and their legacy
As the ice retreated, meltwater pooled behind ice dams and moraines, creating enormous proglacial lakes. Glacial Lake Agassiz, centred on present-day Manitoba, was at its maximum extent larger than any lake on Earth today — larger than all of the modern Great Lakes combined. When its ice dams eventually failed, the drainage of Lake Agassiz sent pulses of cold freshwater into the North Atlantic, events possibly linked to abrupt climate oscillations recorded in Greenland ice cores.
The legacy of these proglacial lakes is flat, fine-grained lake-bottom sediment across wide areas of the prairies. The fertile Red River valley of Manitoba is floored by Glacial Lake Agassiz sediments — one reason it became one of Canada's most productive agricultural zones.
Further reading
The Geological Survey of Canada's Quaternary Geology mapping program has produced detailed surficial geology maps for most of Canada, available through the GSC publications portal. For the Oak Ridges Moraine specifically, the Ontario Geological Survey maintains a dedicated dataset and interpretation documents.