The Quaternary, which started 1.64 million years ago, is the most recent and best known geological period in Canada. The drifting continents were located at approximately their present position. In North America, this period is characterized by climate changes that induced many geological processes such as buildup and decay of continental ice sheets, marine transgressions and regressions, advance and retreat of deserts, extensive erosion of bedrock, removal of pre-existing unconsolidated sediments and deposition of newly eroded materials. The Quaternary Period also corresponds with the advent on Earth of the human race.

Most of Canada has been covered by ice at least once during the Quaternary, and in several parts of the country, there is evidence of at least four major glacial periods. The number of times ice sheets have covered the National Capital Area is unknown, as erosion during subsequent ice advances and during non glacial periods removed evidence of earlier events. Consequently, unconsolidated deposits in the NCA area consist of glacial and related sediments from the last (Wisconsinan) glaciation, marine sediments related to the Champlain Sea, and the reworking of these same sediments by modern geomorphic processes.

During the Upper Wisconsinan (23-10 Ka), a continental ice sheet, called the Laurentide Ice Sheet, covered most of Canada. The Laurentide Ice Sheet is divided three major sectors fed by three major ice domes; one of these domes developed in the Quebec-Labrador area and expanded progressively in all directions in the eastern and central part of Canada (Fulton, 1989). In the NCA, the ice sheet flowed southward across the Gatineau Hills, the Ottawa River valley, and southwestwards across the Frontenac Arch. During the early stages of glaciation, a major ice stream flowed in the Gatineau and adjacent north-south trending valleys but eventually the entire area was covered by an ice sheet that could have been thousands of metres thick (Gadd, 1987). Unconsolidated sediments older than the Upper Wisconsinan glaciation and the upper layers of bedrock were eroded by the Laurentide Ice Sheet and transported along the path of ice flow over distances up to hundreds of kilometres. In turn, the materials deposited locally by the ice sheet, were derived from sources hundreds of kilometres up-ice of the NCA. The materials eroded upstream were deposited in the form of till and related landforms ( Maps - GeoServ ).

Late Wisconsinan ice flow and deposits

Towards the end of the Wisconsinan, as the margin of the continental glaciers retreated from their maximum extent, the Laurentide Ice Sheet thinned over the NCA. The ice sheet separated into various retreating lobes, the highlands started to emerge and glacier tonques became restricted to the valleys of the Gatineau hills. The abundant meltwater from the decaying glaciers transported an enormous amount of debris beyond the ice margin, depositing it on flood plains in valleys, in glacial lakes, and sea.

The weight of the Wisconsinan ice sheet isostatically depressed the earth crust below sea level. At the time of deglaciation, proglacial lakes formed by the meltwater of glaciers were soon replaced by a marine invasion, called the Champlain Sea, along the St Lawrence Valley. The marine invasion lasted 2 500 years, between 12 and 9.5 ka, and locally the marine limit reached approximately 210m above present sea level. During the marine invasion, a layer up to 100 metres thick of marine clays and silts (commonly known as Leda clay), was deposited over the glacial sediments. Isostatic rebound of the earth's crust caused the Champlain Sea ( Maps - GeoServ ) to begin retreating from the Ottawa area at about 11.2 ka. As the sea level lowered, wave action eroded and reworked the surface of marine and glacial sediments, redepositing the coarser sediments nearshore and transporting the fine material into deeper water.

During the retreat of the Champlain Sea (11.1 to <11 ka), the ice front temporarily stabilized approximately 30 km north of Ottawa. The ice retreated from the upper Ottawa valley between 11 and 10 ka, opening a series of outlets that permitted the drainage of glacial meltwaters from the Upper Great Lakes and large glacial lakes in northern Ontario and from the Canadian Prairies through the Ottawa valley. The high rates of flow of the proto-Ottawa River down cut the glacial and marine deposits until approximately 5.5 ka., forming numerous terraces along the valley. The gradual isostatic uplift eventually diverted the drainage of the northern and prairie lakes into the Hudson Bay, and the Upper Great Lakes through the Lower Great Lakes into the St Lawrence valley, leaving a number of river channels of the proto-Ottawa River abandoned. By 4.7 ka, the basin drained by the Ottawa River was approximately the same as today. (Aylsworth et al., 1997; Fulton and Richard, 1987)