Historical Geology

Historical Geology

Ottawa is found a in valley named the St. Lawrence Lowland which is surrounded by Precambrian rocks of the Canadian Shield. The St. Lawrence Lowland encompasses an area from eastern Ontario to western Quebec and northern New York. The St. Lawrence Lowland area is composed of basement Precambrian rock overlain by Paleozoic sedimentary deposits which are overlain by glacial deposits (Wilson, 1956).

The Precambrian basement, including the Gatineau hills, is part of the Grenville Province composed of crystalline limestone. The Gatineau hills were formed by uplift and erosion. Pillow lavas are found in the Grenville province. This indicates that there must have been a time when the area was submerged by water and intruded with volcanic material.

The Paleozoic sedimentary deposits occurred via a number of transgressions and regressions of the ocean as a result of eustatic sea level rise (Lemon, 1993). The Epeiric seas, or seas which covered the continent, covered the St. Lawrence Lowland for a significant amount of time. There was deposition of limestones, sandstones and shales during this time (Wilson, 1956). These are the deposits of the characteristically layered sediment we see all over the city. For instance, the photo of Hogs Back Falls on the home page illustrates the layering of shale and sandstone beds.

The glacial deposits were deposited as the glaciers of the latest period of glaciation began their retreat. Erratics, glacial till and glacial stria can be found easily in the Gatineau hills. The Gatineau hills are so rounded and hilly because of the glaciation. Mountains were broken and crushed by the weight of the glaciers and smoothed out by the mass of boulders and sediment carried at the base of the glaciers.

This is a paleogeographic reconstruction of the Ottawa area during the retreat of the glaciers which occurred during the last period of glaciation.