Postglacial history

The climate of North America today is very different than 20,000 years ago. At that time an immense sheet of ice between 1-4 km thick covered almost all of Canada and a large part of the adjacent United States. The mass of ice overlying the lower St. Lawrence region of Quebec was so great that the elevation of the land surface was forced down hundreds of metres from its present levels. This episode of glaciation had begun about 100,000 years ago, but by about 18,000 years ago the climate had warmed and the ice gradually began to retreat. The melting ice revealed a land surface devoid of vegetation and much transformed by the physical action of the ice sheet. About 12,500 years ago, the surface of Mont St. Hilaire finally emerged above the melting ice sheet. As the ice sheet melted back, ocean waters flowed into the St. Lawrence basin, which was still depressed below sea level by the ice remaining to the north. Our region was flooded by an inland sea, the Champlain Sea, in which the Monteregian Hills were islands. Mont St. Hilaire stood about 100 metres above the sea surface.

We can still see vestiges of this stage in the mountain's history. Many sand deposits exist at elevations on the slopes of the mountain that reflect their original deposition as beaches on the shore of the Champlain Sea. At these same elevations we can find low cliffs formed when storm-tossed waves eroded the steep, rocky parts of the island shore. The flat valley floor below the mountain is the old seabed, with its present soils rich in clay from fine sediments of glacial materials washing from the melting ice and settling to the sea floor. We can find fragments of marine creatures in these sands and clays, all of them associated with cold, salty water. These include shells and even skeletons of beluga whales! As the ice sheet continued to melt north of us, the elevation of the land surface gradually rose to a point where oceanic waters no longer penetrated inland to our region. By 10,000 to 9,000 years ago the Champlain Sea had been replaced by the freshwater Lake Lampsilis, which was fed by the rush of glacial meltwaters still flowing down the Ottawa and St. Lawrence Rivers. Mont St. Hilaire stayed as an island for a while longer, but by about 9,000 years ago the rivers had receded to their present channels and the mountain stood above the dry land of the valley floor.

Since the retreat of the glaciers, the vegetation of Mont St. Hilaire has gone through a series of changes as the climate warmed and soils developed again on the ice-ravaged landscape. Soon after the higher parts of Mont St. Hilaire emerged above the ice sheet about 12,500 years ago, tundra species became established on the mountain. These are sun-loving species tolerant of cold, short growing seasons that are now found in far northern Canada. After a brief period of dominance on Mont St. Hilaire the tundra species were replaced by a mixed forest, including some conifers more typical of boreal regions and some of the deciduous trees we now consider a normal part of the local vegetation. These early forests included spruce, pines, ash, oaks and birch. By about 8,000 years ago, other more temperate trees had become established and the forest began to take on its modern character. A period of co-dominance by oak and hemlock fairly quickly gave way to the co-dominance of sugar maple and American beech that we see today. It is interesting that of the two co-dominant trees today, the American beech recolonized the mountain well before the sugar maple.

The advance and retreat of the glacial ice sheet left marks on the bedrock of Mont St. Hilaire that can still be seen today on the rounded and striated peaks. The ice also left a great diversity of till, beach and clay deposits on different parts of the mountain, which helps explain the origins of microhabitats that contribute to the high diversity of plant species on Mont St. Hilaire.



The advance and retreat of the glacial ice sheet left marks on the bedrock of Mont St. Hilaire that can still be seen today on the rounded and striated peaks. The ice also left a great diversity of till, beach and clay deposits on different parts of the mountain, which helps explain the origins of microhabitats that contribute to the high diversity of plant species on Mont St. Hilaire.

How do we know this history of postglacial vegetation? Researchers utilize the techniques of palynology to reveal the history. Plants produce large quantities of pollen, each species having uniquely sculptured grains of pollen very resistant to decomposition. During plant reproduction, the pollen is released into the environment, where it eventually finds its way into the sediments of lakes and peatlands. By taking cores from the sediments of Lac Hertel and a small peatland on Mont St. Hilaire, researchers can identify the types of pollen found at different depths in the sediments. The deeper the sediment, the older it is; indeed, the specific age at different depths can be determined by radiometric analysis of plant fragments found with the pollen. In this way, researchers have built up a detailed history of the vegetation of Mont St. Hilaire.