Elora and Fergus geology

Saturday, December 24, 2011

What on Earth: Volume 7 2011

A geological view of Elora and Fergus
P. F. Karrow 
Department of Earth and Environmental Sciences 
University of Waterloo 
One of the most scenic features of southern Ontario is the Elora gorge on the Grand River. The river has cut down about 30 m into dolostone (related to limestone) in the time since the last glacier ice melted away about 14000 years ago. The local rocks feature prominently in the older stone buildings in Fergus.

Bedrock geology

The local bedrock is of Silurian age, or about 450 million years old. The rock in the Elora gorge belongs to the Guelph Formation and this is one of the best exposures of these beds. Below the bottom of the gorge are older dolostones of the Amabel Formation, which is exposed in, and forms the resistant caprock of the Niagara Escarpment to the east. Overlying the Guelph Formation is the soft shale of the mineral-rich (salt, gypsum, mined at Goderich, Windsor, and Caledonia) Salina Formation, which extends west to the next bedrock escarpment, the Onondaga, near New Hamburg.  This succession of formations slopes down to the west into the Michigan basin so that progressively older underlying layers outcrop to the east and progressively younger overlying layers are exposed to the west (ie. where you can see them under the glacial deposits). All these formations were laid down in a warm shallow sea, as can be inferred from the fossils they contain, such as corals, clams, snails and crinoids. The last of these, crinoids, are sometimes referred to as sea lilies, but they are in fact animals related to starfish. 
The surface of the bedrock is very uneven, resulting from erosion by rivers and glaciers. Rock formations vary in their resistance to erosion and the soft Salina Formation forms a lower belt underground to the west, under the Waterloo area. The Grand River has cut its gorge headward or upstream into the edge of the higher and more resistant Guelph Formation. That’s why, travelling along the river downstream to the southwest, the gorge wall lowers as the bedrock surface drops below river level. Also, where the river is cutting into the bedrock, its course is relatively straight, but downstream over the Salina Formation it swings back and forth in meanders, because still further downstream  downward erosion is again impeded at Cambridge, where the river is back on the Guelph Formation.
The present gorge is only the latest in such erosion events, as water wells and geophysical surveys reveal a buried bedrock valley crosses the present valley between Elora and Fergus. The buried valley is twice as deep as the present one and has been traced from Belwood Lake to near Inverhaugh, where, like the present gorge, the rock surface drops over the Salina Formation below the ancient river level.

The glaci​al story

After a long gap unrecorded in the local rock record, the story is taken up in the local glacial deposits. After many glaciations in the past two million years, only the local and latest comings and goings of the continental glaciers are recorded here. Evidence of the older glaciations was destroyed by the younger ones. Local glacial deposits only record part of the last 100,000 years, and mostly of the last 20,000 years. These softer, younger deposits are mostly till, the direct deposits of glacier ice, including varied blocks of rock (erratics) carried here from the Precambrian Shield, where the source rocks can be seen at the surface east of Georgian Bay. Varied erratic boulders derived from farm fields are often to be seen in barn foundations and houses in the countryside. Similar Precambrian rocks can be found deeply buried about a kilometer down below Fergus.
Glaciers flow sluggishly, but, like water, conform to the shape of the ground they flow over. The Great Lakes basins depressions caused lobes or projections of the ice front to extend into the low ground and advance onto the land around. Thus at Elora and Fergus the Georgian Bay ice lobe from the northwest met head-on the Lake Ontario ice lobe from the southeast along an “interlobate zone” that passes close by. The higher ground to the north of here, sometimes referred to as “Ontario Island,” because it was surrounded by shrinking ice lobes and growing glacial lakes, tended to deglaciate first. The melting glaciers melted sporadically with climate fluctuations, forming end moraines of till, such as the Paris moraine southeast of Guelph, when melting and flow rates were in balance. Evidence of the flow of the ice is seen in rock surface scratches (striae), and hills of till called drumlins, elongated parallel to ice flow. South of Fergus, dozens of these drumlins form the Guelph drumlin field, and reveal the north westerly flow of the Lake Ontario ice lobe to its limit near Fergus. Drumlins near Westover are ringed by old shorelines of glacial lakes Whittlesey and Warren which existed about 13000 years ago.
The weight of the ice caused the Earths’ crust to be forced down, but as the ice melted and thinned, the crust slowly rose again, and continues to do so today, a process known as “isostatic rebound”. This rebound, greater in the northeast where the ice was thicker, is tilting the Grand River to the southwest, increasing its gradient. Large glacial lakes formed in each of the Great Lakes basins, gradually lowering to the present lakes as lower outlets were uncovered by the retreating ice. The slowly rebounding crust causes Kingston to rise relative to Hamilton about a foot per century. As Kingston is the location of the lake outlet, that’s forcing Lake Ontario to rise on the land. Lake Huron, with its outlet in the south at Sarnia, conversely is slowly lowering on the land at the same rate.
Southern Ontario was deglaciated about 11,000 years ago, and shortly afterward many of the large mammals (horses, mammoths) went extinct. We don’t know why, but several theories compete for attention. As the climate warmed, the vegetation changed from conifer forest to the present mixed – conifer – hardwood forest. It is widely believed another glaciation will follow the last one, thousands of years into the future.

References for further information

Bond, I.J., and Telford, P.G. 1973. Paleozoic geology, Orangeville. Ontario Division of Mines Map 2339.
Chapman, L.J. and Putnam, D.F., 1984. Physiography of southern Ontario. Third Edition. Ontario Geological Survey, Special Volume 2, 270p.
Cowan, W.R., 1976.  Quaternary geology of the Orangeville area. Ontario Division of Mines, Geoscience Report 141, 98p.
Greenhouse, J.P., and Karrow, P.F., 1994. Geological and geophysical studies of buried valleys and their fills near Elora and Rockwood, Ontario. Canadian Journal of Earth Sciences 31, 1838-1848.
Karrow, P.F., 1968. Pleistocene geology of the Guelph area. Ontario Department of Mines, Geological Report 61, 38p.