Beachville geology

Saturday, December 24, 2011

What on Earth: Volume 7 2011

Residents of the Beachville area should be well aware of the importance of geology in the local economy. The bedrock yields limestone from several nearby quarries, whereas the overlying glacial sediments are the foundation of the arable soils supporting the agricultural industry.

Bedrock

Deep borings and drill holes provide information on what lies below the ground surface. The lowest and oldest rocks are igneous and metamorphic, faulted and folded crystalline rocks of Precambrian age (over 1 billion years old) which can be seen at the surface on the Canadian Shield east of Georgian Bay, but here are over 1000 m below the surface. Most of the depth to these rocks is a sequence of Paleozoic age rocks (Ordovician, Silurian, and Devonian age, 500 to 350 million years ago) consisting of thick layers of sandstone, shale, and limestone. These rocks are the surface bedrock in southern Ontario and include deposits of salt and gypsum and oil and gas. The local quarries provide high quality limestone of Devonian age, which is used in chemical and construction industries to make roads and buildings. Some of these rocks contain various types of fossils such as snails, clams, and corals that show they were deposited in warm shallow seas. No rock between 350 and 2 million years old is known from southern Ontario.

The “drift”

The last 2 million years (the Quaternary Period – the Ice Age) saw the climate cool and fluctuate. The cold times formed great ice sheets that covered most of Canada. The most recent was the Wisconsin Glaciation (120000 to 10000 years ago). What we know of the drift (overburden to quarry operators) is learned from creek banks, pits and quarries and other temporary excavations, as well as borings for water supply and building construction. The chief sediment deposited by glaciers is till, a mixture of glacial debris varying from boulders, gravel, and sand to silt and clay. Because glacial erosion ground up the bedrock over which the ice flowed, our glacial deposits commonly contain 50% lime, which causes our groundwater to be very hard. The economic value of the drift is really enormous, as it is the main source of groundwater, yields sand and gravel for construction, clay for making bricks, is the foundation material on which we build, and is the parent material for the soils on which our crops are grown. All this is facilitated by the fact that the drift is relatively young and soft, so it is easily excavated and cultivated.
 
As the glaciers advanced and retreated, ice melting formed meltwater streams that eroded valleys and deposited outwash gravel. Where the drainage was dammed by the ice, temporary glacial lakes were formed in which thick beds of clay were deposited. Just as there are the large Great Lakes today, there were even larger lakes in each of the great lake basins, such as the Erie basin to the south. The existence of the Great Lakes basins affected the shape of the ice front as ice, like water, flows downhill, and formed large ice lobes in each of the basins. Beachville, being inland was affected by ice of the Erie lobe flowing from the southeast, and latest by the Huron lobe from the northwest forming the round hills known as drumlins. Being near the margins of these ice masses we describe the area as being interlobate. Meltwaters flowing between the lobes created the early Thames River valley and eroded the drift down to expose the bedrock. Thus, the limestone quarry industry could develop in the floor of the Thames Valley near Beachville.
 
Notable exposures of the sequence of drift deposits are seen in the quarries at Zorra and Beachville, where it can be seen from the many layers of till and intervening gravel and clay that glacial history was complex, with many ice fluctuations over the past few tens of thousands of years. Near the base of the sequence fossiliferous (plants, molluscs, insects) sediments are under study to learn more about past environments. Since the glaciers left this area about 14000 years ago, the landscape has been weathering to form the agricultural soils and eroding to form the stream valleys.
 

References for further in​formation

Chapman, L.J., and Putnam, D.F., 1984.  The physiography of Southern Ontario.  Ontario Geological Survey Special Volume 2, 270p.
 
Cowan, W.R., 1975. Quaternary geology of the Woodstock area, Southern Ontario. Ontario Division of Mines, Geological Report 119, 91p.
 
Hewitt, D.F., and Vos, M.A., 1972.  The limestone industries of Ontario.  Ontario Division of Mines Industrial Mineral Report 39, 79p.
 
Krzyozkowski, D., and Karrow, P.F., 2001. Wisconsinan inter-lobal stratigraphy in three quarries near Woodstock, Ontario. Geographie physique et Quaternaire 55, 3-22.
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