Woodstock geology

Saturday, December 24, 2011

What on Earth: Volume 7 2011

Woodstock Geology​
P.F. Karrow
Department of Earth and Environmental Sciences
University of Waterloo
Geology plays a very important part in the history and economy of the Woodstock area. There is a long history of limestone quarrying in the area (Innerkip, Beachville), water supply from the ground satisfies our thirst, and our food is grown on soils formed from glacial deposits.

Drill a hole into the bedrock

Although the solid bedrock is exposed along the Thames Valley and in quarries, most of what we know about the bedrock is from drill holes for water, oil and gas, and building construction. A few holes are deep enough (over 1000 metres or a kilometer) to reach the buried surface of the Precambrian shield, whose grantic rocks can be seen at the surface in “cottage country” (Parry Sound- Huntsville) and near Kingston. Pieces of Precambrian rock of a great variety were transported here by glaciers (glacial erractics) and are often used in barn foundations and farm houses. Those crystalline rocks contain interesting mineral deposits such as iron ore, feldspar, mica, and syenite, complexly folded and faulted in ancient, now eroded flat, mountains over a billion years old. They are buried by nearly flat-lying layers of shale and limestone, much younger at several hundred million years of age. Numerous fossils of coral, crinoids, and brachiopods show these rocks were deposited in warm shallow seas. Later, the seas withdrew and rain eroded the rocks into valleys and uplands before the coming of the “Ice Age” a couple of million years ago. The crest of the buried Onondaga Escarpment is seen at the Innerkip quarry, where rocks of Devonian age overlie those of Silurian age. The latter contain valuable salt and gypsum deposits, while Devonian limestone is the focus of local quarrying.

Take a walk along a creek

That’s what geologist Dick Cowan did in 1970, and found an old buried peat bed near Innerkip, later dated at over 50 000 years old. Fossil plants, insects, and bones have been studied in the peat that reveal a mixture of non-glacial environments, with further studies underway. Dr. Cowan’s mapping showed these organic sediments underlie several glacial till layers deposited by successive ice advances during the last main glaciation about 27,000 to 13,000 years ago.
The “icing on the cake” of glacial deposits was formed by glacial advances of the Erie basin ice lobe flowing from the southeast and the Georgian Bay ice lobe flowing from the northwest to form the streamlined round hills of the Woodstock drumlin field. Pauses in the Erie lobe ice retreat formed a succession of hummocky end moraine ridges such as the Ingersoll and St. Thomas moraines. During the ice melting there was an abundance of water flowing between the Erie and Georgian Bay ice lobes (what we call the interlobate zone) to erode the Thames Valley to bedrock, which made the limestone cheaper to quarry. Farther south, meltwater trapped by the retreating Erie ice lobe formed glacial lakes larger than Lake Erie during the 14,000 to 12,500 – year time interval.
The irregular, hilly ground left behind by the disappearing ice sheet had many surface depressions, which formed wetlands. These became swamps, bogs, and marshes, in which accumulations of plant remains provide a record, through study of pollen grains, of changing climates and vegetation up to the present. Near the bottom of these deposits there are rarely found remains of mastodons, the local elephant that became extinct for unknown reasons about 10,000 years ago. Some mastodon bones can be seen at the Woodstock museum. By then, Paleoindians had arrived to begin the human history of the area.


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