Ridgetown geology

Geological Notes on the Ridge of Ridgetown

P.F. Karrow

In southeastern Kent County is an elevated area called the Blenheim moraine. As it forms a banana-shaped ridge about 20 metres above the generally low and flat areas of former lake bottom that form much of the county, it is the county’s most prominent land form. Ridgetown is situated on the north edge of the moraine and takes its name from its setting, whereas the name of the moraine is after the town of Blenheim, located at the southwest end of the moraine.

The oldest solid rocks under the area are only known from drilled holes.  About 1200 metres down are granite-like Precambrian rocks about one billion years old, which appear at the surface east of Georgian Bay. Glacial erratics of such rocks are seen locally as carried here by glaciers about 15,000 years ago (recycling is not new to humans!). The youngest overlying rocks (Paleozoic age, about 350 million years old) include the Hamilton Formation shale with outstanding coral and brachiopod fossils seen near Arkona and Thetford, and the black shale Kettle Point Formation, named from its spherical lime concretions exposed at Kettle Point on Lake Huron. Paleozoic rock formations include important mineral products such as limestone near Woodstock and St. Marys, used for cement manufacture, and salt and gypsum, mined at Goderich, Windsor, and Caledonia. The oldest oil wells were drilled at Petrolia in the 1850s. Oil and gas continue to be important resources, but like all mineral resources, they are finite and with their enormous rates of consumption will greatly decline in coming decades.

The soft sediments covering the bedrock were deposited by glaciers and glacial lakes during the later part of the Quaternary Period (or “Ice Age”) which spans the last two million years of geological time. These deposits vary from 20 to more than 50 m in depth. Quaternary climate varied cyclically from warm like the present, to cold when thick glaciers formed. Advancing ice eroded the bedrock and left behind debris called till, consisting of mixtures of all sizes of material from boulders (stony fields), gravel, and sand to silt and clay. Incorporated ground up limestone makes the groundwater “hard”. The “drift” is an essential part of the local economy by yielding groundwater and forming the parent material for agricultural soils. We tend to take for granted the fact that the drift is also what we build on.

Three main landforms are associated with the drift. The first is the Blenheim moraine, a long ridge with clayey Port Stanley Till on its surface. The moraine formed at the edge of the ice during the melting back of the Lake Erie basin ice lobe. The moraine marks a time of stability of the ice front when melting and forward flow were in balance.

Glacial meltwater was blocked from escaping eastward by the Erie ice lobe and instead drained across Indiana and Michigan to the Mississippi River. A succession of large glacial lakes (Arkona, Whittlesey, and Warren, 13,500 to 12,500 years ago) formed eroded bluffs and beach ridges of sand and gravel on the slopes of the Blenheim moraine as water levels dropped.  Offshore from the moraine, deeper-water deposits of silt and clay were laid down. Former lake floor was exposed to wind and small sand dunes formed. These extensive lake floors are the second important landform of the area and are the basis of its flatness. The third major landform is the Pointe aux Pins spit at Rondeau Park, which encloses a lagoon with marshy shore. This is believed to have formed by wave erosion of the Erieau moraine, which extends across the floor of Lake Erie; the spit reached its present form in the last 4000 years.

The great weight of glacial ice caused the Earth’s crust to subside by hundreds of meters. As the ice melted away and ever since, the crust has been tilting back up to its former position. This geological process, technically called isostatic rebound, is barely perceptible to human eyes, but over thousands of years can have marked effects. Because of this process, Kingston, at the outlet of Lake Ontario, is rising relative to Hamilton about 30 cm. per century, raising the lake level by that much at Hamilton. Similarly, in Lake Erie, the outlet at Fort Erie is rising and raising the water level in Lake Erie. It was that rising water level that eroded the Erieau Moraine and eventually submerged it to its situation today.

 As land appeared above the waters of the lowering glacial lakes, plants and animals (including humans) moved in. This poorly drained flat land retained many wetlands, the draining of which for farming has revealed the presence of animal bones. The Highgate mastodon is unusual in that it was accompanied by bones of the giant beaver, also extinct as of about 10,000 years ago. Some of their fossil bones and related information can be seen at the Thamesville and Fairfield museums. Recently, a single tooth of a mammoth was found on the beach at Rondeau Park.

Chapman, L.J., and Putnam, D.F, 1984.  The physiography of southern Ontario.  Third Edition.  Ontario Geological Survey Special Volume 2, 270p.

Coakley, J.P., 1989.  The origin and evolution of a complex cuspate foreland:  Pointe-aux-Pins, Lake Erie, Ontario.  Géographie physique et Quaternaire 43, 65-76.

Cooper, A.J., and Baker, C., 1978.  Quaternary geology of the Bothwell - Ridgetown area, Southern Ontario. Ontario Geological Survey.  Preliminary Map P-1973.

 Hewitt, D.F., 1966.  Paleozoic geology of Southern Ontario.  Ontario Department of Mines Geological Circular 15, 11p.