The rocks that form the Canadian Shield were formed about four billion
years ago during the Archeon Eon of the Precambrian Era. Erosion of
this extremely rugged, mountainous landscape deposited enormous
quantities of clays, silts, sands and gravels into the surrounding
waters. Compressed by their sheer cumulative weight and the heat of the
shifting Earth's crust, sedimentary and metamorphic rocks formed during
the Proterozoic Eon of the Precambrian Era.
More recent rocks that were formed above these ancient layers have since been largely removed by the scouring action of glaciers that covered northern North America in the several ice ages in the past 100,000 years.
The last ice age scraped the rocks in a NNE (north-north-east) to SSE (south-south-east) direction. At the end of the last ice age, all the waters in central Ontario (and the great lakes) drained to the east, toward the St Lawrence River. After the weight of the glaciers left this area, the land slowly began to rise.
During the last Ice Age, much of Toronto was covered by Glacial Lake Iroquois, with a series of steep escarpments marking the lake's former shoreline. Thses are most noticeable in three areas: along the current lakeshore at the Scarborough Bluffs, most prominent between Victoria Park Avenue to Highland Creek, and near St. Clair Avenue West between Bathurst Street and the Don River, and north of Davenport Road (around Casa Loma). Although not remarkably hilly, The city has a 200 metre (640 ft) elevation drop from Steles Ave in the north to Lake Ontario, and over the last 10,000 years erosion has created a number of steep banked ravines along the city's waterways.
North of Toronto, between Whitchurch-Stouffville and Aurora is the Oak Ridges Moraine, formed by sediments dropped by the glaciers about 14,000 years ago. Today, the moraine has gently rolling landscape, low hills, kettle lakes, kettle bogs and wide river valleys. Even today, about 30% of its area remains forested, and the Oak Ridges Moraine offers refuge for forest birds as well as significant flora and fauna and is a significant wildlife corridor in well-populated Southern Ontario.
More history of North York