Ontario is home to one of the world's largest collection of glacial features, scars left over from its amazing glacial history. Glaciers are gigantic sheets of ice whose weight is so large that they have enough momentum to move slowly across the land. Their immense weight acts as a giant scrapper, leaving behind its mark on the bedrock below.
Ice ages are produced by fluctuating global temperatures. They also have recurrent cycles of glacial advancement and retreat of about 26 000 years. The last ice age started about 1.6 million years ago forcing glaciers up to 3.5 km thick over Ontario. Ontario was last fully covered some 20 000 years ago, until the glaciers began retreating 6 000 years later.
Evidence of these events are clear all around us. Basins formed by glacial erosion provide us with places to swim, fish and boat. Sediments deposited by the glaciers give us healthy soils great for growing fresh crops. Exposed Precambrian bedrock from glacial retreats gives us beautiful hills and outcrops to walk and drive over.
Common Ontario Rocks:
Granite: An igneous rock formed from the slow cooling of magma within the Earth's mantle. It consists mainly of coarse interlocking mineral crystals of quartz and feldspar. Its colour may vary from grey to pink depending on the amounts of these minerals present and others such as mica. The general rule of thumb is the cooler the conditions during solidification, the more resistant the rock will be to weathering. This is likely why the granite Canadian Shield is so stable.
Basalt: An igneous rock formed by rapid cooling. This occurs when molten rock from the Earth's interior is directly injected onto the surface or near to it. Its quick cooling results in a much more uniform structure than granite. It is dark in colour and is found in some parts of northern Ontario.
Limestone: A sedimentary rock formed when calcium carbonate is dissolved in water and is allowed to precipitate. Accounts for very much of our regions bedrock and is apparent by the abundance of limestone structures.
Marble: A metamorphic rock, meaning that it was either igneous or sedimentary rock and has changed over an incredible amount of time and pressure. It can be recognized by its bright white appearance.
Sandstone: Another sedimentary rock formed from cemented sand grains. Rich in quartz, time and pressure would be transform sandstone into quartzite. Quartzite is also used to describe sandstone containing more than 85% quartz. Sandstone is usually red in colour but can be a striking white colour when a lot of quartz is present.
Shales: Sedimentary rocks made from clays.
Clays: Very fine-grained, silica-rich minerals that are important components of many soils.
Slate: The metamorphic equivalent to shale.