From the age of glaciers

A glacier is a thick ice mass forming over thousands of years.

How are glaciers formed? If for thousands of years, global temperatures stay cold enough your-round for snow to accumulate and become compacted, common valley and continental glaciers develop.

The Ice Age is a reference to glaciers of long ago covering much of the Earth. Yet, technically the Ice Age is a long interval of time lasting millions of years. Within this time there were shorter-term periods of glacial advances and retreats, the last of which ended ~15,000 years ago. This last period of glacial advance has been informally called the “Ice Age” where in North America the continental glacier grew and advanced from the Canadian North to slightly further south than the present-day Great Lakes (see Laurentide Ice Sheet below).

As a body, glaciers actually move - the valley glaciers behaving as rivers of ice and larger continental types migrating out from a central point. Sometimes 1-2 miles thick, these slowly moving glaciers carve and grind the bedrock at their base consuming rock fragments in their icy mass. As the glaciers grow and advance, such rocks are transported hundreds to thousands of miles from their source and left as aliens in a new land (also called “erratics”). When the glacier melts with a warming climate, these rocks are deposited in place. Only the hardest rocks will survive such a journey where other types are ground to fine fragments and rock flour.

The Great Lake region of North America, themselves shaped by the erosional forces of the glaciers, today behave as a reservoir of glacial rocks (erratics) from the Canadian Shield (in red, below). This geologic region of Canada contains some of the oldest rocks of the world with an age range of 2-4.5 billion years old. Some are igneous but much of it a colorful metamorphic rock type.

Today, one walking along the beaches of the Great Lakes can encounter many of these glacial “gems” washed up on the shore by normal wave action or a recent storm.