How Were The Great Lakes Formed?

Lake Ontario with the Toronto skyline in the background. Image credit: Andres Garcia Martin/
Lake Ontario with the Toronto skyline in the background. Image credit: Andres Garcia Martin/
  • The formation and location of the Great Lakes is a direct result of ancient glaciation and geology, yet the precise age of the lakes is not known. It is estimated that they formed anywhere within the last 7,000 to 32,000 years.
  • The glaciers that formed the Great Lakes once covered 97% of Canada in thick ice.
  • The Great Lakes are the source of food, transportation, and drinking water for millions living in Canada and the United States.

The chain of five deep, freshwater bodies known as the Great Lakes make up 20% of the world’s surface water supply. In fact, the Great Lakes are so large they are often referred to as inland seas. For thousands of years these lakes have been the source of food, transportation, and drinking water for populations of First Nations, early European settlers, and, today, millions of people in Canada and the United States. But how and when were the lakes Superior, Michigan, Huron, Erie, and Ontario formed? Read on to discover the origins of the Great Lakes. 

What Makes The Great Lakes So Great? Their Size

With a larger volume of water, Russia’s Lake Baikal is technically the largest singular lake in the world, but with a combined 94,000 square miles - that’s larger than all of Great Britain - the Great Lakes are the largest surface area of freshwater on the planet. Eight American States border the lakes, from Minnesota, Wisconsin, Illinois, and Indiana in the midwest to Michigan, Ohio, Pennsylvania, and New York in the east, as well as the Canadian provinces of Quebec and Ontario. Plus, there are hundreds of thousands of smaller lakes, rivers, ponds, and connected waterways within the 295,710 square miles of drainage basin that today extends all the way to the Atlantic Ocean via the channels and locks of St. Lawrence Seaway. The Great Lakes form a natural border between Canada and the United States, and since the signing of a 1909 treaty between the two countries, no fortifications or military warships patrol the water border.

Great Lakes Come From Great Glaciers

The icy and cold rocky shore of Lake Erie In Northwest Ohio. Image credit: Michael Shake/

The formation and location of the Great Lakes is a direct result of ancient glaciation and geology, yet the precise age of the lakes is not known. Scientists estimate that they are anywhere between 7,000 to 32,000 years old, with the lakes changing in shape and size throughout the millennia. But all agree the process began millions of years before that with the movement of continents and a series of enormous glaciers that cut across the landscape. Beginning in the Palaezoic Era which included the Cambrian, Ordovician, and Silurian Periods wherein new life forms began to appear, large inland seas divided portions of North and South America, submerging the landforms we know today completely under water. As the Palaeozoic seas withdrew, powerful geographic forces continued to shift the continents and alter the landscape.

Although glaciation occurred numerous times during the Earth’s history, it is only the process that took place in the last two to three million years that scientists are the most certain about. During that time, nearly 30% of the Earth was beneath glaciers, during what is referred to as the Pleistocene Ice age. These massive ice sheets formed in mountainous regions along the northern latitudes, and expanded down across continents digging out deep valleys and slopes. These ice age glaciers were so large that approximately 97% of Canada, the second largest country in the world, was covered in thick ice.


The sunset over the coast of the Lake Michigan. Image credit: Ehrlif/

It is the process of year round snow accumulating and becoming buried under subsequent layers, with the weight of each new layer compressing previous layers into recrystallized, icy grains that slide forward or recess, depending on the temperature. No one knows how many glaciations took place during the Pleistocene Ice Age, although many researchers suggest there were four major periods of about 100,000 years each, interrupted by warmer periods of melting. The oldest glaciation during this ice age in North America is called the Nebraskan, followed by the Kansan, Illinoian and Wisconsinan. This repeated glaciation resulted in the abrasion and quarrying of the ground below, as well as leaving huge depressions in the bedrock where millions of gallons of icemelt collected into massive lakes. As glaciers continued to retreat, vast amounts of drainage and soil deposits continued to the east. At one time, the waters were so high that Huron and Michigan combined into one large lake, while at the westernmost point both Lake Superior and Lake Duluth drained all the way east to the Mississippi. As drainage continued, sediment and waterflow ultimately reached the Atlantic Ocean and the shape of the Lakes was established. 

The Great Lakes Today

One of the most recognizable features on a modern map of North America are the five unique shapes of the Great Lakes. While the movement of glacial ice caused the lakes to form, it is the geology beneath them that gives each its unique shape. The largest, Lake Superior, was shaped by buckles and junctures in its precambrian geology, while the deep trough of the Appalachian geosyncline not only gave Lake Erie its shape but also the vast drop-off and resulting torrent of water known as Niagara Falls. Today, over 30 million people totalling 30% of Canada’s population and 10% of the United States, live close to the Great Lakes and rely on them for drinking water, transportation, and their livelihoods.


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