Glacial Lake

Illustration of the area once covered by Glacial Lake AgassizIllustration of the area once covered by Glacial Lake AgassizIllustration of glacier water running off to form Glacial Lake AgassizIllustration of glacier water running off to form Glacial Lake AgassizTo understand why all of the snakes have made the Narcisse dens their home, we must learn how the dens were formed. We start our journey by venturing back thousands of years ago to a time when the center of North America was covered by a huge lake larger than all of the present-day Great Lakes combined. Approximately 10,000 years ago, at the waning of the last ice age, the area of North America that is now northwest Minnesota, northeast North Dakota, southern Manitoba and southwestern Ontario was covered by a large glacial ice sheet. As this glacier melted, the large amounts of water created the great Lake Agassiz. This lake became the largest body of fresh water in the world but was just like every other lake in that it contained living organisms that made the waters their home. These living organisms began and ended their life cycles within the waters. While the organisms used the water to sustain themselves, their life cycles also impacted the waters. These organisms secreted calcium carbonate to form their protective shells. As the creatures died, their shells would fall and collect on the lake bottom. This particular lake bottom was made of limestone rock created by similar organisms that made their home in the shallow seas that invaded areas of North America over 570 million to 225 million years ago. Over time, these shells were compacted and formed layer upon layer of limestone rock and other sediments. Even today, almost all of the limestone found throughout the world was created by sea organisms.

As the limestone floor was being created, the Earth was going through major climate changes, many of which were presumably due to the large amounts of cold glacier water that would spill out of Lake Agassiz into neighboring rivers, as well as into the Atlantic Ocean. These changes caused the size of the lake to change rapidly as glaciers melted or water spilled from the lake. Over a few thousand years, Lake Agassiz became smaller and smaller, and eventually became Lake Winnipeg, Lake Manitoba and Lake Winnipegosis. Even though most of the water had retreated, the limestone sea floor was left. This sea floor is now the ground on which most people walk within Manitoba and where the red-sided garter snakes make their home.