The Great Lakes hold the largest collection of liquid fresh water in the world and provides water to nearly 10% of the United States. Our fabulous City of Chicago (and our luxury Chicago apartments) rests on Lake Michigan’s shores, and the vast waterway creates a passage all the way to the Atlantic Ocean that was vital to the modern development of the region.
But how did these massive lakes get here? How did five of the biggest lakes on the planet end up in the middle of the North American Midwest?
Here’s a brief glimpse into the history and geology of the region that helps us explain the origin of Lake Michigan and, of course, the Great Lakes as a whole.
How did Lake Michigan and the Great Lakes form?
The world back then
In order to understand the processes that went into forming Lake Michigan, we have to travel back in time a little bit to around 12,000 B.C., the end of the Last Glacial Period or, as we more commonly know it, the last ice age.
The world’s long, long history has been dotted with several ice ages, the last of which lasted from around 115,000-11,700 years ago — a fairly recent geological period in terms of Earth’s several-billion-year-old history. This ice age is what caused the Great Lakes to form, and it's all thanks to one ice sheet in particular.
The Laurentide Ice Sheet was a massive continental glacier that covered most of Canada, stretched across the North American Continent and extended as far south as central Illinois and New York. Just as alpine glaciers, which flow down mountains by the force of gravity, continental glaciers expand outward in all directions as the sheer weight of the ice forces the edges away from the center. Both glacier types use gravity to travel, and both violently erode the surface of the Earth as the ice digs into bedrock and scrapes away anything in its path.
This particular Laurentide Ice Sheet measured around 2,000 miles in length and had an average thickness of about a mile in many places, weighing up to 3.8 billion tons per square mile. As this massive ice sheet scraped across the continent, it’s terrifying weight and force gouged and annihilated the ground beneath, leaving behind clues of its existence for us to find today.
The glacier retreats
As the last ice age came to an end and global temperatures warmed, ice sheets and glaciers all over the world began to melt and retreat, starting with their lower and warmer ends. This includes our very own Laurentide Glacier up here, which began to “pull back” North into Canada.
Glaciers leave debris both as they spread and as they recede, though both processes create slightly different buildups of material. The best way to explain it is to compare it to a bulldozer clearing a pile of dirt. As the scoop drives the dirt forward, a relatively smoother trail is left behind with, perhaps, some tire tracks and markings from large boulders being scraped along the ground. As the bulldozer reaches the end of its path, it stops and reverses backward along its original path again, leaving the mound of dirt and debris in one pile at the end of its forward route.
Glaciers work in similar ways, although the mounds of dirt and debris left behind are known as moraines, and they can be as large and solid as hills or as small as sand dunes.
We can still see the moraines left behind by the Laurentide Ice Sheet’s retreat in northwestern Indiana about eight miles south of Lake Michigan’s current shores, where ancient strips of hills and ridges mark the southernmost ends of the ice sheet. These piles of rock and sediment acted as dams for the melting water left behind from the receding glacier, creating what we know today as Lake Michigan.
So, how did Lake Michigan get there?
If Lake Michigan formed from an ice sheet that covered most of North America, then it would stand to reason that large lakes would dot the landscape all across the Canadian-United States border, right? While we certainly can find evidence of moraines and debris buildup all over the northern United States, the Great Lakes region stands alone in its exceptional collection of, well, great lakes.
Long, long before the Laurentide Ice Sheet existed, the world was going through a period of massive renovation, to say the least. Around 385 million years ago the entire region was covered by a warm, shallow sea, as the continent itself was originally located below the equator! Organic material from decaying coral reefs left deposits of limestone on the ocean floor, and those deposits remained intact as the continents reorganized into the configuration today.
About 2.5 million years ago, the region was in much the same position it was in today, though the basin of limestone deposits had been covered by sediment, soils and other organic matter over time. Three large rivers known as the Huronian, Laurentian and Erigan Rivers, flowed into the Atlantic ocean from this limestone basin area, leaving behind deep valleys and channels covered in silt and loose debris.
It was these valleys that channeled the receding ice and pooled them behind the moraines, creating massive lakes that are located and that flow in the exact same place as the old rivers. The limestone basin far below kept the ice from gouging the valleys too far down, and the moraines kept the lakes from dissipating into the plains below.
Thus, Lake Michigan was born.
Lake Huron and Lake Erie are also located within this basin and along the same rivers and, as a result, have similar creation stories. Lake Superior and Lake Ontario, however, have no limestone basin and no pre-existing valleys, giving them an entirely different story of origin.
That story, however, we’ll save for our next part of the series!
Until then, enjoy!
Featured photo courtesy Pixabay/JillWellington