The historical geology of the Great Lakes straddling the border of the United States and Canada is a study in the awesome shape-shifting power of nature. It all began in the Precambrian Era roughly three billion years ago. If you were here, by any chance, for the last 40 million average human life spans you could have witnessed the start of that process. But in geological time, the Precambrian Era accounts for approximately the first five-sixths of the life span of planet Earth and was marked by incredibly intense volumes of volcanic activity that would be hard to fathom today. During this era, massive shifts in the Earth's surface created the gorgeous and towering mountains across the globe. Poetically speaking, this era could be deemed the "basement of time" because in many places, including the Canadian Shield, volcanic and sedimentary rocks were heated and folded to form essentially a base layer.
That layer of rocks eroded later to expose what we see even today as small mountains and rolling hills in that section of Canada. Since none of the hills are that high, the area was uniquely situated to set the stage for the massive lakes that would be created later with the process described below. The most common fossils are found in the sedimentary rock which provides the evidence scientists need to study ancient geological developments. The Canadian Shield forms the giant Northern and Northwestern basin of what are now the Great Lakes. Coarse-grained igneous rock consisting of feldspar, mica, and at least 20 percent quartz (otherwise known as granite) make up the eastern and southern portions of the basin. This metamorphic layer is underneath the more recent and softer layer of Paleozoic sedimentary rocks.
Although it's hard to imagine today, the historical geology of the Great Lakes during that Paleozoic Era involved a dramatically different landscape and climate than that of today. In fact, a great deal of central North America was covered by tropical seas that provided habitat for crinoids, corals, mollusks, and brachiopods. By the way, if you’re unfamiliar with the major geological eras, a quick look at divisions of geologic time will set you straight. As in many of the world's seas, the ocean floor consisted of clays, lime silts, shells, salts, and sands which over time morphed into harder shale, limestone, sandstone, gypsum, and halite.
Image courtesy of geology.com
On Come the Glaciers from the North to Dig The Great Lakes Basin
The Pleistocene Epoch brought the southerly advance of the gigantic glaciers from the north. These glaciers, some of which were a few kilometers thick, ripped through the area inches at a time like colossal bulldozers. While they advanced, they scoured giant heaps out of the Earth and formed the deep basins that would eventually hold the water of each of the six Great Lakes. After a period of thousands of years, the climate there warmed and melted the glaciers filling the basins with fresh glacial waters. The glaciers then underwent alternating periods of refreezing and melting stages for some time. When the glaciers retreated for good, lakes that were far bigger than the present day Great Lakes were left in the deep basins that they had scoured out of the Earth. Eroded bluffs and ridges hundreds of meters above lake levels of the present day tell that story.
As the ancient waters drained, the lake depths decreased. The Great Lakes drained through several routes to the south in the Illinois River Valley (towards the Mississippi River), the Kawartha Lakes (Trent River), the Hudson River Valley, and the Ottawa River Valley before entering their present outlet through the St. Lawrence River Valley. As is obvious to most people, the magnificent Niagara Falls are the last significant drainage route for all that water. A fascinating look at the geology of Niagara Falls will delve into how they were formed through this process as well. Today, 20% of the world's supply of fresh water is in the Great Lakes and a staggering 99% of that is from the melted glaciers.
The historical geology of the Great Lakes is not over with as it is still evolving ever so slowly today. The uplift (meaning the land is slowly rising) is still taking place in the northern portion of the basin. That factor together with climate change indicates to scientists that the conditions of the lakes are not quite static just yet.
First image courtesy of commons.wikimedia.org
Second image courtesy of wikimedia.org
Natural Processes in The Great Lakes, United States Environmental Protection Agency: https://www.epa.gov/glnpo/atlas/glat-ch2.html
Niagra Falls Formation: Niagarafallsstatepark.com