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