The History of the Great Lakes: Salt Water or Fresh?

The Great Lakes, a collection of five large freshwater bodies in North America, are often considered a bastion of purity and freshness in today's world. However, their geological history reveals that they might have had a different story. Were they once filled with salt water, or have they always been fresh?

Current State of the Great Lakes

Today, the Great Lakes are indeed fresh water bodies, with Lake Superior being the purest of them all. It contains approximately 10% of the world's fresh water. However, despite this purity, it's not immune to contamination. The area near Ashland on the south shore of Lake Superior is designated as a Superfund clean-up site due to past pollution. Another concern is the presence of Entamoeba hartmanni (Giardia), which can cause severe gastrointestinal issues if ingested. For this reason, it's recommended to use a mechanical filter when drinking water from Lake Huron and Lake Erie.

Geological Past of the Great Lakes

Geological evidence shows that the area where the Great Lakes sit has had a much more dynamic history. Four hundred million years ago, the region was covered by an inland sea. This sea, now dried up, left behind vast amounts of salt deposits, which are still evident today. You can find these salt deposits under Lake Huron, Lake Erie, and other nearby areas. Cities like Detroit have underground salt mines with over 100 miles of tunnels underneath them. This demonstrates that, while the current Great Lakes are freshwater lakes, there was a time when they might have been brackish or contained salt water.

Formation and Evolution of the Great Lakes

The Great Lakes, as we know them today, were formed roughly 14,000 years ago during and after the last glacial period, the Wisconsinan glacial period. As the Laurentide ice sheet melted, it left behind vast depressions that gradually filled with meltwater, forming the Great Lakes. This process explains why the lakes are not only large but also contain substantial freshwater resources.

Geological Definitions and Other Interpretations

It's important to note that the term "sea" in the context of the Great Lakes is not limited to salt-water bodies. In English, "sea" can also refer to large, inland waters of any salinity. Another definition suggests that a "sea" is "a large lake or landlocked body of water." Given this, the Great Lakes fit this definition perfectly, even though they are freshwater. This broad definition helps us understand that the Great Lakes might have had different compositions throughout their geological history.

Further, the bedrock in Central Ontario and the area around Lake Erie and Lake Huron is limestone. This sedimentary rock is a result of the deposition of dead marine creatures in shallow tropical seas. The Dolomite layer in the Niagara Escarpment is particularly hard and erosion-resistant, contributing to the formation of the famous Niagara Falls and the escarpment itself.

Conclusion

While the current Great Lakes are indeed freshwater, their geological history reveals a more complex story. During their formation and evolution, they might have contained salt water, reflecting the vast changes that the region has undergone over millions of years. The salt deposits, fossilized marine life, and geological formations in the area provide us with fascinating insight into the diverse and dynamic history of these important water bodies.