The Great Lakes of the United States and Canada comprise the largest area of inland sea on planet Earth. For the most part the Great Lakes were formed 7,000 to 32,000 years ago during the Pleistocene Epoch, better known as the Ice Age. The earth's temperature remained very low during this period, giving rise to massive amounts of snow. Since the earths temperature stayed low, no warming trends appeared and the snow did not melt. As the years passed the layers of snow built up, and the tremendous weight of the upper layers compressed the lower layers into ice sheets - or glaciers - some of which were 10,000 feet thick.
As these glaciers moved southward they carved deeply into the earth's surface, displacing billions of tons of soil and rock (evidence of the power of these mighty glaciers is visible today at Glacial Grooves State Park on Kelley's Island). The glaciers advanced to the southernmost part of Wisconsin, and then as the earth's temperature gradually began to rise, the glaciers started their long northward retreat.
As the ice melted the residual water filled in the basins created by glacial erosion. Slowly, the Great Lakes began to assume their present configuration. Lake Erie, being the southernmost of the Great Lakes, was first to appear, soon followed by Lakes Ontario, Michigan, Huron and Superior.
Lake Erie is the fourth largest of the Lakes, with Lake Ontario being the only one smaller. It is 240 miles long and 30 to 60 miles wide. Lake Erie is the only Great Lake with its bottom above sea level. The lake is composed of three separate basins. The eastern basin extends from Buffalo, New York to Erie, PA, having an average depth of approximately 120 feet; the central basin extends from Erie, PA to Cedar Point, OH with an average depth of 80 feet; and the western basin stretches from Cedar Point to Toledo, OH having an average depth of only 35 feet.
Although the average depth of Lake Erie is 60 feet, it is obvious that there is considerable variation in depths, with the western basin being much shallower than the eastern. This has occurred primarily because of the composition of the basins. The western basin is composed of a hard, resistant limestone, whereas the eastern end is composed of a softer shale. As glacial and erosive forces worked on these basins, they naturally carved deeper into the softer shale at the eastern end.
Western Lake Erie supports about 20 islands as well as numerous reefs and shoals. These islands form what is known as an archipelago, a word derived from the Greek language meaning "a sea studded with many islands".
The islands are mostly composed of two types of rock, limestone and dolomite. Limestone, which was formed about 350 million years ago during the Devonian period, is comprised by limey mud and sea animal remains. Dolomite, formed about 400 million years ago during the Silurian period, is similar to limestone in composition, but also contains a mineral called magnesium.
As the islands were forming during geologic times, an event occurred which affected their present day positioning. There was an uplifting of the earth which created an arch, known today as the Cincinnati Arch. Because of this arch the rock layers are exposed to dip toward the east, therefore, the older layers are exposed progressively westward. This is evident by looking at an overall view of the islands. Pelee, Middle, and Kelley's Islands are composed of the younger Columbus limestone, whereas the East Sister and Hen Islands along with North, Middle, and South Bass, are in a similar line but are composed of the older dolomite.
South Bass Island is the third largest of the Lake Erie Islands and is an extension of the Marblehead Peninsula. It is composed from west to east, of Tymochtee, Put-in-Bay and Raisin River dolomite. Tymochtee dolomite is visible at the base of the cliffs at the northwest limit of South Bass State Park. The light grey Put-in-Bay dolomite, characterized by its bedding layers, forms the western portion of the island. The location of Perry's Monument marks the area of contact between the Put-in-Bay and the Raisin River dolomite.
One of the most interesting geologic features of the island is the many caves. When the island was tilted during the upheaval of the Cincinnati Arch, the rock was weakened and joints formed. This allowed water to enter, dissolving the minerals in the shale and forming caves. Most of the domed caves are small and many have collapsed. The largest, Perry's Cave, is 165 by 200 feet and 9 feet high. The water level in the cave rises and falls with that of the lake. Crystal Cave is unique in that it is not a dome shaped cave, but a huge geode. The walls are covered with blue celestite crystals from 8 to 15 inches long. Originally the cave was only 3 feet high but was excavated and the crystals sold for the manufacture of fireworks. Both these caves are now open to the public.
Today the Lake Erie Islands and their surrounding environment provide a natural and scenic setting for Perry's Victory and International Peace Memorial. Although geologically speaking the islands are constantly changing and will never rest, the Memorial symbolizes an everlasting peace between the English speaking nation of the United States and Canada, that share their lives among the Lake Erie Islands.
