Geology

During the Devonian Period of Earth's history, from about 419 million years until about 359 million years ago, the planet looked much different than it does today. Plate tectonics had not yet moved the continents into their current locations, and the area that would become the United States was periodically covered by warm, shallow seas and was actually south of the Equator. Many invertebrates such as corals, brachiopods, clams, snails, and trilobites lived in these waters, and as they died, their remains settled to the sea floor, forming thick deposits of calcium carbonate that later consolidated into limestone as much as 100 feet thick. Limestone is found throughout Ohio, but one exceptionally pure formation is called the Columbus Limestone, which can be seen in Pickaway County south of Columbus and runs north, ending under Lake Erie near Pelee Island in Ontario, Canada; Sandusky, Marblehead, and Kelleys Island are part of it. In some places this limestone lies close to or at the surface. Until the last glacier began to recede about 14,000 years ago, modern Lake Erie did not yet exist, and although the lakes are geologically recent, the Wisconsin Glacier carved the limestone bedrock beneath them, leaving features such as the Glacial Grooves on Kelleys Island, among the best examples of ice carving in the world, where the glacier may have been up to one mile thick. As Ohio was settled by Europeans during the 18th century, settlers recognized the value of the region's easily accessible limestone, a durable and workable material long used for construction.

Marblehead, Ohio · United States