Geology

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 hadn't 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., While geologists have differing opinions about what the world looked like exactly, this map shows a fair approximation of the world during the Middle Devonian., Many invertebrates such as corals, brachiopods, clams, snails and trilobites lived in these warm, shallow waters and as they died, their remains settled to the sea floor forming thick deposits of calcium carbonate (CaCO3). As time and pressure consolidated these deposits, they became limestone as much as 100 feet thick., Limestone rock is found throughout Ohio, but one formation of exceptionally pure calcium carbonate is called the Columbus Limestone. It 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 the Columbus Limestone., In some areas, the limestone occurs very close to or is exposed at the surface. Until the last glacier began to recede about 14,000 years ago, modern Lake Erie didn't yet exist. Compared to the millions of years that limestone took to form, the lakes we see today are very recent geological features. But as the Wisconsin Glacier ground over the limestone beneath, it carved its mark into the bedrock. The Glacial Grooves on Kelleys Island are some of the best examples in the world of this ice carving. The glacier at this location could have been up to one mile thick., As Ohio was settled by Europeans during the 18th century, settlers realized that the relatively easy access to large quantities of limestone was a valuable resource. Limestone has been used for construction since the Egyptian pyramids were built and it is an easily worked yet durable material.