Geological History of MD Explained by Rock Expert

By a Broadmead Resident

Geologist Dennis Coskren gave Broadmead residents a fast-paced tour of a slow-moving process: the geological history of Maryland, which goes back 1.2 billion years. The state sits on the North American Plate, one of a number of plates which float on the Earth's crust. Their movement produces continental drift, explained by plate tectonics.

Colliding continents formed "super continents". Pangea formed the Appalachians, then broke apart. An earlier supercontinent was Rodinia, formed when South America came north. A billion years ago, all the continents formed one supercontinent, a phenomenon which has occurred several times. An interesting sidenote is that plate tectonics shows that if the east coast of North America were put against the British Isles, the Appalachians would continue there.

The talk was illustrated by an impressive collection of rocks of varied sizes. Coskren showed a piece of Baltimore gneiss, ametamorphic rock formed by collision zones of continents that formed Rodinia. Catoctin greenstone, which shows the breaking up of Rodinia, is a basalt formed from volcanic islands smashing into the coast, forming the Appalachians. He showed an example of greenstone basalt. The formation and breaking up of Rodinia and Pangea can beseen in a microcosm between Hagerstown and Frederick. As the plates moved, loose, sandy material alternated with limestone deposition, each signifying a collision. Displayed was a piece of rock from Sideling Hill in Washington County, near Hancock, which formed this way. An asteroid hit 35.5 million years ago to form the Chesapeake Bay.

Following a few questions, some of the audience examined the rocks.

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