Topic: Profiles

A Half-Billion Years in the Making

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Conflict is nothing new to Swanton Red marble, as the rock itself was born of a cataclysmic continental collision. Five hundred million years ago, western Vermont lay beneath a shallow inland sea where billions of generations of tiny marine creatures lived and died, littering the ocean floor with their chalky shells. The shells eventually became compressed into a 30- to 120-foot-thick layer of limestone-like rock called dolomite, which today is part of the Dunham Formation.

Fifty million years later, the continents of Africa and North America converged, subjecting the rocks of the Champlain Valley to intense heat and pressure that metamorphosed shale into slate, limestone into calcite marble, and dolomite into dolomitic marble — some of which was colored red by iron impurities.

This slow-motion continental impact also thrust up immense slices of crust, like so many tilted dominoes, along a series of faults that run the length of Vermont. One of the most famous of these, the Champlain Thrust Fault, heaved the Dunham dolomite up from a depth of 1.5 miles and drove it 50 miles west, where it came to rest above much younger rocks. There it would lie, awaiting the quarrier’s saw and the sculptor’s chisel to alter it once again, this time into a highly polished floor tile or mantel — a mere half-billion years in the making.

An excellent exposure of the Champlain Thrust Fault, which stretches 200 miles from Canada to the Catskills, is visible at Lone Rock Point in Burlington, ideally from the Lake Champlain ferry shortly after it leaves the dock.

For more information, visit: anr.state.vt.us/dec/geo/chthrust.htm


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