This is Catoctin Greenstone which is the hard igneous stone that can be found at the top of many of the highest peaks in the northern Blue Ridge Mountains, especially along the cliffs that dot the border of Maryland, Virginia, West Virginia, and Pennsylvania.
The flat benches and cliffs we see in the northern Blue Ridge are generally due to the presence of hard greenstone retarding erosion underneath. The Blue Ridge Mountains were created over 1 billion years ago, making them among the oldest mountain in the world, second only to South Africa's Barberton greenstone belt.
Greenstone was originally black basalt, but over time, and with heat and pressure, it has been invaded by other minerals such as chlorite and epidote which has colored the hard stone and turned it shades of grey and dark green
Catoctin Greenstone was laid down about 570 million years ago when tectonic plates began to spread apart creating a series of rifts thousands of miles long. Over the course of several million years, molten rock rose up through these rifts and spilled out across 4,000 square miles of older igneous and metamorphic rocks in flows 20 to 100 feet thick.
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