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As a resident of South Carolina, I was fascinated by this article, which refers to an investigation of an ancient crater buried beneath the Piedmont sediments of our state. However, there wouldn’t be deep sediments overlying basement rock in the Piedmont. There, “basement rock” is close to or at the surface.

Julian R. Harrison
Charleston, S.C.

There’s a difference between the uppermost levels of solid rock beneath soil–sometimes referred to as bedrock–and basement rock. Basement rock is the ancient foundation of a continent, upon which all of its newer rock rests .–S. Perkins