The area of the Copake, New York, quadrangle lies centrally in what has been described formerly as a klippe of a large overthrust sheet of Cambro-Ordovician argillaceous rocks, thrust westward about 40 miles over carbonate rocks of similar age. The argillaceous rocks are divisible into an older (Late Cambrian to Early Ordovician) group, here named the Elizaville shales, and a younger group of Trentonian shales lying unconformably above the Elizaville shales and the Cambro-Ordovician carbonate rocks. A thin limestone at the base of the Trenton shales wedges out westward.

The structure is imbricate. Most faulting was pre-Trenton; further movement involving the Trenton rocks has occurred mainly along faults in the underlying rocks. No evidence supports the klippe hypothesis, and the facies changes which engendered it can be explained as associated with offshore reef-girt islets or perhaps back-reef conditions.

The Berkshire schist in the eastern edge of the quadrangle is the metamorphosed equivalent of the Trentonian shales.

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