Point Gratiot Disconformity
December 14, 2020
Photographer: James R. Ebert
Summary Author: James R. Ebert
Point Gratiot, a rocky headland in Dunkirk, New York, protrudes one kilometer into Lake Erie, one of the Laurentian Great Lakes. The point is underlain by the Upper Devonian Dunkirk Shale, a black shale deposited under anoxic conditions in the Appalachian Foreland Basin during the Acadian Orogeny. Here, the Dunkirk Shale is overlain by Pleistocene glacial deposits producing a disconformity that represents the loss of approximately 375 million years of Earth’s history.
The eastern side of Point Gratiot (upper picture) is usually difficult to access. However, during fall 2020, a small beach formed at the base of the cliff enabling access. The shale clearly shows two sets of joints (stress release fractures) that are nearly perpendicular to each other.
The lower photo shows the disconformity between the Devonian shale and the Pleistocene glacial sediments. The top of the shale is marked with glacial striations and grooves accented by sand blown from the adjacent beach. This erosional surface is overlain by 10-20 cm of cobble-bearing glacial till deposited directly by the ice. Above the till is over one meter of well-sorted silt that displays spectacular contorted bedding. Contorted bedding results from the rapid dewatering of sediment as it compacts. This silt was deposited in the Pleistocene precursors of Lake Erie. Photos taken October 13, 2020.