Preservation of a lens of Hillsboro-like sandstone at the Silurian-Devonian disconformable contact in Madison County, Ohio, coincides with the presence of a dolomite pebble basal conglomerate with a red clayey matrix. Analysis of the minerals in the red matrix and study of the features of the underlying dolomite suggest that a portion of a pre-Devonian weathered surface has been preserved. The presence of such a clay mineral assemblage in other similar geologic circumstances may provide a criterion for the identification of an unconformity.

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