Because biostratigraphic and obvious structural markers are lacking, correlation and interpretation of the exclusively clastic Cretaceous section underlying Nassau County, Long Island, New York, have previously been based solely on the nature and distribution of various sediments.

Detailed clay mineralogical analysis of drill-core samples now establishes an extensive horizon of chlorite-bearing clay in the Raritan Formation, which is interpreted to constitute a time-rock unit. The structural attitude of the chlorite zone with respect to overlying rocks suggests the presence of an angular unconformity.

The contact between the Raritan and the Magothy Formations — traditionally recorded at the bottom of a laterally discontinuous gravelly layer — is placed at the unconformity.

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