The paper deals with the Upper Cretaceous beds of the Sacramento and San Joaquin valleys between the latitude of Redding on the north and the latitude of Lost Hills on the south. On the basis of foraminiferal study of a number of surface sections and of core samples obtained from 128 wells the Upper Cretaceous strata of the Great Valley have been divided into twelve zones, four of which are assumed to be represented each by two different ecological facies. The zones are grouped into seven stages. Foraminiferal and lithologic characteristics of the zones, their areal distribution, and stratigraphic relations are discussed and illustrated by cross sections and a diagrammatic map. Another map shows contours of the Cretaceous surface. Eight small-scale maps elucidate paleogeography of the zones and their ecological variations.

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