This article discusses the lithology of the Upper Cretaceous sediments (Chico series) along the west side of Sacramento Valley, as displayed in a series of measured and examined surface sections between Winters, Yolo County, and Willows, Glenn County. Certain recognizable lithologic units on the outcrop are briefly described, defined, and named as a step toward the breakdown of the thick Chico series into formations adaptable for use in this region. These formational units in descending order are as follows: Forbes formation, Guinda formation, Funks formation, Sites formation, Mills formation, and Golden Gate formation.

Relationship between measured sections, as well as variations in the sections, is discussed and shown by means of correlation charts and drawings.

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