A cross section of the stratigraphic succession on the American side of the Rio Grande conforms to Kane and Gierhart's (FOOTNOTE 1) formational divisions which were established for the most part from Trowbridge's original work. The Cook Mountain has been subdivided into three members. The subsurface top of Cook Mountain (uppermost occurrence of Ceratobulimina eximia) is about 500 feet below the top of the Cook Mountain as mapped at the surface.

Cycles of deposition in the Yegua and Fayette are found to be very similar. The Mier and Alamo sandstones of the Yegua and the Salineno, Roma, and Sanchez sandstones of the Fayette have a marine facies where they cross the Rio Grande into Starr and Zapata counties. Northward these marine sandstones wedge out and the shale members between become increasingly non-marine. It is suggested that each sandstone wedge and its associated shales represent a cycle of transgression and regression of the sea.

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