The geologic section of the Rio Grande embayment of Texas and Mexico has several features peculiar to that locality that have not been adequately studied and presented in the literature. This paper is offered as a contribution to the geologic literature of that region. Areal conditions are described and subsurface information from numerous wells is presented. The term "Rio Grande embayment" is used partly in a geographic sense, as the paper begins with a discussion of the Glen Rose formation, which antedates the genesis of the embayment.

The Upper Cretaceous geology of the region is peculiar in the exceptional thicknesses of the formations; the probable extension and interlapping of faunal ranges; and the occurrence of two or more non-marine series of Navarro age, only one of which appears at the surface.

The Tertiary formations are described and the Bigford as mapped by Trowbridge is shown to extend much farther than the territory covered by his report and map, continuing north into Zavala County, thence east into Frio County, where the formation lenses out and the Carrizo sands and Mount Selman become contiguous.