Cretaceous sedimentary formations and Eocene volcanic rocks crop out in, and adjacent to, the Barrilla and Davis mountains of trans-Pecos Texas. The Lower Cretaceous (Comanche series) is represented by a basal section of Trinity (?) and Fredericksburg age; a medial section consisting of Comanche Peak limestone, the Edwards formation, and the Kiamichi clay all of Fredericksburg age; and an upper portion of Washita age made up of the Georgetown limestone and the overlying Grayson clay and Buda limestone. Only the top member of the Washita group, the Buda limestone, crops out contiguous to the mountain front.

The Upper Cretaceous (Gulf series) is represented by the Eagle Ford, Austin, and Taylor formations. The basal Upper Cretaceous, the Woodbine sand, is absent and Eagle Ford strata are in contact with the underlying Buda.

Of considerable interest is the unconformity between the Cretaceous and Eocene. Before the volcanic rocks encroached, the land surface was very flat and gently inclined as great uniformity characterizes the lava flows. A few lake deposits are associated with the basal igneous section. Each lava flow maintains constant lithologic characteristics and can be recognized from hand specimens with one exception.

These volcanics can be used to good advantage in structure mapping because they reflect the folding affecting underlying sediments and extend over wide areas with relatively uniform change in thickness and interval.

It is evident that the source of these extrusions was at the west and southwest. A constant increase in thickness and quantity of intrusive activity in that direction tends to substantiate this assumption.