The consolidated strata of the Devil Ridge area in Trans-Pecos Texas are Cretaceous. They are surrounded by Cenozoic alluvial fill in the adjacent basins and are intruded by rhyolite sills and dikes of Tertiary age.
The Lower Trinity Yucca formation (1567+ feet thick) is composed of red and gray limestone and red shale with sandstone and conglomerate. Conformably over-lying the Yucca are arenaceous limestone and sandstone of the Bluff formation (1080 to 1369 feet thick) of Glen Rose age.
The Cox formation (557 to 1066 feet thick) above the Bluff is composed of sandstone, quartzite, and thin limestone and ranges in age from perhaps the upper Glen Rose through the Lower Fredericksburg. The Finlay formation (346+ feet thick), a limestone belonging to the Fredericksburg group, rests conformably on the Cox.
Georgetown and possibly Grayson strata of the Washita group (1021+ feet thick) are limestone and sandstone.
The Eagle Ford formation (1419+ feet thick) of the Upper Cretaceous rests conformably on the Washita beds and is composed of black shale, sandstone, and thin beds of chalky limestone.
Northward the younger beds successively overlap the older ones.
The area lies along the northeastern front of the Sierra Madre orogenic belt in Trans-Pecos Texas. Structurally it is divided into three blocks by two northwest-trending thrust faults—the Devil Ridge and the Red Hills—with a total displacement of at least 7½ miles. There are four minor thrust faults, numerous small normal faults, and some small folds.