Deeper drilling and better well records in this area since 1934 have added much to the knowledge of subsurface stratigraphy in southern Arkansas. Wells ranging in depth from 3,000 feet to 8,327 feet have disclosed 4,000 feet or more of sediments below the Travis Peak (Trinity) formation which are generally believed to be Mesozoic in age.

Approximately 1,800 feet of the Eocene series lies unconformably on the Gulf series. The underlying Comanche series of formations includes Fredericksburg, Paluxy, Glen Rose, and Travis Peak. This series, with an aggregate thickness of 4,600 feet in the southwest corner of the state, is completely absent within 85 miles northeast, due to pre-Gulf truncation.

Also truncated at this time, were the underlying 2,250 feet of Cotton Valley formation variegated shales and sands; the 270 feet of Buckner red shale and anhydrite; the 870 feet of Smackover limestone; and the 1,800 feet of Eagle Mills red shale and salt formation. These lower beds are thought to be Mesozoic in age; however, the Eagle Mills may be older.

The underlying Paleozoic formations are steeply folded and are probably a part of the Ouachita Mountain system.