Persimmon Gap, the northeast entrance to Big Bend National Park, lies astride a reverse-faulted monocline of a northwest-trending segment of the Santiago Mountains. Preserved Cretaceous rocks, about 762 m of Comanchean, and an incomplete section of Gulfian, consist of limestone, marl, and sandstone. Slip reversal, between Laramide and basin-and-range deformation using part of the Santiago thrust, is well-displayed in this range.

Laramide deformation produced the southwest-facing, N58°W-trending, reverse-faulted monocline of the Santiago Mountains. Structural relief across this faulted monocline is about 914 m. On the upthrown side, northeast of the Santiagos, are en echelon folds that suggest a left-lateral component of movement along the fault. The northwest end of this segment turns north where it becomes an unfaulted monocline with about 762 m of structural relief. To the southeast, structures in the Santiagos turn south and the monocline is unfaulted, with structural relief being about 762 m. The transition southward from the narrow northwest-trending Santiagos to the relatively broad, asymmetric anticlines with faulted limbs of the northern Sierra del Carmen involves both Laramide and basin-and-rang episodes of deformation.

Basin-and-range deformation has reactivated the steeply-dipping Laramide fault planes and displacement is reversed. Throw along these faults ranges from 30 to 945 m. This area lies along the southeastern extension of the Texas lineament and suggests a left-lateral component of movement during Laramide time and a right-lateral component during basin-and-range time.

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