One of the best expositions of the structure of the Santiago Mountains is given by Persimmon Peak just before the entrance to Big Bend National Park. Conglomerate and coarse sandstone of the Glen Rose Formation at the base of the Cretaceous are wrapped round an overturned anticline on the left. Within the anticline, a thrust fault has itself been folded, resulting in Maravillas strata lying over younger Tesnus strata. At the base of the peak, Glen Rose sedimentary rocks are separated from overturned upper Cretaceous strata by the main Santiago Mountains thrust fault, which runs from Red Mountain to Dog Canyon.
The Tesnus Formation belongs to the upper Pennsylvanian period, some 340 million years old, younger than the Ordovician Maravillas Formation, around 450 million years old, but much younger than the Glen Rose Formation, 140 million years old.
The photograph, diagram and description are from Big Bend Vistas.
