The other major landmark in the southern Marathon Basin is Santiago Peak (6,521 feet), seen here in a photograph taken 6.4 miles south of Highway 90. The peak is one of the most striking landmarks in the Big Bend, rising 3,250 feet very steeply the Maravillas Creek valley below. The upper part is a nepheline syenite intrusion 1,250 feet thick and about three-quarters of a mile in diameter. Debris covers the intrusion’s lower boundary so it is not possible to say whether the intrusion is a plug or the remnant of a larger sill such as the ones capping Nine Point Mesa and Elephant Mountain. Its shape suggests that it is a plug. The intrusion overlies 900 feet of volcaniclastic sandstones, the most easterly occurrence of tuff-derived material.
The mountain in the left foreground is Simpson Springs Mountain (4,685 feet), showing steeply dipping beds of Devonian? (416-359 million years old) Caballos Novaculite on its crest and flanks. The poorly outcropping strata between the novaculite is mapped as Dagger Flat Sandstone, Cambrian (542-488 million years old) and Ordovician (488-444 million years old) in age. The Marathon Basin provides the most complete sequence of Paleozoic rocks in Texas, the only period missing being the Silurian (444-416 million years old).