Geo Tex

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).

Continuing my journey around the Marathon Basin, this photograph, taken one mile south of the Highway 90/385 junction, is a good illustration of the basin south of Hwy 90. Many of the hills there are capped by a chert bed known as the Caballos Novaculite, Caballos from Horse Mountain in the basin where the chert is particularly thick and prominent. The term “novaculite” comes from Arkansas where this rock also crops out. There it is used for whetstones. The novalculite is very hard, microcrystalline, chemically inert, and brittle. It doesn’t erode chemically, only mechanically, and so caps hills. Note the scallopped outcrops in mid-picture, called “flatirons” by geologists. Flatirons are are found on the flanks of several hills in the basin.

The mesa on the right horizon is Elephant Mountain, prominent to the west of the basin. The mountain is capped by an enormous nepheline syenite sill, four miles long, two miles wide and 1,200 feet thick, weighing about 3 billion tons. The mountain was named for its shape, which resembles an elephant’s back when viewed from some angles. For more see River Road Vistas.