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.

I have been taking photographs in the Marathon area in preparation for a small book on the basin. The original Big Bend Vistas had a section on the Marathon basin but I had to drop it in the Second Edition to keep the book down to an affordable size.

Cathedral Mountain in the Glass Mountains (confusingly there is another Cathedral Mountain south of Alpine) is capped by the Capitan Limestone, one of three erosion-resistant beds in the Permian strata of the mountains, that have created cuesta ridges. A cuesta is a hill or ridge with a steep slope or escarpment on one side and a gentle slope parallel to the strata on the other.

The Capitan Limestone is a fossil reef, rather like the coral reefs of the Caribbean, but built by sponges, algae and by calcium carbonate cement precipitated from seawater. It is found all the way round the south coast of the Delaware Basin, and reaches its most spectacular development in the Guadalupe Mountains, 130 miles northwest of Marathon, where it takes its name from the Capitan Peak. It crops out over most of the west-facing Glass Mountain slopes and caps the three highest peaks, Gilliland Peak (6,513 feet), Old Blue Mountain (6,286 feet), and Cathedral Mountain (6,220 feet).

Cathedral Mountain is north of Highway 90, 11 miles west of Marathon.

Chisos Mountains Lodge and Casa Grande (7,325 feet) in the early morning mist. Casa Grande is a square-topped monolith of bare volcanic rock with sheer, towering cliffs overlooking the Lodge some 2,000 feet below. The volcanic rhyolite dome capping the mountain is slow to erode, and forms solid cliffs. Below it, thinly layered surge deposits and air-fall tuffs erode more easily and form gentler slopes.

Each of the volcanic domes on Casa Grande, Toll Mountain and Emory Peak lies above a volcanic vent and so each can considered an extinct volcano.

The photograph was taken from the Window View Trail in July 2008 at 7:30 a.m.

This view of the old fort has Sleeping Lion Mountain (5,202 feet) on the left horizon with the buildings in front of Hospital Canyon and the columnar lava cliffs just coming into view on the right. The lavas are porphyritic rhyolite of the Sleeping Lion Formation, about 200 feet thick here. It erupted in a single lava flow 35.9 million years ago and is 630 feet thick at maximum. For more see Davis Mountains Vistas.
The buildings in front, formerly the enlisted men’s barracks, now house the offices and visitor center. Behind them, at and to the left of the flagpole are two bungalows that housed officers.

Ranger Peak (6,246 feet) on the left with Twin Peaks (6,133 and 6,112 feet) on the right, photographed yesterday on a beautiful fall afternoon. All three are igneous intrusions into lavas of the Decie Formation.

In mid-photograph, Lizard Mountain is another intrusion. For more see Davis Mountains Vistas.

We stayed at the newly opened Hotel El Capitan in Van Horn over the weekend. The hotel was built by Charles Bassett of El Paso and opened in 1930. It was one of five hotels that he built in west Texas and eastern New Mexico for the Gateway Hotel chain. He commissioned notable architect Henry Trost of Trost and Trost in El Paso for the architectural design. Although smaller, the interior is similar to the El Paisano in Marfa, also designed by Henry Trost for Gateway Hotels.

The hotel was bought by the Van Horn State Bank in the 1970s and converted into an office building. Restoration to an hotel, which began a year ago, involved installing new bathrooms – the bank had removed the originals.

I took this photograph yesterday from the Davis Mountains Scenic Loop about 4 miles from the 166/17 junction just south of Fort Davis.

The Haystacks on the left horizon are twin trachyte intrusions (6,895 and 6,670 feet). Their age is unknown; a nearby intrusion was dated at 34.6 Ma, about 700,000 years after the main phase of volcanic activity in the Davis Mountains ended. The peaks are called Twin Mountains on U.S. Geological Survey topographic maps.

On the right horizon, the Puertacitas Mountains are an eroded series of basalts and tuffs, 1,250-feet thick, that erupted from nearby vents in one of the later volcanic episodes. One vent has been identified between the Puertacitas Mountains and the Haystacks.

The Mano Prieto Mountains are in mid-photograph, a series of small hills composed of Sleeping Lion and Barrel Springs volcanic strata about 35 million years old.

Younger basalt lava outcrops can be seen on the far left of the photograph.

Another favorite image is this one of Mule Ear Peaks from Big Bend Vistas. As I said there,

“Part of the mystique of Big Bend National Park comes from the bizarre shapes that igneous rocks can assume, shapes that are not found anywhere else in Texas. The Mule Ear Peaks are two rhyolite dikes that have eroded into the shapes of mule ears. The dikes were intruded into Bee Mountain Basalt, which forms the ridge in front, and gray Chisos tuffs seen between the peaks and at the bottom of the photograph.

Army Air Corps pilots stationed in the park used to enjoy flying between the peaks in the 1930s.”

Photographed from Mile 17.5 on the Ross Maxwell Scenic Drive in Big Bend National Park.

A favorite image from Davis Mountains Vistas, the opening in the volcanic cliffs is Sunny Glen, photographed three miles north of Alpine on Highway 118.

The cliffs are capped by the Decie Formation with the break in the cliffs being a thin tuff horizon between lavas of the Morrow and McIntyre formations.

The light-colored patches above the left steer are of tuffs interbedded in the underlying Cottonwood Springs Formation. The top of this formation is about 5,100 feet on the cliffs compared to about 3,686 feet in a water well drilled near the highway. The difference of 1,414 feet is due to a fault that runs along the base of the cliffs.