Continuing down the Marathon Basin on Hwy 385, the southern margin on the left of the highway is seen in the above photograph, taken 24 miles south of Hwy 90. On this margin, the Cretaceous strata dip only very slightly to the south, perhaps because they are a distance from the center of the uplift. The knob in center is a small intrusion. Several similar intrusions occur along the margin.

A close-up view of Santiago Peak from the south, 50 miles down Hwy 385 from Hwy 90. The northeast wing of the intrusion can be seen from this angle, about 6,250 feet at the peak, 270 feet lower than the main body. The shapes of the two summits suggest that they are part of a plug, not an eroded sill. The flat top seen in the previous photo would therefore be the result of the uprising intrusion coming in contact with a resistant rock layer, perhaps a thick lava bed.

It is hard to believe that there were lava beds at this altitude when there are no traces of volcanic rocks of the main volcanic phase east of this point, but Elephant Mountain, 12 miles to the north is definitely a sill, the top of which is at 6,230 feet, so there has been a great amount of volcanic material removed in the last 30 million years.

Continuing down Hwy 385 in the Marathon Basin, one of the most spectacular sights are the flatirons on East Bourland Mountain, photographed 8 miles south of Hwy 90. A flatiron is a short, triangular hogback forming a ridge or spur on the flank of a hill that looks like a flatiron. A flatiron is usually a plate of steeply inclined resistant rock, in this case Caballos Novaculite, called a flatiron from its shape. A flatiron is a short, triangular hogback forming a ridge or spur on the flank of a hill that looks like a flatiron. Here, the light-colored flatirons of novaculite stand out against dark Maravillas Chert on the higher parts of the mountain. The Maravillas Chert Formation is of bedded black chert and dark gray to black limestone, 100 to 400 feet thick, the next youngest to the novaculite in the Marathon Basin succession.

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.

Indian Lodge from below – the design is pueblo revival, something quite foreign to West Texas except though the work of architects like Henry Trost. The Civilian Conservation Corp, who built the lodge, employed its own architects, in this case Arthur F. Fehr, who also worked on restoring the San Antonio missions.

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.