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.