The mountain is composed of 43-million-year-old volcanic rock which formed columnar jointing as displayed on the west face.

[4][5] Although modest in elevation, topographic relief is significant as the summit rises 1,000 feet (305 m) above Highway 118 in one-half mile (0.8 km).

[1] The mountain's toponym has been officially adopted by the United States Board on Geographic Names,[3] and has been listed in publications since at least 1904.

[5] Willow Mountain is the most spectacular display of rock joints in the southern Big Bend area.

[8] The mountain mass is an intrusive plug, which cooled below the surface and was subsequently uncovered by erosion.