The monoliths (large, freestanding rock formations) are composed of the earthy, buff-pink Entrada Sandstone. Deposited 160 million years ago in the Jurassic period, this fine-grained sandstone formed by the deposition of sand and silt in tidal flats. It crumbles easily to a fine sand which is rapidly removed by water; therefore, talus (debris) slopes do not form and Entrada cliffs tend to rise sheer from their base. Above the Entrada, the grayish-green sandstone and siltstone of the Curtis Sandstone forms a hard cap rock on some of the monoliths and higher cliffs and buttes, protecting them from erosion. Above the Curtis is the thinly-bedded, reddish-brown siltstone of the Summerville Formation.
