RAINBOW CLIFFS. The unique geological formations called Rainbow Cliffs, a landmark for centuries, are located four miles south of Marathon, Texas, in central Brewster County. The outcroppings served as protection against Indian attacks and harsh winter winds for nearby Camp Peña Colorado, a United States Army post from 1879 to 1893. The cliffs are composed of the slate from which whetstones are made. The fine-grained sedimentary siliceous rock, without the vegetation that now covers it, produces a rainbow of colors from sunshine. This area of Brewster County became the property of David St. Clair Combs in 1900, when his Combs Cattle Company included the Post Ranch (site of Camp Peña Colorado) and the Rainbow Cliffs. The designation Rainbow Cliffs was used as early as 1879, when Col. B. H. Grierson, who commanded the District of the Pecos from Fort Concho, ordered the establishment of a military post at "Pena Colorado (Rainbow Cliffs) Texas."