Figure 5.10These sedimentary rocks exposed at El Capitan, in the Guadalupe Mountains of West Texas, were formed in an ancient ocean about 260 million years ago. The lower slopes of the mountains contain siliciclastic sedimentary rocks formed in deep-sea environments. The overlying cliffs of El Capitan are limestone and dolostone, formed from sediments deposited in a shallow sea when carbonate-secreting organisms died, leaving their shells in the form of a reef.