Sentinel Rock is a geologic treasure composed of two types of rock, a fine sandstone and a gravelly conglomerate. Its various layers were deposited 275 million years ago, when ancient rivers flowed out of the ancestral Rocky Mountains and sand dunes were beginning to creep across the landscape. The ripple marks on the rock are a sandstone cast of one stream bed layer, and alternating layers of coarse and fine gravels were deposited when streams ran with high or low energy. The streambed is now vertical, showing that the rock layers were tilted upward long after the sediments turned into solid stone. Since taking on their vertical positions, the rocks have continued to change shape as water freezes in cracks and wind erodes the soft sandstone into unusual, bizarre forms. Millions of years from now, the rocks may once again become a pile of sand and pebbles.