Blown across the land by wind or carried along by water and ice as the land continued to remake itself, loose sediments eventually compressed and cemented into rock and left messages in stone for us to decipher. Sediments include the mud at the bottom of streams, the sand dunes at the foot of the mountains, the chemical precipitates of salt in shallow seas, the beaches at the edge of inland seas, and the graveyards of tiny fossils at the bottom of tropical oceans. In these sedimentary layers, such as the Book Cliffs in western Colorado, the imprints of changing life forms in an ancient world are faithfully recorded.
In a geologic “ugly duckling becomes a swan” saga, sediments that were originally just gunk were later patiently sculpted by wind and water, pressed, and finally lifted to prominence as some of the state’s most imposing landmarks. The sandstones of Colorado National Monument, the reddish-brown siltstones and mudstones of Owl Canyon, and the Flatirons that flank Boulder are all sedimentary rocks. Other sedimentary deposits include massive limestone formations around Leadville, the evaporites of the Eagle Valley, chalks of the eastern plains, coals near Trinidad, oil shale in western Colorado, and the thick shale of eastern Colorado.
How do geologists know that a particular sedimentary rock formed in a particular environment? In 1795 a geologist developed a concept that is the next best thing to being there: “The present is the key to the past.” The idea is that by studying the characteristics found in modern depositional environments and comparing them to similar features found in ancient rocks, one can solve the mystery. For example, in Colorado, we can study the features of modern dunes in the Great Sand Dunes National Park and compare them to the ancient deposits of the 250 million-year-old Lyons sandstone found along the eastern flank of the Front Range. Using the same theory allows us to decipher the rock in the images below to be mudcracks, ripple marks, coarse-grained conglomerate, and raindrops.