All of Earth's rock types fall into one of three categories: igneous, metamorphic, and sedimentary. Most of the Grand Canyon's exposed rocks belong to the third category: sedimentary rock. When mineral grains are eroded from surface materials, transported by water or wind, and eventually laid down on the ground or seafloor, a loose deposit of sediments forms. Over time, the mounting pressure and heat from subsequent layers compact these soft sediments. Squeezed of their water, the sediment grains are cemented together, or lithified, to form solid -- though brittle -- rock.

The telltale feature of sedimentary rocks is horizontal bedding, or layering, that ranges in thickness from several millimeters to several meters. As a rule, deeper layers are older than those above them. Because sediments form from different minerals and may be deposited on land or in water, there are many kinds of sedimentary rocks. Sand becomes sandstone, mud becomes shale, and the calcified remains of marine organisms become limestone.

Fossils, the preserved body parts or impressions left by once-living organisms, can be found in sedimentary rocks. Though discoveries of bones and even whole bodies entombed in ice, tar, or amber may be more celebrated, far more common in the fossil record are trace fossils -- burrows, footprints, and other impressions left by everything from plants to dinosaurs in soft, sandy, or muddy sediments. When these traces are quickly filled in by younger sediment and later lithified, they can be protected from destruction and preserved until they resurface through uplift, erosion, and weathering of ancient sedimentary strata.

By studying fossils, scientists learn a great deal about life forms that once inhabited Earth. From its fossilized skeleton and impression, researchers can often discern how an organism looked, moved, and obtained food. By studying the rock layer in which the fossil was found, they can also understand the climate in which the organism lived, and even the circumstances by which it died.

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