Sedimentary Rocks

Sedimentary rocks cover 80 percent of Earth’s exposed surface and contain nearly all of our record of past life, past environments, and climate change—not to mention supplying all of our society’s fossil fuels (coal, oil, and natural gas). A number of the geologic subsciences (such as geomorphology, oceanography, paleontology, stratigraphy, and sedimentology) are concerned wholly or partly with their study. These studies are frequently called “soft-rock geology,” as opposed to the study of igneous and metamorphic rocks, which is called “hard-rock geology.”

Sediments may accumulate either on the continents or in the ocean basins. They may be of two main types:

• Detrital—actual particles carried in suspension or rolled by water, wind, or ice to their place of deposition. Detrital sediments are commonly divided on the basis of grain size into seven categories—boulders, cobbles, pebbles, granules, sand, silt, and clay. The last three are by far the most abundant and important.

• Chemical—sediments that are precipitated out of water and are derived from dissolved ions carried in solution.