The material moved by all of the geologic agents of transportation—gravity, streams, wind, ice, ocean currents, and waves—both the solid fragments and the material carried in solution, is deposited to form sedimentary rocks. Some of the material is deposited on the land surface to form nonmarine sediment, and some is deposited in the ocean as marine sediment. The ocean is the final resting place of most transported debris; the great bulk of sedimentary rocks are marine. Over broad areas these marine sediments have been lifted up to form the surface of the land. Sedimentary rocks (and metamorphic rocks derived by the alteration of sedimentary rocks) comprise only about 5 percent of Earth’s crust as a whole, but they account for 75 percent of the surface of the continents.
The marine sedimentary rocks now exposed on the continents were not formed in the deep ocean. They were deposited in shallow seas that covered submerged parts of the continents. Rarely has the deep ocean bottom been upheaved to form land. Most sedimentary rocks are deposited in layers, or strata. An individual layer may be less than a centimeter, or several tens (rarely hundreds) of meters thick. Most of these layers (strata, or "beds") were originally laid down in nearly horizontal positions, but later deformation of Earth’s crust has broken and tilted them until they may now lie in any position—steeply inclined on end, or even upside-down.
Many sedimentary rocks contain fossils—the remains of ancient organisms buried in the muds and sands of the ancient sea bottom and former lakes and streams, or under the shifting sands of ancient deserts. Most fossils are the remains of the hard parts of actual organisms, the shells or bones of animals, or the trunks or leaves of trees. Some, however, are other sorts of organic traces, such as filled-in worm borings or trails of animals. For instance, during the Triassic period, some 250–200 million years ago, dinosaurs wandered about the mudflats of what is now the Connecticut River valley in New England, and left their footprints, which were buried by succeeding layers of mud and sand and preserved as fossils.