Geologists believe that sedimentary rock was forming three and a half billion years ago. Sedimentary rock is formed when mineral matter of plants and animals settle out of water and, less commonly, air or ice. The most common materials for sedimentary rocks are fossils, formed when sediment covered dead plants and animals as the sediment changed into rock. The remains are outlines of the dead plants and animals. Some limestone is made entirely of fossils, microscopic sea life, and is deposited in oceans. Sedimentary rock covers about three fourths of the land area, and most of the ocean floor. Where the earth's crust is deformed or eroded, large areas of buried sedimentary rock may be exposed. In some places, such as the mouths of rivers, the sedimentary rock is 12,000 meters thick.
After thousands of years, sedimentary rock is formed by many compact layers of rock building up pressure from water and the weight of other layers of overlying rock squeezing the rock until it molds together. Water that trickles slowly through layers of coarse sand and gravel, deposits mineral cement around these particles, cementing the layers together to form rock. The layers, which vary from one another in composition or texture, distinguish sedimentary rock from igneous and metamorphic rock. For sedimentary rock to bed or to form broad, flat layers from the collection of grains of clay, silt, or sand settling in river valleys, or on the bottoms of lakes and oceans, there has to be a parent rock for the sediment to form around. This parent rock can be any type of rock, igneous, metamorphic, or sedimentary.