Sedimentary Rocks Always Form in Layers
Although sedimentary rocks can form on dry land most form beneath water. They form from larger rocks which over time have broken down into very small pieces. The resulting sediment which is a mixture of sand, mud, gravel and clay is then carried away by the elements. Where the sediment settles on water it sinks to the bottom. As the pile increases in size it becomes heavier and with the increase in pressure the water is squeezed out. This stage of the process is known as compaction.
During the final stage minerals in the water surrounding the mass precipitate and begin cementing the layers into rock. Precipitation is a process whereby dissolved minerals crystallise as they come out of water. An example could be a salt water solution. Once the water evaporates you'll just be left with crystallised salt.
The breaking down of rocks and minerals on Earth's surface is known as weathering and over many thousands of years huge mountains can be reduced to sediment. Erosion is the process of carrying the sediment away. Wind, rain, ice, acidity, salts and changes in temperature all contribute to weathering and erosion.
The sediment often includes living organisms which become trapped in the layers. For this reason sedimentary rocks usually contains fossils. The layers which are correctly known as strata are often distinctive and those at the bottom will always be the oldest.
Some of the most common types of sedimentary rock include sandstone which is made up of minute grains of sand, mudstone which froms from clay particles that have been compressed together and limestone which consists mainly of fragments of shell and skeleton from dead marine organisms. Chalk is a white type of limestone.
The oldest sedimentary rocks in Greenland which are 3.8 billion years old are known to have formed from sediment which precipitated from sea water.