Rocks break into smaller pieces by many different processes, but no matter what process it has gone through, the smaller pieces of rock are called sediment. The sediment can also include organic material, such as plant parts. A sedimentary rock is a rock that is formed from these small pieces. This does not mean that the smaller piece of rock will fit in a hand. Some of them can be boulder-size or even the size of a barn. The larger rock that it broke off of could have been the size of a mountain.
Sediment may be carried from one place to another in many different ways. It may be carried down a river, wind may carry it a distance away or gravity may pull it down through an avalanche or a landslide. While the sediment is traveling it can be broken, or eroded, into smaller pieces by being jostled by other rocks. The longer that the sediment is transported the more erosion can occur.
After being transported, many pieces of sediment rest in one place, whether at the base of a hill, or on a river bank, or on a beach. A big piece of sediment takes more effort to move, so it will have stopped moving before the smaller ones. Along a delta of a river, there may be large pieces of sediment which have settled, while the smaller pieces continue traveling in the water for many miles until it settles in the ocean somewhere.
The change of sediment into rock takes several thousands of year to happen. Many of these rocks have calcite and quartz which hold them together. As time passes, high temperatures and pressure leads to this sediment becoming cemented together. New layers of sediment could be deposited over the older layer which would add weight and pressure on the older layer. The new layer also raises the temperature of the older layer as it acts as insulation.
Sedimentary rocks generally form in layers due to the process of adding new layers on top of older ones. This aids scientists in discerning what the climate was like in an area for hundreds, or even thousands of years in the past. They are able to see when floods, or wet seasons occurred, or if an area had a pond or lake on it in previous eras. These layers of sedimentary rock, or strata, can help geologists discover the history of an area, through the layers.
Different kinds of sedimentary rocks form from different materials. Limestone, after being under very high temperatures and pressure metamorphoses into marble. Sandstone changes into quartzite, mud stone into slate, and flint forms from quartz.