The rock cycle is a theoretical model of the constant recycling of rocks as they form, are destroyed, and then reform. We began our discussion of the rock cycle with igneous rock (chapters 3 and 4), and we now discuss sedimentary rocks. Metamorphic rocks, the third major rock type, are the subject of the next chapter.

You saw in chapter 12 how weathering produces sediment. In this chapter, we explain more about sediment origin, as well as the erosion, transportation, sorting, deposition, and eventual lithification of sediments to form sedimentary rock. Because they have such diverse origins, sedimentary rocks are difficult to classify. We divide them into clastic, chemical, and organic sedimentary rocks, but this classification is not entirely satisfactory. Furthermore, despite their great variety, only three sedimentary rocks are very common - shale, sandstone, and limestone.

Sedimentary rocks contain numerous clues to their origin and the environment in which they were deposited. Geologists determine this information from the shape and sequence of rock layers and from the sediment grains and the sedimentary structures such as a fossils, cross-beds, ripple marks, and mud cracks that are preserved in the rock.

Sedimentary rocks are important because they are widespread and because many of them, such as coal and limestone, are economically important. About three-fourths of the surface of the continents is blanketed with a relatively thin skin of sedimentary rocks. Concentrated in sedimentary rocks are important natural resources such as crude oil, natural gas, ground water, salt, gypsum, uranium, and iron ore.