The finer sediments are called _sand_, _silt_, _mud_, and _clay_. When cemented, the sand grains become _sandstones_, the silt particles become _siltstones_, and the mud and clay particles become _shale_. The sedimentary rocks that are made up of these rock fragments are called _clastic_ or _fragmental_ rocks. Sedimentary Rock Materials in Solution As they are weathered, some rocks dissolve and go into solution. For example, a number of the Texas creeks and rivers carry calcium carbonate in solution because they flow through areas where limestone rocks, which consist mostly of calcium carbonate, are being weathered. (Water that contains a large amount of dissolved rock material is called _hard_ water.) _Cementing materials and chemical sediments._— Some of the waters containing dissolved rock material seep through loose sediments where the dissolved material may come out of solution and form a _cement_, which binds the sediments together. For example, when loose sand sediments are cemented, they form sandstone. Three of the most common cements are iron oxide, calcium carbonate, and silicon dioxide, although a number of other materials also serve as cements. Dissolved rock materials come out of solution not only to serve as cementing agents but to form the chief mineral of some sedimentary rocks as well. Sedimentary rocks of this kind form mostly in lakes and seas into which much dissolved material is carried by rivers. When the dissolved material comes out of solution, it is said to be _precipitated_ and the mineral sediments it forms are the _chemical_ sediments. Some limestones originate this way. You can see examples of precipitated materials by noting the crust-like deposits that form inside some water pipes and teakettles, as dissolved material in the water comes out of solution. [Illustration: Precipitated sediments are commonly observed lining a teakettle.] _Sedimentary rocks formed by plants and animals._— The dissolved rock material can come out of solution in another way. Some plants and animals are able to take dissolved calcium carbonate out of the sea water and use it to build their shells and other structures. Some of these organisms, such as corals and algae, can grow upward from the sea floor in large groups to form reefs that later become reef limestones. Other limestones are made up of the remains of plants and animals that collect on the sea floor and become cemented together. METAMORPHIC ROCKS