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The finer‐grained clastic sedimentary rocks are called shale, siltstone, and mudstone. If a coarse‐grained sandstone consists of over 25 percent feldspar grains it is called an arkose.įiner‐grained rocks. Graywacke is a type of “dirty” sandstone that consists of more than 15 percent silt‐sized or clay‐sized (finer‐grained) material that imparts a darker or speckled appearance. The grains in a quartz sandstone are at least 90 percent quartz. Sandstone is a medium‐grained rock that contains rock particles (mostly quartz) about the size of sand. A conglomerate is formed from coarse‐grained, rounded pieces of gravel. Sedimentary breccia contains an abundance of coarse, angular fragments of gravel that were deposited very near the source area. This process is called sorting.Ĭoarse‐grained rocks. Large, coarse, angular pieces of sediment will be deposited near the source area well‐rounded sand grains will have been transported a considerable distance before being deposited silt, mud, and clay have been carried even farther. This smoothing of rock fragments during transportation is called In general, the greater the distance traveled, the smaller and more rounded the sediment particles will be. Grain size is largely a function of the distance the particle was transported. Clastic sedimentary rocks are classified according to the grain size of the sediment and the kinds of rock fragments that make up the sediment (Table