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Grains And Rock Fragments:
The quartz grains of sandstones are well rounded and have frosted (dull) surfaces if the sands are wind-blown and of desert origin, less well rounded if they are water-transported, and angular if they are of glacial or fluvio-glacial origin. Arkoses are feldspathic sandstones with more than 25 per cent of feldspar, mainly potash feldspar. Graywackes are dark-colored, poorly sorted rocks with numerous small rock-and-mineral fragments set in a clayey, calcareous matrix, and generally with more sodic plagioclase than potash feldspar. Both arkoses and gray-wackes have been deposited near their sources and the grains are generally angular.Residual Deposits.—These are generally poorly sorted and consist of larger, angular grains and rock fragments, together with much fine clay material. They are made up of material left behind when the rest of the formation was carried away. Soil and subsoil are the most important residual deposits, but soils may be both residual or transported. Laterite is a red soil formed in tropical and subtropical climates.
The particles in gravel are derived from many kinds of rocks. When rocks are picked up by moving water, they are gradually reduced in size by the breaking away of edges and corners. (The current's velocity must increase greatly to carry successively larger sizes.) The fragments become more or less rounded after long periods of rolling and skipping along stream bottoms. The rounded fragments that make up gravel range from particles 2 millimeters (about 0.08 inch) in diameter to pebbles, cobbles, and boulders. The spaces between the fragments may be more or less filled with grains of sand; gravel beaches, or shingle beaches, however, generally contain little sand.
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