BED, in geology, a term used when rocks are arranged in more or less distinct layers; these are the beds of rock or strata. Nor mally the bedding of rocks is horizontal, or very nearly so; when the upper and lower surfaces of a bed are parallel, the bedding is said to be regular ; if it is thickest at one point and thins away thence in every direction the bedding is lenticular. Beds may be many feet thick, as in some sandstones, or so thin as to be like sheets of paper; e.g., paper shales, such thin beds being often termed layers or laminae; intermediate regular varieties may be called flags, flagstones, or tilestones. Bedding is confined to rocks which have been formed under water or by the agency of wind; these are the "stratified" rocks.
Where rapidly moving currents of water (or air) are transport ing or depositing sand, etc., the bedding is generally not hori zontal, but inclined more or less steeply; this brings about the formation of what is variously called "cross-bedding," "diagonal bedding," "current bedding" or, improperly, "false-bedding." (See SEDIMENTARY ROCKS.) Igneous materials, when deposited through the agency of water or air, exhibit bedding, but no true stratifica tion is seen in igneous rocks that have solidified after cooling, al though in granites and similar rocks the process of weathering fre quently produces an appearance resembling this structure. Miners not infrequently describe a bed of rock as a "vein," if it is one that has some economic value ; e.g., a vein of coal or ironstone.