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Bed (Geology)

1911 Encyclopedia Britannica

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In geology, a term for certain kinds of rock usually found to be arranged in more or less distinct layers; these are the beds of rock or strata. Normally, 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 thick (50 ft. or more) 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. In fine-grained rocks the bedding is usually thinner and more regular than in coarser rocks, such as sandstones and grits. Bedding is confined to rocks which have been formed under water or by the agency of wind; these are the "stratified" rocks.

The deposition of rock material by moving water is not as a rule uniform, slight changes in the velocity produce an immediate change in the size of the particles deposited upon a given area; thus a coarse sand layer may be succeeded by a finer sand or a mud, or two sandy layers may be separated by a thin layer of muddy shale. Bedding is most often induced by a change in the nature of the contiguous strata; thus a sandstone is followed by a shale or vice versa - changes which may be due to the varying volume or velocity of a current. Or the nature of the deposit may be influenced by chemical actions, whereby we get beds of rock-salt or gypsum between beds of marl. Or again, organic activities may influence the deposit, beds of coal may succeed layers of shale, iron-stone may lie between limestones or clays, a layer of large fossils or of flints may determine a bedding plane in massive limestones. Flaky minerals like mica frequently assist in the formation of bedding planes; and the pressure of superincumbent strata upon earlier formed deposits has no doubt often produced a tendency in the particles to arrange themselves normal to the direction of pressure, thus causing the rock to split more readily along the same direction.

Where rapidly-moving currents of water (or air) are transporting or depositing sand, &c., the bedding is generally not horizontal, but inclined more or less steeply; this brings about the formation of what is variously called "cross-bedding," "diagonal bedding", "current bedding" or improperly "falsebedding." Igneous materials, when deposited through the agency of water or air, exhibit bedding, but no true stratification is seen in igneous rocks that have solidified after cooling, although in granites and similar rocks the process of weathering frequently 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." (J. A. H.)