joint, in geology, fracture in rocks along which no appreciable movement has occurred (see fault). Nearly vertical, or sheet, joints that result from shrinkage during cooling are commonly found in igneous rocks. Similar joints occur in thick beds of sandstone and gneiss, with the sheets resembling the structure of a sliced onion. The prismatic joints of the Palisades of New Jersey and Devil's Tower, Wyoming, are examples of joints caused by contraction during the cooling of fine-grained igneous rock masses. Deep-seated igneous rocks often have joints approximately parallel to the surface, suggesting that they formed by expansion of the rock mass as overlying rocks were eroded away. Some joints in sedimentary rocks may have formed as the result of contraction during compaction and drying of the sediment. In some cases, jointing of the rock may result from the action of the same forces that cause folds and faults. In relatively undisturbed sedimentary rocks, such joints are often in two vertical sets perpendicular to one another. Commonly, streams develop along zones of weakness caused by joints in rocks, and thus the regional pattern of joint orientation often exerts a strong control on the development of drainage patterns.
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