Aside from the bedding planes, the sedimentary rocks, as well as the other groups, are crossed by joint planes, which, with the bedding planes, cause the rock to break naturally into rhombic or cubical blocks, greatly aiding in quarrying operations. Most of the jointing in sedimentary strata, and much of that in the igneous and meta morphic rocks, seems to be due to disturbances in the rock, which cause strains. Under violent strains the rocks are often folded and faulted, especially among mountains. This folding is are even overturned or recumbent. Under able conditions the rock under strain breaks in place of bending, forming faults. Some of the faults are dislocations of only a few inches, some of thousands of feet. Ordinarily the plane of faulting is approximately vertical, but in some cases, as when folds are overturned and the fold ing continues to the point of breakage, faults are developed with nearly horizontal planes. Such faults are called overthrust faults, and the plane a thrust plane, because the rocks on the upper side are thrust over those on the lower side.
Ordinarily the sedimentary strata are hori zontally deposited in the sea; and when lifted to form a part of the land they are usually still approximately horizontal. With mountain dis turbance. however, the rock layers are thrown into inclined positions. In plains and plateaus, on the other hand, the rocks are prevailingly horizontal and there is little disturbance. Owing to the volcanic activity accompanying mountain formation, and to the great pressure under which the strata are placed in folding, both volcanic and metamorphic rocks are common among mountains, but are rarely found in plains. By reason of the instability of the earth's crust, land is often lowered below sea-level subsequent to a period of denudation. Then sedimentary de
posits are laid down on the submerged surface, after which the area may be raised once more into land. The plane between the new deposit and the old land marks an unconformity, and the upper rocks are said to rest unconformably upon the lower. An unconformity thus represents a gap in rock formation and in the life record, and irregularities; it may have lakes, because there has not been time enough for the rivers to fill them; and its tributaries are liable to be few and its divides poorly developed, for want of time. A mature stream has lost these character istics. It has many tributaries and well-defined divides, but no waterfalls, excepting possibly in the head-water regions. Lakes are absent, and the valley is broad and its side slopes mod erate. This is the normal development; but acci dents may occur to interfere with this develop ment. For example, lava floods may cross the valley or fill it; glacial deposits may be laid down to embarrass the stream; and the land may be raised qr depressed. A mountain or a plain, or any other land feature, when newly formed, and hence young, will have, therefore, certain characteristics; but with increasing age these will be changed. For example, drainage, at first vigorous, will dissect the land form, making it more irregular. A plain may then become a hilly region, and a mountain chain will become very rugged. Weathering and erosion will later reduce the irregularities, causing the mountain to become more level and the plain once more to approach a level condition. See PHYSIOGRAPHY.