EARTH MOVEMENTS The study of earth movements in general forms one of the most important and interesting branches of geology ; but for its proper comprehension it needs some acquaintance with certain features of geological structure—the results of disturbances of the rocks—so that here it is necessary to begin by arguing in a circle. We can to some extent get over this difficulty by starting with large-scale earth-movements that do not involve small differential movements. Certain necessary definitions can then be introduced as required.
Relative Nature of Movements.—Many of the facts of geology show that the relative levels of land and sea have changed. Strata of marine origin are now found forming part of very high moun tains (e.g., at i6,000 ft. in the Himalayas), and historical geology shows that alternations of submergence and emergence have been frequent in many areas. In the British Isles at least three marine periods and four continental ones can be traced (see STRATI GRAPHY, below). It is a moot point whether the middles of con tinents and the abysmal depths of the oceans have ever changed places, but it is clear that the margins of continents and certain epicontinental areas have been drowned and have emerged many times. In studies of this kind there is always the difficulty that we have no fixed datum line ; we cannot measure the distance of any given point from the centre of the earth, and so we cannot tell in any case whether the land rose or the sea sank. All such movements have to be treated as relative; hence we speak of emergence and submergence, not of rising and sinking. These relative movements are exemplified on a smaller scale by raised beaches and drowned valleys (see above) ; and in such cases there need not necessarily have been any horizontal component in the movement ; mere rising or sinking of the land would produce all the effects, and a possible cause is contained in the doctrine of isostasy (q.v.).
It has been suggested by American geologists that an actual lowering of the sea-level, simulating a rise of the land, might be brought about during a glacial period, by the locking up on a continent of a vast mass of water as snow and ice, since all pre cipitation is ultimately derived from the sea. This seems very probable and has been applied to explain some actual examples of old sea-beaches and wave-cut platforms. It is evident that during upward or downward movements of the land, the earth's crust might be fractured along the edges of the moving blocks, thus giving rise to faults (q.v.) but much more complicated and, there fore, more difficult to explain are those movements that are wholly or in part horizontal, for these may give rise either to compression or stretching of the crust, usually with local frac ture and relative displacement of the fractured blocks. Under the influence of such forces the earth's crust often behaves as if it were plastic, so that originally flat and horizontal strata are oc casionally crumpled into the most fantastic forms, some instances worked out in detail in great mountain chains, such as the Alps, or even occurring in hand-specimens of rocks only a few inches square, almost passing the bounds of belief. It is probable that extreme crumpling of this kind (for detailed consideration of which see FOLD), mainly occurs in depth under a heavy load of rock, whereas near and at the surface fracture is the dominant feature.