Many factors tend to modify the apparent simplicity of the above history. When hard rocks overlie soft ones, the latter frequently wear back faster, causing an undermining of the hard projecting layer. Many cliffs are of this origin. When such a cliff lies athwart a river course a fall or rapid results which re treats upstream by undermining. A good illus tration of this is Niagara, which is retreating about four feet a year. In arid regions where there is little precipitation erosion may go on very slowly. What rainfall there is, however, frequently comes in sudden storms and, as there is no protecting vegetation, may be very effective. (See Daseaws). Where the rocks are fladying, or homogeneous, the familiar branching treelike or dendritic pattern of drainage results. Tilted layers of hard and soft rock frequently result in long parallel ridges and valleys, a, pattern of drainage known as latticed or trellised. Streams working. on the softer layers of rock have an advantage in that they cut faster and ultimately capture their less fortunate neighbors. (See STREAM PIRACY). Thus there is brought about what is known as structural adjustment or the close control of drainage by rock structures. Changes of level may take place at any stage of the cycle. A base-levelled region when uplifted enters upon a second youth, is said to be reju venated and may again pass through another cycle. Gradual depression of a region, on the other hand, tends to slow up erosion or even to inaugurate desposition.
The fine material furnished in the course of the erosion cycle by weathering and by the constant attrition of the stream itself is car ried away by the running water, most of it to ultimately find its way to the ocean. Soluble carbonates, sulphates and chlorides are carried in solution. Calcium carbonate (limestone) is abundant in most river waters. Very finely divided clay or sand is carried in suspension and causes the turbidity of river water. Coarser sand and gravel are rolled along the bottom. The total amount of sediment carried by the rivers of the United States in a year is greater than the total tonnage of all our rail roads. A most significant factor in transpor tation by running water is the sorting which the sediment undergoes. Because it is more easily carried, the finer material is borne away, the coarse left behind. If a residual soil from a granite consists of a mixture of sand and clay (see section on chemical work of the atmosphere), running water tends to sort the finer clay from the coarser sand and ultimately clay and sand come to rest in widely separated areas. Alternations of shales (consolidated clay) and sandstone (consolidated sand) in the rocks of any region are a resultant of the sort ing power of moving water.
Where rivers emerge from mountainous regions and flow across plains the velocity is suddenly checked and deposits are built up, which, if steep, are known as alluvial cones, if low and flat, as alluvial fans. Alluvial fans
are common at the base of practically all moun tain ranges. Where several adjacent fans coalesce, the sheet of debris is spoken of as a piedmont alluvial plain, or sometimes as a mountain apron. Intermontane valleys in arid regions frequently fill to great depths by this process, because, since there is no drainage to the sea, all the material brought down by water from the melting snows is perforce lodged in the adjacent valleys. In the lower courses of many streams, as they pass from maturity into old age, the valleys become wide flood plains over which streams meander slug ishly and the velocity may not be enough to carry off the sediment carried down from the more vigorous upper reaches. Consequently broad sheets of fine sediment may be laid down over these plains. At their seaward edge these deposits grade into deltas, built by rivers in lakes or the ocean where the velocity is checked by quiet water. The delta plains of large rivers are low and swampy and like flood plains subject to frequent overflows. The soil is usually fine since the velocity of the stream in its lower reaches is not sufficient to trans port coarse material. Flood plains and deltas, when they can be properly drained and pro tected against flood, are usually rich asricul tural areas. See DELTA; FLOOD PLAIN ; PHYSIOGRAPHY' RIVERS; TERRACE; etc.
Work of Glaciers.— Wherever more snow falls than melts perpetual snow, fields occur and the snow may gradually be compacted till it becomes granular neve, as it is called, and ultimately 'pass into ice. If the accnmulation becomes sufficiently thick it may begin a slow outward creep from the point of origin and become a glacier. In mountains these bodies of ice often occupy valleys and are termed valley or alpine glaciers. In polar regions they frequently spread over vast areas and are then spoken of as ice caps or continental glaciers.
See GLACIER.
One school of glaciologists holds that glaciers erode powerfully carving out deep valleys and removing great masses of sediment; another believes that glacial modification of topography is not profound. Be that as it may, it is a well-known fact that the appearance of a region is much changed by the passage over it of a great ice sheet. As the ice, into the base of which are frozen sand and bowl ders, scours over the rocks it scratches long parallel marks known as striae which reveal the direction of movement. Rock hills and ledges (roches moutonnees) are worn smooth with the more gentle slope in the direction from which the ice came. Projecting spurs and promontories are truncated and valley bot toms rounded into characteristic U-shapes. By deepening and widening trunk valleys tribu taries are left hanging high above the floor of the main stream, resulting in discordant junctions or hanging valleys. By freezing to the walls of the valley heads and plucking away the loosened bowlders, glaciers carve amphitheatre-like bowls, known as cirques.