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Chapter VII — THE WORK OF THE OCEAN | Index | Chapter IX — THE MOVEMENTS AND DEFORMATIONS OF THE EARTH'S BODY (DIASTROPHISM) |
[p. 332]
Many of the phenomena of the ocean are repeated on a smaller scale in lakes. The waves of lakes and their attendant undertows and littoral currents are governed by the same laws and do the same sort of work as the corresponding movements of the ocean. Tides are absent, or insignificant; but slight oscillations of level, known as seiches,[1] have been observed in many lakes. They are probably caused, in most cases, by sudden changes in atmospheric pressure. While they are generally very slight, they sometimes amount to as much as a foot, and occasionally to several feet. The seiches are oscillatory movements, and their period is influenced by the size and depth of the lake. They have been studied most carefully in Switzerland. Currents corresponding to those of the ocean are slight or wanting in lakes, but since most lakes have inlets and outlets, their waters are in constant movement toward the latter. In most cases this movement is too slow to be readily noted, or to do effective work either in corrasion or transportation. The work of ice is relatively more important in lakes than in the sea.
Changes taking place in lakes. The processes in operation in lakes are easily observed and readily understood. (1) The waves wear the shores, and the material thus derived is transported, assorted, and deposited as in the sea, and all the topographic forms resulting from erosion or deposition along the seacoast are reproduced on their appropriate scale in lakes. (2) Streams bear their burden of gravel, sand, and mud into lakes and leave it there. (3) The winds blow dust and sand into the lakes, and in some places pile the sand up into dunes along the shores. (4) Animals of [p. 333] various sorts live in the lakes, and their shells and bones give rise to deposits comparable to the animal deposits in the sea. (5) Abundant plants grow in the shallow water about the borders of many ponds and lakes, and as they die, their substance accumulates on the bottom. (6) At the outlet, the water is constantly lowering its channel. The lowering of the outlet is often slow, especially if the rock is coherent, for the outflowing water is usually clear, and therefore inefficient in corrasive work. These six processes (except the last, which does not apply to lakes without outlets) are essentially universal, and all conspire against the perpetuity of the lakes. (7) In lakes wnere the temperature is low enough for ice to be formed, it crowds on the shores and develops phenomena peculiar to itself (Figs. 183-4). The ice of the sea may work in similar ways, but its work is restricted to high latitudes. (8) In lakes in arid regions, deposits are often made by precipitation from solution.
The first five and the last of these processes are filling the basins of the lakes. As sediment is deposited in a lake, a corresponding volume of water is displaced, and forced out of the basin if the lake has an outlet. The sixth process is equally antagonistic to lakes, while the seventh has little influence on their permanence. Given time enough, these processes must bring the history of any lake to an end. The lowering of the outlet alone will accomplish this result if the bottom of the basin is above base-level. Many lakes have already become extinct, either through the filling or draining of their basins, or through both combined. It does not follow, however, that lakes will ever cease to exist, for the causes which produce them may be in operation contemporaneously with those which bring lakes now in existence to an end.
Lacustrine deposits. The beds of sediment deposited in lakes are similar in kind, in structure, and in disposition, to beds of sediment laid down in the sea; but river-borne sediment is more commonly concentrated into deltas, since waves and shore-currents are less effective in lakes than in the sea. Even the limestone made in the sea has its correlative in limestone made in some lakes. Some of it was made of the shells of fresh-water animals which throve where the in-wash of terrigenous sediment was slight, some [p. 334] of it from the calcareous secretions of plants,[2] and some of it was precipitated from solution.2 While still soft, such deposits are called mar[3]. Salt and iron-ore deposits are also sometimes made in lakes.
Extinct lakes. The former existence of lakes where none now exist may be known in various ways. If a lake basin was filled, its former area is a flat which bears evidence of its origin in its composition, its structure, and often in its fossils. Such a flat is commonly so situated topographically that the basin would be reproduced if the lacustrine deposits were removed. To this general rule there are exceptions, as where a glacier formed one side of the basin when it was filled. If, on the other hand, the lake was destroyed by the lowering of its outlet, or by the removal of some barrier such as glacier ice, or by desiccation, shore phenomena, such as beaches, terraces (Fig. 269), spits, etc., may be found, even though there is no well developed flat corresponding to the bed of the lake. In time, such features are destroyed by subaerial erosion, so that they are most distinct soon after a lake becomes extinct.
[p. 335]
Many lakes, some of them large [4] and many of them small, are known to have become extinct, while many others are now in their last stages, namely, marshes. Many others have been greatly reduced in size. Such reductions are often obvious where deltas are built into lakes. Thus the delta built by the Rhone into Lake Geneva is several miles in length, and has been lengthened nearly two miles since the time of the Roman occupation. The end of Seneca (N. Y.) Lake (PL XXI) has been crowded northward some two miles by deposition at its head. Similar changes have taken place and are now in progress in many other lakes.
Salt lakes. A few lakes, especially in arid or semi-arid regions, are salt, and others are “bitter.” Beside common salt, salt lakes usually contain magnesium chloride, and magnesium and calcium sulphates, as well as some other mineral substances. “Bitter” lakes usually contain sodium carbonate, as well as sodium chloride and sulphate, and sometimes borax. The degrees of saltness and bitterness vary from freshness on the one hand to saturation on the other. The water of the Caspian Sea (lake) contains, on the [p. 336] average, less salt than that of the sea; that of Great Salt Lake contains about 18%; that of the Dead Sea, about 24%; and that of Lake Van, in eastern Turkestan, the densest body of water known, about 33%.
Many salt lakes, such as the Dead Sea and Great Salt Lake, are descended from lakes which were fresh, while others, like the Caspian Sea, are probably isolated portions of the ocean. Lakes of the former class have usually become salt through a decrease in the humidity of the region where they occur. The water begins to be salty when the aridity is such that evaporation from the lake exceeds its inflow. The inflowing waters bring in small amounts of saline and alkaline matter, which is concentrated as evaporation takes place. The concentration may go on until saturation is reached, or until chemical reactions cause precipitation.
Deposits of salt and other mineral matters once in solution are making in some salt lakes at the present time, and considerable formations of the same sort have been so made in the past. Buried beneath sediments of other sorts, beds of common salt or of other precipitates are preserved for ages. Lime carbonate has been precipitated in quantity from some extinct lakes (Fig. 271).
Lakes which originate by the isolation of portions of the sea are salt at the outset. If inflow exceeds evaporation, they become [p. 337] less and less salty, and may ultimately become fresh; otherwise they remain salt. If evaporation exceeds inflow they diminish in size and their waters become more and more salt or bitter.
Indirect effects of lakes. Lakes tend to modify the climate of the region where they occur, both by increasing its humidity and by decreasing its range of temperature. They act as reservoirs for surface-waters, and so tend to restrain floods and to promote regularity of stream flow. They purify the waters which enter them by allowing their sediments to settle, and so influence the work and the life of the waters below.
Origin of lake basins.[5] Lake basins arise in many ways, some of which have been pointed out on preceding pages. Most of them arise through processes of gradation. Some are formed by rivers (p. 185), some by waves and shore currents (p. 313), some by glacial erosion, some by glacial deposition (p. 268), and some by a combination of glacial erosion and glacial deposition. Others are formed by volcanic action, as when a lava flow dams a valley, or when a volcano leaves its cone with a depressed crater. Still others are formed by warpings of the earth’s surface, and a few in other ways.
Chapter VII — THE WORK OF THE OCEAN | Index | Chapter IX — THE MOVEMENTS AND DEFORMATIONS OF THE EARTH'S BODY (DIASTROPHISM) |
Forel, Compte Rendu, 1875, 1876, 1878, 1879, and P. DuBois, 1891. Also Forel’s Lac Leman. ↩︎
C. A. Davis, Jour, of Geol., Vol. VIII, pp. 485-497, and 498-503, and Vol. IX, pp. 491-506. ↩︎
Russell, Lake Lahontan, Mono. XI, U. S. Geol. Surv., Chap. V; also Third Ann. Rept., pp. 211-221. Gilbert, Lake Bonneville, Mono. I, U. S. Geol. Surv., p. 167. ↩︎
Gilbert, Lake Bonneville, Mono. I, U. S. Geol. Surv.; Russell, Lake Lahontan, Mono. XI, U. S. Geol. Surv.; and Mono Lake, Eighth Ann. Rept., U. S. Geol. Surv., Pt. I; Upham, Lake Agassiz, Mono. XXV, U. S. Geol. Surv.; Salisbury and Kiimmel, Lake Passaic, Rept. of the State Geologist of N. J., 1893, and Jour, of Geol., Vol. Ill, pp. 533-560. ↩︎
Salisbury’s Physiography, larger edition, p. 303. ↩︎