The mechanical sediment which accumulates in a lake basin is of two kinds, (1) that which is brought in by tributary streams, and (2) that which the lake itself acquires by cutting back its shores; of these the former is much the greater in volume. Almost without exception, rivers entering lakes form deltas, which spread out fan-like from the stream mouths, and, if sufficiently numerous, may fringe the entire lake shore with delta deposits. Part of the materials will be distributed by waves and currents, but the coarser material remains to form the foreset beds, the inclination of which depends upon the depth of the lake at the stream mouth and upon the coarseness of the debris. If deposited in deep water, the beds may be inclined at a considerable angle; if in shallow water, they form a very gradual slope. In small lakes the coalescence of deltas, or the advance of a single one, will eventually fill up the basin, forming first swamps and then smooth, grassy meadows, through which flow the streams, keeping their own channels clear. Such filled-up lakes are common in many mountain valleys.
In large lakes the process is, of course, much slower.
Fig. 101. - Gravel beach, Lake Ontario. (U. S. G. S).
Away from the deltas the combined action of the waves and currents fringes the lake with coarse deposits of boulders, gravel, and sand, which form the beach, the sand extending some distance out into shallow water. In large lakes the heavy surf cuts a terrace on the shore and the debris thus obtained builds out the terrace, which is therefore said to be "cut and built." A succession of terraces indicates the various levels of the water, for lakes are often subject to great fluctuations of level. The finer materials are carried out into deeper water and deposited in successive layers over the whole lake bottom, the finest materials in the centre. The coarse and fine sediments grade into each other dovetail and overlap, because in heavy storms, or when the streams are in flood, the coarser sediments are carried farther out and deposited on the fine, and these changes of material in any given vertical section, not too far from shore, may be often repeated. Special lines of accumulation for the coarse substances also occur in the form of shoals, spits, embankments, and the like. If the lake is subject to fluctuations of its level, with the water much higher at one time than another, even more wide-spread changes in the character of the deposits will occur.
The deposits now forming in the great Laurentian lakes, which-occupy a relatively small drainage basin and receive no large tributaries, are principally blue muds and clays, partly made up of kaolirtite and partly of the debris of other minerals in an extremely fine state of subdivision, but not decomposed chemically. In Lake Superior the clay has generally a pinkish tinge. Owing to the way in which the materials are arranged, lake deposits betray the form of the basin in which they were laid down. .Around the old shore line are masses of coarse materials, with deltas interspersed, to mark the mouths of streams, while towards the middle of the basin, quantities of fine mud and clay have accumulated. An excellent example of such a deserted lake basin is that known as Lake Bonneville in Utah, of which Salt Lake is the shrunken remnant. The drying up of this lake, which was once fresh and had an outlet northward to the Snake River, is an event geologically so recent, that its form and size, its shores and islands, its high and low stages, in short, its history, can be made out with great clearness, as has been admirably done by Mr. Gilbert of the United States Geological Survey. At its time of greatest extension, Lake Bonneville had an area of 19,750 square miles and a maximum depth of 1050 feet, while Salt Lake (which is variable) had in 1869 an area of 2170 miles and an extreme depth of 46 feet.
Around the ancient shores are beautifully preserved the terraces, embankments, and deltas of the various stages of water, with the gravels and sands appropriate to the shallow water. The principal part of the basin is a level plain, filled to a great but unknown depth with beds of clay and marl (Fig. 104). In still more ancient lakes the terraces, embankments, and other shore features have been swept away by the processes of denudation, but the outline of the lake may frequently be reconstructed from the character of the deposits.
Fig. 102. - Outlet of Lake Bonneville, Utah. (U. S. G. S).
Fig. 103. - Terraces of Lake Bonneville, Utah. (U. S. G. S).
Chemical Deposits are not common, nor of much importance in fresh lakes. In a few, chemically precipitated carbonate of lime is found, and more abundant is limonite (2 Fe203,3 H20). This is carried into the lake by streams that contain dissolved ferrous carbonate (FeC03), which, becoming oxidized and hydrated, is no longer soluble, and accumulates on the bottom. In Sweden ores of this kind are dredged out of the lakes and employed as a source of iron.