Many lakes, some of them large[188] 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 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.
Lake ice.[189]—Since fresh water is densest at 39° Fahr., ice does not commonly form on the surface until the temperature from top to bottom is reduced to this point. Cooled below this temperature, the surface-water fails to sink, and with sufficient reduction freezes. If the lake be small, and especially if it be shallow, it is likely to freeze over completely in any region where the temperature is notably below the freezing-point for fresh water for any considerable period of time. It is under these circumstances that the ice becomes most effective.
Fig. 331.—Ice crowding upon low shore. Clear Lake, Ia. (Calvin.)
Suppose a lake in temperate latitudes, where the range of temperature is considerable, to be frozen over when the temperature is 20° Fahr. If now the temperature be suddenly lowered to −10°, and such change of temperature is not uncommon in the northern part of the United States, the ice contracts notably. In contracting, it either pulls away from the shores or cracks. If the former, the water from which the ice is withdrawn quickly freezes; if the latter, water rises in the cracks and freezes there. In either case, the ice-cover of the lake is again complete. If the temperature now rises to 20° the ice expands. The cover is now too large for the lake, and it must either crowd up on the shores ([Fig. 331]) or arch up (wrinkle) elsewhere. It follows the one course or the other, or both, according to the resistance offered by the shore.
If the water near the shore is very shallow, the ice freezes to the sand, gravel, and bowlders at the bottom. If the adjacent land is low, the ice in expanding may shove up over it, carrying the débris frozen in its bottom. It may even push up loose gravel and sand in front of its edge if they be present on the shore. Where bowlders are frozen to the bottom of the ice, the shoreward thrust in expanding has the effect of shifting them in the same direction, and even of lifting them a little above the normal water-level. This constant process of concentrating bowlders at the shore-line gives rise to the “walled” lakes, which are not uncommon in the northern part of the United States. The “wall” does not commonly extend entirely around a lake, though it exists at various points on the shores of many lakes. In making the walls, the ice shoved up by winds, especially in the spring when the ice is breaking up, coöperates.
Fig. 333.—Calcareous tufa domes. Pyramid Lake, Nev. (Russell.)
If the lake be bordered by a low marsh, the ice and frozen earth of the latter are really continuous with the ice of the lake, and the push of the latter sometimes arches up the former into distinct anticlines, the frozen part only being involved in the deformation. A succession of colder and less cold periods may give rise to a succession of such anticlines.[190] If the shore be steep and of non-resistant material, the crowding of the ice produces different but not less striking results. Where the thrust of the ice is against a low cliff of yielding material, such as clay, it disturbs all above the shore-line. Where the cliff is sufficiently resistant, it withstands the push of the ice, and the ice itself is warped and broken.
Saline lakes.—A few lakes, especially in arid or semi-arid regions, are salt, and others are “bitter.” Beside sodium chloride, salt lakes usually contain magnesium chloride, and magnesium and calcium sulphates. “Bitter” lakes usually contain much sodium carbonate, as well as some 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 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 (eastern Turkestan), the densest body of water known, about 33%. See accompanying table.