LAKE. 701 LAKE. and there by undermining, and little basins or siiik-lioles are formed toward wliieli the water drains. If the hole in the centre becomes filled, ponds are caused. Far the most common cause for lakes is some accident to a stream, so interfering with its normal development as locally to transform its valley lo .i hasin. An avalanche across a river dams back a lake, and the growth of a moun- tain harrier makes a still greater dam. The war|)ing of valleys during mountain growth also makes basins. Lake Geneva in the Alps has been ascril)pd to this origin. There are basins where the rocks across a valley have l)een faulted — for example, in southern Oregon, in Ireland, and in the ease of the Dead !Sea. Wiien the land has subsided and the sea entered the mouths of river valleys, the building of bars across the drowned valleys often shuts in the water, forming lakes. These may be made salt by the occasional overllows of the sea, or they m.ay be completely disconnected from the sea. Such lak^s are illustrated by the shut-in bays on the south shore of the Great Lakes. Sinking of parts of the land during earthquake shocks forms basins, as in the 'sunk' country of Arkan- sas, in the Mississippi Valley, that was shaken by the eartlujuake of 1812. Lava dams hold back river water, as is illustrated in the -Vuvergne region of Central France ; by Snag Lake, near Mount Shasta ; by the Lake of Tiberias, in the .Jordan Valley; and by many other lakes in volcanic regions. After the vol- canic energj- has subsided, volcanic craters are occupied by lakes, as in the Eifel region of Germany, Lake Xemi. near Rome: .verno, near Naples; and many other places. Such lakes are especially large and deep when the cra- ter bottom has subsided, as in Crater Lake, Ore- gon. But perhaps the most important single cause for lakes is the glacial accident. By moraine dams and by dams of other classes of glacial deposits a vast number of lakes in Xortheastern .merica and Northwestern Europe have been formed. Without doubt the number of glacial lakes and ponds in Northeastern America and Northwest- ern Europe is several hundred thousand. There are eslimaled to be lO.Odf) lakes in Minnesota alone, due in one way or another to the glacier. In consequence of the interference of drainage by the glacial acciilent, glaciated regions are cliaracterize<l by an abundance of lakes, while unglaeiated regions have relatively few. In addi- tion to the deposit of materials forming a dam across stream valleys, glaciers have scoured out many, basins, known as rock basins. Seneca and Cayuga Lake valleys in central New York have been thus formed, the (ireat Lakes owe at least a part of their depth to this cause, and the same is true of some of the Alpine lakes, notably the Italian lakes Como, Lugano, and ^laggiore. Many lakes are the result of a combination of causes. For example, the Great Lakes are evi- dently in old river valleys, deepened to some ex- tent by glacial erosion, further deepened by a warping of the earth's crust, and with their depth still further increased by dams of drift in the preexisting valleys. The Alpine lakes also seem to combine valley warping, glacier erosion, and glacial-drift dams among their causes. Destruction of Lakks. Lakes are normally of brief duration, from the standpoint of geo- logical time. Consequently lakes are mostly of recent origin, and are especially abundant in re- gions where .some recent accident has hap[)ened to drainage; as, for example, where glaciers have been, or where lava Hows have recently overspread areas of country. Since lakes act as catchment basins for sediment, they are soon filled by the contributions brought by the incom- ing streams, by rain-wash, by wimls, and by waves. Much of the liner sediment settles in the lake at a distance from the shore ; but most of the coarser material accumulates near the shore, and especially in the river, deltas. These grow out into the lake, forming Hats at the head of the lakes and protruding deltas on the mar- gin. In some cases, where the sediment supply is abundant, as in the Swiss lakes, which receive glacier-fed streams, the growth of deltas from opposite sides of the lake has cut a single lake in two parts. This is dearly illustrated in the case of lakes Thun and Brienz, which are divided by the delta deposit on which Interlaken is situ- ated. As lakes are shallowed by sediment deposit, organic contributions help linally to fill them. Various forms of vegetation, including lilies, reeds, rushes, cane, and sphagnum, are very effective in this last stage of lake destruction. When finally filled the lake becomes a swampy ])lain ; the rivers then build the plain up into dry land in their establishment of a slope or grade across the plain. Lakes are not always destroyed soleh- by fill- ing. The outlet stream is always eroding at the barrier, though this work is usually slow, because the lake water has filtered out the sedi- ment, so that the outlet is robbed of its cutting tools. Thus Niagara, in (lowing from Lake Erie, has cut away but little more of the barrier than the loose soil, and consequently (lows practically on the surface of the i)lain. Where the outlet passes through unconsolidated material it may rapidly lower the lake-level. If a condition like that at Niagara could exist near a lake, the eating back of the falls would in time reach the lake and rapidly drain it. This will not happen in the case of Niagara, because the layer which causes the cataract dips toward the south, and the fall will disappear before Lake Erie is reached. A lake may also be destroyed by a change of climate. Thus a series of lakes existed in the great basin of Western United States during the Glacial period, when the climate was cooler and damper. (See L.VKE Boxxeville: Lake Lajiox- TAX. ) During the Glacial period large lakes also existed along the margin of the ice wherever the glacier formed a dam across north-flowing stream valleys. An enormous lake of this origin ex- isted in the valley of the Red River of the North, to which the name Lake Agassiz (q.v. ) has been given. Similar lakes appeared In the basin of the Great Lakes, while the Saint Law- rence Valley was ice-filled. The shore-lines of these glacial lakes are plainly seen along the mar- gin of the Great Lakes, and record a very com- plex history, with various outflows Avhich were made available as the ice-front melted farther and farther back. The deposits made in these extinct lakes form much of the soil of the land along the southern margin of the Great Lakes. Such methods of lake destruction are abnormal ; the natural and usual method is filling, com-
