202 LAKES 



temperature is low enough for ice to be formed, it crowds on the 

 shores and develops phenomena peculiar to itself (Figs. 126-127). 

 (8) In some lakes in arid regions, deposits are made by precipitation 

 from solution. 



Several of these processes are filling the basins of lakes, and 

 as sediment is deposited in a lake, a corresponding volume of water is 

 forced out if the lake has an outlet. The sixth process also is antag- 

 onistic to lakes. 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 already have 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 operate contemporaneously with those 

 which tend to destroy lakes' now in existence. 



Lacustrine deposits. Beds of sediment deposited in lakes are 

 similar in kind, structure, and disposition, to beds of sediment laid 

 down in the sea; but in lakes river-borne sediment is more com- 

 monly concentrated into deltas, since waves, tides, and shore-cur- 

 rents are less effective than in the sea. Even the limestone of the 

 sea has its counterpart 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 of it from the calcareous secre- 

 tions of plants, 1 and some of it was precipitated from solution. 2 

 While still soft, such deposits are called marl. Salt deposits also 

 are made in some lakes, and iron-ore in some marshy ones. 



Extinct lakes. The former presence of lakes where none now 

 exist is known in various ways. If a lake basin was filled, its former 

 area is a flat, the material of which bears evidence of its origin in its 

 composition, its structure, and in its fossils. Such a flat com- 

 monly is so situated topographically that the basin would be repro- 

 duced if the 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 the lake was destroyed by the lowering of its out- 

 let, or by the removal of some barrier such as glacier ice, or by 

 desiccation, shore phenomena, such as beaches, terraces (Fig. 202), 

 spits, etc., may exist, even though there is no well developed flat 



1 C. A. Davis, Jour, of Geol., Vol. VIII and Vol. IX. 



2 Russell, Mono. XI, U. S. Geol. Surv., Chap. V; also Third Ann. Kept., pp. 

 211-221. Gilbert, Mono. I, U. S. Geol. Surv., p. 167. 



