LAKES AND SHORES 147 



into lakes always make the lakes smaller. 2. Waves of lakes cut 

 into their shores at some points most of the time, and the larger 

 part of the material worn from the land is deposited in the lake. 

 This makes the average depth of the water less. While rivers and 

 waves are doing more than other agents to fill lake basins, filling 

 goes on slowly in other ways. 3. Numerous shell-bearing animals 

 live in lakes, and when they die their shells are left on the bottom. 

 4. Plants grow in lakes, especially in the shallow water about their 

 shores, and when they die they fall into the water. 5. Winds 

 blow dust and sand into lakes. In these various ways, all lake 

 basins would be filled in time, and the lakes would then cease to 

 exist. 



The water flowing out of a lake cuts down the level of its outlet, 

 and as this is lowered the depth of the basin below the outlet 

 becomes less. A lake may be destroyed by the lowering of its 

 outlet if its bottom is above base-level; but a lake whose bottom 

 is below base-level can never be destroyed by the lowering of its 

 outlet. In such cases, filling and cutting may do what cutting 

 alone could not. As a result of the cutting down of their outlets 

 and of the filling of their basins, all existing lakes must finally 

 become extinct. New ones may be formed, however, as old ones 

 are destroyed. 



Lakes are occasionally destroyed by drying up. This sometimes 

 results from a change of climate, but it may also result from the 

 turning away of inflowing waters. 



Extinct lakes. Many lakes have become extinct, but their 

 sites may be recognized by various features. If a lake became 

 extinct by having its basin filled, its former position is marked by 

 a flat covered with sediment of the sort deposited in lakes. Such 

 a flat is a lacustrine plain. Lacustrine plains occur in mountains, 

 on plateaus, or on plains of a larger type. If a lake became extinct 

 by the lowering of its outlet or by evaporation, its former bed 

 might not be flat. 



The former borders of an extinct lake are often marked by 

 deltas, terraces, beaches, shore-cliffs, etc., (p. 154). All features 

 developed by lakes about their shores will be destroyed sooner or 

 later by the agents of erosion. 



