264 PHYSICAL GEOLOGY 



Functions of lakes. The geological functions of lakes are 

 several in number. (1) They tend to increase the rainfall and 

 to equalize the temperature of their surroundings, making the 

 summers cooler and the winters warmer than they would be 

 otherwise. Since the character of the climate affects various 

 geological processes, this tendency is not unimportant. (2) 

 Mechanical and chemical deposits, discussed further below, 

 are being made in lakes. Although but a few lakes are of 

 great area (some ten only exceed 10,000 square miles in extent), 

 yet the aggregate area of lake sedimentation is very large. 

 (3) Lakes filter the waters of their tributary streams and regu- 

 late the volume of outflowing streams, preventing, or tending 

 to prevent, destructive floods. Thus they influence erosion 

 throughout the areas affected by the streams which flow from 

 them. (4) Many kinds of plants and animals dwell in lakes, 

 and the bodies of land-inhabiting animals are often washed in by 

 streams. So far as these are capable of preservation, they may 

 be converted into fossils in the growing lake sediments. Thus 

 the lake deposits of former ages often afford a valuable record 

 of the lake and land life of the times when the lakes existed. 



Processes in operation in lakes. The changes in progress 

 in lakes correspond closely to those taking place in the ocean, 

 and discussed in preceding pages. (1) Winds generate waves 

 more easily in fresh than in salt water, because the former is 

 lighter, and when lakes are sufficiently large and deep, strong 

 storm waves develop. Where they wear the shore, cliffs 

 and terraces are formed (Figs. 282 and 283). The material 

 worn from the cliffs is swept by the undertow into deeper 

 water, or transported alongshore by waves and wind-driven 

 currents and built into beaches, spits, bars, etc. In the pro- 

 cess the shore drift is assorted and worn. The general effect of 

 the work of waves and currents is to increase the area of 

 the lakes, but at the same time to make them shallower. 

 (2) Streams and rains wash material into lakes from the tribu- 

 tary slopes. In many cases the sediment brought in by rivers 

 accumulates at their mouths to form deltas. Lake deltas built 



