CHAPTER VII 

 LAKES 



Many of the phenomena of the ocean are repeated on a smaller 

 scale in lakes. The waves of lakes and their attendant undertows 

 and littoral currents are governed by the same laws and do the 

 same sort of work as the corresponding movements of the ocean. 

 Tides are insignificant; but slight oscillations of level, known as 

 seiches, have been observed in many lakes. They are probably 

 caused by sudden changes in atmospheric pressure. Currents 

 corresponding to those of the ocean are slight or wanting in lakes, 

 but since most lakes have inlets and outlets, their waters are in 

 constant movement toward the latter. In most cases this move- 

 ment is too slow to be noted readily, or to do effective work either 

 in corrasion or transportation. The work of ice is relatively more 

 important in lakes than in the sea. 



Changes taking place in lakes. The processes in operation in 

 lakes are easily observed and readily understood, (i) The waves 

 wear the shores, and the material thus derived is transported, 

 assorted, and deposited as in the sea, and all the topographic forms 

 resulting from erosion or deposition along the seacoast are repro- 

 duced on their appropriate scale in lakes. (2) Streams bear their 

 burden of gravel, sand, and mud into lakes and leave it there. 

 (3) The winds blow dust and sand into them, and in some places pile 

 the sand up into dunes along their shores. (4) Animals of various 

 sorts live in lakes, and their shells and bones give rise to deposits 

 comparable to animal deposits in the sea. (5) Numerous plants 

 grow in the shallow water about the borders of many ponds and 

 lakes, and as they die, their substance accumulates on the bottom. 

 (6) The outlets of lakes which have outlets are constantly lowered 

 by the outflow. The lowering is generally slow if the rock is coher- 

 ent, for the outflowing water is usually clear, and therefore inefficient 

 in corrasive work. These six processes (except the last, which does 

 not apply to lakes without outlets) are essentially universal, and all 

 conspire against the perpetuity of the lakes. (7) In lakes where the 



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