RECORDS MA PL BY R1\'LRS AND LAKLS 



rapidly and build a layer of light-colored sediments. Then in 

 the winter time when ice covers the lake or the land is so 

 frozen that little or no sediment is washed from it, the very 

 fine particles of silt and minute organic matter, which re- 

 mained in suspension in the water all summer, gradually 

 settle to the bottom in the cold quiet water and build a thinner 

 layer of dark-colored sediments on top of the thicker summer 

 layer. Such seasonal layers of alternating fine dark -colored 

 and coarser light-colored clays are known as varves. It is 

 evident that if a section cut through layers of varved clays 

 to the original bed of a lake can be examined, the number of 

 seasons the lake lived can be determined, just as the age of a 

 tree is determined by the seasonal rings of wood that it ac- 

 quires. If a lake exists long enough all depressions of its bot- 

 tom will be filled to a common flat surface. At times the 

 level of a lake may be raised, its area become larger, and its 

 storm waves more powerful. The waves bite into the beaches, 

 notching them on the lakeward side, cut into high lands along 

 its shores, and make water-worn cliffs. If the higher stage 

 exists long enough the lake may completely destroy the shore 

 forms made at a lower level. If the borders of the lake are 

 resistant rocky cliffs, the waves cut grottos and sea caves in 

 them and pound the loosened debris on the shore into a narrow 

 rocky beach. Outlets of lakes also have their distinctive 

 characteristics. The lake beaches curve into the outlet and 

 disappear in the channel, as the outlet assumes the character 

 of a river and slopes away from the beach lines; thus old 

 abandoned outlets can be identified. 



If the lake dries up or is drained away all these records — 

 beaches, strands, dunes, shore cliffs, sea caves, grottos, varved 

 clays, flat lake-beds of sand and clay — are left high and dry, 

 showing where the lake once was and revealing its history. 

 The strength of these features indicates the length of time 

 the lake endured. Obviously a short-lived lake would make 

 weak, ill-defined shore features, whereas a long-lived lake 



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