STRATIFICATION 



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be inclined at a considerable angle ; if in shallow water, they form a 

 very gradual slope. In small lakes the coalescence of deltas, or the 

 advance of a single one, will eventually fill up the basin, forming 

 first swamps and then smooth, grassy meadows, through which 

 flow the streams, keeping their own channels clear. Such filled-up 

 lakes are common in many mountain ranges. In large lakes the 

 process is, of course, much slower, and the lake is apt to be drained 

 by geographical or climatic changes, long before the basin is choked 

 with sediment. 



Away from the deltas the combined action of the waves and 

 currents fringes the lake with coarse deposits of boulders, gravel, 

 and sand, which form the beach, the sand extending some dis- 

 tance out into shallow water. The finer materials are carried out 

 into deeper water and deposited in successive layers over the 

 whole lake bottom, the finest materials in the centre. The coarse 

 and fine sediments grade into each other, dovetail and overlap, 

 because in heavy storms or when the streams are in flood, the 

 coarser sediments are carried farther out and deposited on the 

 fine, and these changes of material in any given vertical section, 

 not too far from shore, may be often repeated. Special lines of 

 accumulation for the coarse substances also occur in the form of 

 shoals, spits, embankments, and the like. If the lake is subject to 

 fluctuations of its level, with the water much higher at one time 

 than another, even more wide-spread changes in the character of 

 the deposits will occur. The deposits now forming in the great 

 Laurentian lakes are principally blue muds and clays, partly made 

 up of kaolinite and partly of the debris of other minerals, in an 

 extremely fine state of subdivision, but not decomposed chemically. 

 In Lake Superior the clay has generally a pinkish tinge. 



Stratification. — It has been incidentally remarked that the 

 sediment accumulated in lakes is divided into layers, which, 

 except locally, are laid down in a horizontal position. This divi- 

 sion into layers is stratification, and the sediment is said to be 

 stratified. Not that stratification is peculiar to lake deposits ; on 

 the contrary, it is characteristic of all accumulations made under 

 water. It is due to the sorting power of water, by which, so long 



