380 W. A. Johnston — Sedimentation in 



Character and Significance of the Stratification of the Sediments. 



Seasonal banding in the sediments being formed in the 

 lake is to be expected because of the manner in which 

 sedimentation takes place. Considerable quantities of 

 silt are carried into the lake during the summer months 

 and very little during the winter months. The coarse 

 material quickly settles to the bottom, and the very fine 

 material remains in suspension, so that by the end of the 

 summer the water is slightly turbid and has the greenish 

 shade characteristic of silt-laden glacial waters. During 

 the winter this very fine material settles to the bottom 

 and by spring the water is clear and has the blue color 

 characteristic of pure water. 6 There is very little cur- 

 rent through the lake, and, therefore, practically all the 

 material deposited on the bed of the lake, except in the 

 delta, is fine enough to be carried in suspension in nearly 

 still water. A small amount of coarse material, which 

 accumulates on the ice during the winter, and is derived 

 from slides, wind-action or by streams flowing over the 

 ice, falls to the bottom when the ice melts. It is to be 

 expected, therefore, that the annual layers consist of 

 two parts ; that the material is mostly fine-grained and 

 that there is a gradation in fineness from the coarser 

 lower part of the layer to the upper finer-grained part. 

 It is to be expected also, as pointed out above, that the 

 thickness of the annual layers in the central part of the 

 lake basin does not greatly exceed 1/6 inch. In the sub- 

 aqueous part of the delta, at the upper end of the lake, 

 conditions are different from those obtaining in the main 

 body of the lake. The highly colored, turbid water of 

 the creek flows out over the subaqueous part of the delta 

 and extends into the lake for varying distances, depend- 

 ing upon varying conditions. The water flowing into the 

 lake during the summer is nearly at the temperature of 

 maximum density, but varies in temperature at different 

 times, being colder in the morning than in the evening 

 and varying at other times according to weather condi- 

 tions. As the great mass of the water in the lake is at 

 the temperature of maximum density and as the river 

 water usually has nearly the same density — the silt 



6 William H. Sherzer, opus cit. p. 60. 



