6o6 PRINCIPLES OF STRATIGRAPHY 



bodies free or nearly so from growing vegetation, are of more 

 significance, since they add a decided individualistic note to the ter- 

 restrial hydroclastic series with which they are commonly asso- 

 ciated. It is true that many of their most pronounced characteris- 

 tics are due to peculiarities in the composition of their waters 

 which will give rise to deposits of special chemical character, such 

 as have been discussed in preceding chapters. In other respects, 

 again, the nature of the sediment is not very different from that of 

 the ocean, except in so far as the absence of the tides, the differ- 

 ence in composition and specific gravity of the water and the 

 difference in size of the water body influence such deposition. The 

 last falls practically out of consideration in lakes of great size, such 

 as the American Great Lakes. 



The clastic sediment of lakes is chiefly derived from two 

 sources, that resulting from the erosion of its shores by waves, 

 and that brought in by the tributary streams. Erosion is very 

 marked, especially in the larger lakes, such as Erie, Ontario, Huron, 

 etc., and the product is distributed along the shore as shingle, or 

 heaped up into storm terraces, as on Lake Michigan. Coarse mate- 

 rial is seldom carried far out into the lake, but accumulates along 

 the shores, where it is subject to constant wave attack. The finer 

 sand and mud, however, resulting from such wave attack are 

 carried out from shore and slowly settle all over the lake bottom 

 and sides. This also happens to some of the fine slime brought 

 in by streams, but this tends for the most part to sink to the lake 

 bottom. The coarsest of the river-borne sediments will build up a 

 delta at the shore, but the finer mud, which is held in intimate sus- 

 pension in the stream water, is carried beyond this point. Its 

 presence in the stream water renders that water heavier, and it 

 will, therefore, not mingle with the lighter warm water of the sur- 

 face of the lake, but will sink to the deeper, cooler and denser 

 strata. Thus a mud-laden stream passing into a lake will become 

 submerged, often passing along the lake bottom, and occasionally 

 forming a channel there, bounded by submerged mud banks on 

 either side. The force of the current will finally be dissipated in 

 the deeper waters of the lake and the sediment will slowly sink to 

 the bottom, forming horizontal and well-stratified layers of mud- 

 rock, free from irregularity of bedding and of uniformly fine 

 grain. ]\Iingled with sediments of this type are the muds which 

 were held suspended for a time in the upper waters and which 

 settled all over the lake bottom. 



With the seasonal variation in the strength of the streams there 

 must be a corresponding variation in the grain of the sediment. 



