MOVEMENTS OF LAKE WATERS. 41 



Mcolianioal sediments. — The coarse setliment brought to lakes l)y 

 streams is either l>uilt into deltas or swept along the coast In' shore cur- 

 rents and mingled with the pebbles and sand derived from the wear of 

 the land by shore waves. The finer products of the wash of the land, 

 and of shore erosion, are carried lakeward and deposited in stratified lay- 

 ers over the lake bottom. In general, the sheet of material thus spread out 

 is thickest and coarsest near shore and becomes finer and thinner as the 

 distance from land increases. When sedimentation goes on uninterrui)t- 

 edly until a basin is tilled, the result is a more or less regular lens-shaped 

 l)ody of sediments, having a broad central area of fine material, which 

 graduates into a fringe of coarser character about its borders. The coarse 

 strata in the shore deposits overlap and dovetail along their lakcAvard 

 margins, with the outer Ijorders of the layers of fine sediment in the cen- 

 tral part of the basin, for the reason that the coarser material is carried 

 farther from land during storms than when the Aveather is calm. This 

 general relation of coarse shore and fine off-shore deposits is of interest, 

 especially in the study of extinct lakes, and may enable one to draw their 

 former boundaries with considerable accuracy even when all distinctive 

 features of their shore topography have been obliterated. 



The sediments of the existing lakes of America, so far as they have 

 been studied, are })rin('ipally clays, which var}" in character according to ' 

 the nature of the rocks and soils on the neighboring land. The sediments 

 of the Laurentian lakes and of lakes generall}', particularly in humid 

 regions, are characteristically blue clays. The Pleistocene clays of the 

 Erie and Ontario basins are tenacious blue clays, similar to those now 

 accumulating in the same basins ; but the clays deposited during a 

 former Inoad extension of Lake Superior are fine, evenly laminated pink- 

 ish clays, and owe their distinctive tint to the color of the rocks from 

 which they were derived. 



The sediments now accumulating in the lakes of the arid regions, 

 but more especially in the temporary or playa lakes, are usually light- 

 colored, and have a yellowish tint when dry. 



In regions of deep rock decay, like the southern Api)alachians. the 

 debris swejit into lakes would have the characteristic tints of Icrra roxsa, 

 as the highly oxidized jjroduct of [)rolonged rock decay is termed, unless 

 it was mingled Avith organic matter in sutlicicnt (piantity to deoxidize 

 the iron to which its richness of color is due. 



Tlie generalization that all lake sediments arc of a reddish tint, for- 

 merly ailvaiiced by certain fjiglisli geologists, does not tiiid sii|i|Mirl t'loni 



