LACUSTRINE SEDIMEXTS. 201 



is thus again indicated that tlie time occupied by the recession of the ice- 

 sheet Avas comparatively brief. 



Lacustrine sediments. — In front of the deha pLains of g-ravel and sand, 

 the finer silt and clay brought into the glacial lake by the same tributaries 

 were spread over the lake l)ottoni, covering- the till on large tracts adjacent 

 to the great deltas. Oidy small contributions of fine sediment, usually 

 inappreciable, as before stated, on the greater part of the lake basin, Avere 

 supplied from the shore and sublittoral erosion of till, whicli yielded the 

 gravel and sand of the beaches; l)ut some of these areas of wave ero- 

 sion, reaching a -quarter of a mile off shore, are plentifully strewn with the 

 residual bowlders. 



Because of their relation to the receding ice-sheet, the glacial lakes 

 might be expected to receive noticeable deposits, including bowlders, from 

 floating bergs and from floes of the ice foot which Avould be formed in 

 winter along their nortliern barrier. It is certain, however, that no 

 deposits which can be referred to such origin are spread generally over 

 the lake basins. Bowlders are absent or exceedingly rare in the- beaches, 

 deltas, and finer lacustrine sediments. In a few places, however, I have 

 observed bowlders in considerable numbers on esker ridfjes of ffravel and 

 sand (pp. 186, 188), where they were e\'idently brought and stranded by 

 floating ice masses from the melting- ice border, whose distance could not 

 have exceeded a few miles at the farthest, and, indeed, probably was not 

 so much as 1 mile Avliile the bowlders were being stranded. 



Where terminal moraines cross a glacial lake, their knolly and hillv 

 contour, as deposited on land, is changed to a smoothed, slightly undulat- 

 ing surface, and their pro})<)rtion of bowlders exposed to view is diminished. 

 The lake leveled the till that would otherwise have formed knobs and hills, 

 in which process many of its bowlders were covered. 



After the drainage of the glacial lakes by the complete departure of 

 the ice-sheet, the lower portions of their basins, in depressions and along 

 the present river courses, have become filled to a considerable extent bv 

 fluvial beds of fine silt. These are similar in material with the lacustrine 

 sediments bordering- the deltas, from which they are distinguishable by 

 their containing in some places shells like those now living- in the shallow 



