248 W. UPHAM GLACIAL LAKES IN CANADA. 



from erosion of the eastern and western shores of the lake, whence it has 

 been borne southward by shore currents, especially during northern gales. 

 None of the beaches of our glacial lakes are large enough to make dunes 

 like those on Lake Michigan, though the size and depth of Lake Agassiz* 

 its great extent from south to north, and the character of its shores, seem 

 equally favorable for their accumulation. It is thus again indicated that 

 the time occupied by the recession of the ice-sheet was comparatively brief 



Lacustrine Sediments. — In front of the delta plains of gravel and sand, the 

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

 spread over the lake-bottom, covering the till on large tracts adjacent to the 

 great deltas. Only small contributions of fine sediment, usually inappre- 

 ciable, as before stated, on the greater part of the lake basin, were supplied 

 from the shore and sublittoral erosion of till, which yielded the gravel and 

 sand of the beaches ; but some of these areas of wave erosion, 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 would be formed in winter along 

 their northern 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 consid- 

 erable numbers on osar ridges of gravel and sand, where they were evidently 

 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 one mile while the bowlders were being 

 stranded. 



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

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

 surface, and their proportion of bowlders exposed to view is diminished. 

 The lake levelled 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 by fluvial 

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

 bordering the deltas, from which they are distinguishable by their contain- 

 ing in some places shells like those now living in the shallow lakes and 

 streams of the region, remains of rushes and sedges and peaty deposits, and 

 occasional branches and logs of wood, such as are floated down by streams 

 in their stages of flood. In the valley of the Ked river of the North these 

 recent fluvial deposits have commonly greater thickness and extent than 



