12 LAKES OF NORTH AMERICA. 



escape. They are unusually turbid with silt Ijrought to them by glacial 

 streams, and leave important deposits to mark their sites when the condi- 

 tions are no longer favorable to their existence. 



The most widely known example of the formation of terraces about 

 the borders of a glacial-dammed lake, is furnished by the Parallel Roads 

 of Glen Roy, on the Avest coast of Scotland. The origin of these terraces 

 was a fruitful source of controversy for many years ; but the explanation 

 that they are due to the action of the waves and currents of a lake held in 

 a lateral valley by a glacier flowing past its entrance, has finally been 

 accepted as satisfactory. 



It is worthy of note, that lakes of the type just described, not 

 only occur in mountain valleys, but also about the ends of mountain spurs 

 projecting into encircling ice sheets, as on the northern border of the 

 Malaspina glacier. The deltas and terraces formed in such lakes may 

 remain in unexpected places, as high up on the side of a mountain, Avhei? 

 the retaining glacier is melted. 



When the land bordering an ice sheet slopes towards the ice, the 

 escape of the waters formed by the melting of the glacier, as well as 

 streams from the adjacent areas, is checked, and marginal lakes, some- 

 times of large size, are formed. Two small examples of this class of 

 water-bodies were seen by the writer at the northern base of the Chaix 

 hills, Alaska. During the close of the Glacial epoch, when the ice-sheet 

 occupying northeastern North America was retreating, there came a time 

 when the southern margin of the ice faced a northward-sloping land- 

 surface, and lakes far larger than the present Laurentian lakes, were 

 formed. The largest of these ancient seas, named Lake Agassiz, covered 

 the region in Minnesota and Canada now drained by Red river, and 

 others were formed in the Laurentian basin. 



When glaciers melt, the rock surfaces left exposed are frequently 

 planed, grooved and polished. In such instances, the evidences of the 

 friction of the flo\v'ing ice and of the sand and pebbles frozen into it, are 

 pronounced and unmistakable. These marks of abrasion are frequently 

 buried and concealed by deposits of debris of various kinds which were 

 transported on the surface of the living glacier or enclosed in its mass, 

 and left as superficial deposits when the ice melted. In the lower por- 

 tions of mountain valleys previously occupied by ice streams, and over the 

 outer Ijorder of regions formerly covered hj continental ice sheets, the 

 deposits of debris are in many instances so abundant that the worn rock 

 surfaces beneath are completely concealed. 



