454 



HISTORICAL GEOLOGY 



waters of the lake were drained off, leaving only much smaller 

 lakes, as Winnipeg, in the deepest parts of its basin. Its exist- 

 ence is now known from the many terraces and sandy beaches 

 made by its waves, and by the broad, flat bottom built of fine 

 silts which were deposi- 

 ted in the lake. This 

 alluvial plain is now one 

 of the richest wheat- 

 growing districts in the 

 world. 



Features of the latest 

 drift sheet. We have 

 already said that the 

 older sheets of drift have 

 been trenched by many 

 valleys, so that the origi- 

 nal moraines and other 

 purely glacial features 

 are no longer easily rec- 

 ognized. The last ice 

 sheet (called the Wiscon- 

 sin) disappeared so re- 

 cently that, in general, 

 erosive agencies have 

 not had time to mar the 

 surface of the deposits 

 which it left. 



Where the edge of the 

 ice sheet lingered we now find terminal moraines. There the 

 drift is usually thicker than elsewhere, and rough hills alter- 

 nating with undrained hollows are characteristic. Many of 

 the hills are composed of rudely stratified gravel heaped up 

 in conical form. These kames are often excavated for road 

 material and railroad ballast. On account of the roughness 

 and bowldery soil of the terminal moraines, they are not com- 

 monly cultivated, but are left as woodland and pastures. 



FIG. 472. A modern glacier on the coast 

 of Alaska, showing a marginal lake in- 

 closed by a terminal moraine which is in 

 turn fringed by an outwash plain. (Mod- 

 ified after Maddren, U.S. Geol. Sum.) 

 Is the glacier retreating or advancing ? 



