92 LAKES OF NORTH AMERICA, 



basins thus become terraced valleys, with streams winding through them 

 in broad curves, and in civilized regions afford rich farming lands and 

 charming sites for towns and cities. 



At a later period, if some outside influence does not change the course 

 of history, the alluvial deposits are dissected to the bottom, the terraces 

 of soft material are removed, and all records of the once beautiful lake 

 may be lost. This transformation ma}' require tens of thousands of years 

 for its completion, yet the end is inevitable. The various stages in this 

 general history might be illustrated by an abundance of examples. Thou- 

 sands of lakes in the formerly glaciated region of northeastern America 

 still retain the freshness of youth, and their nearly level bottoms may be 

 considered as unborn lacustral plains. The terraced borders of Lake Cham- 

 plain, and of the Laurentian lakes, mark the former extent of water 

 bodies that have passed the youthful stage. Many terraced valleys in the 

 Cordilleras record the former presence of lakes in basins that are now 

 completely drained. In other localities, as in the " Parks " of Colorado, 

 no terraces may be distinguished, but vestiges of lacustral sediment still 

 floor their bottoms. Many valleys in the same region drain through 

 narrow stream-cut gorges, but all other evidence of their having been 

 formerly water-filled has vanished. The time required for these muta- 

 tions is vast when reckoned in years, but to the geologist they are 

 transient phases in the topographic development of the land. 



The even course of history, outlined above, may be varied somewhat, 

 as when the outflowing stream is rapid and especially when falls occur in 

 its course. Waterfalls are formed especially where streams flow over 

 nearly horizontal strata where a hard surface layer rests upon shales or 

 other easily eroded beds, as is typically illustrated at the Falls of Niagara. 

 The undermining of the hard capping layer is effected by the removal of 

 the soft beds beneath, and blocks from the brink of the precipice fall to 

 the pool below and assist the swirling water to deepen a basin. A fall 

 thus cuts back the ledge over which it plunges with comparative rapid- 

 ity, — in the case of Niagara the rate of recession is from 4 to 6 feet per 



year, and may lead to the drainage of a lake before its basin has been 



deeply filled with sediment. The succession of the principal events in the 

 history of a valley may thus be hastened, but the ultimate results will be 

 essentially the same. 



Many small lakes, especially in forested countries, where the surface 

 waters filter through layers of vegetable debris before gathering into rills 

 and brooks, are filled mainly by organic agencies. Water plants, and 



