NIAGARA FALLS AND VICINITY 39 



downward, i. e. till the grade of the river bottom is a very gentle 

 one, when lateral swinging widens the gorge by undercutting the 

 banks, and atmospheric degradation quickly destroys the steep cliffs 

 which the river does not keep perpendicular. 



During the process of drainage development, numerous side 

 streams come into existence, which join the main stream as branches. 

 These begin as gullies formed by the rainwater running over the 

 sides of the banks into the main stream. A slight depression in the 

 surface, or a difference in the character of the material composing 

 the banks, may determine the location of such a gully, but, 

 once determined, it will become the cause of its own farther 

 growth. For the existence of this gully will determine the 

 direction of flow of succeeding surface waters, and so in the 

 course of time the gully will become longer and longer by 

 headward gnawing, till finally a channel of considerable magni- 

 tude is produced. Streams of this type are known as subsequent 

 streams, and they very generally have a direction varying from a 

 moderately acute to nearly a right angle with reference to the main 

 or consequent stream. 



As the dissection of the Niagara coastal plain continued, the higher 

 portions of the strata, i. e. those nearer the old-land, were slowly re- 

 moved, and the beds lying beneath these were thus exposed. The 

 latter strata were generally of a more destructible character than 

 the overlying ones, and on this account great lowlands, parallel 

 to the old shore line, or the line of strike of the strata, were worn 

 in them by subsequent streams. The more resistant beds, 

 meanwhile, favored the formation of more or less prominent cliffs 

 or escarpments which faced the lowlands, and being undermined 

 slowly retreated southward, thus increasing the width of the low- 

 lands. These features are today repeated in the Niagara escarp- 

 ment which faces the Ontario and Georgian bay lowlands, and the 

 escarpments formed by the outcrops of the Ordovicic limestones 

 farther north. The diagram, fig. 4, illustrates the probable con- 

 dition during early Mesozoic time. The great master consequent 

 streams indicated are: the Saginaw, the Dundas and the Genesee, 

 flowing from the old-land on the northeast, southward or southwest- 



