Page ten 



EVOLUTION 



Tke Story of Niagara 



THE seven mile gorge below Niagara 

 Falls has been dug by the tumbling 

 waters undercutting the hard capping 

 rocks making the cataract crest. Un- 

 derneath are softer rock layers that yield 

 and wear away in the powerful swirl of the 

 plunge basin, thus undermining the harder 

 top layers until great blocks fall from the 

 overhanging ledge £or want of support. 

 These blocks, swept around by the mighty 

 whirlpool of the falls, arm the water with 

 effective cutting tools that dig the more 

 swiftly into the rocks below. Thus the 

 falls cut their way backwards, slowly re- 

 ceding upstream. 



Several factors have altered the rate and 

 depth of gorge cutting, the chief of which 

 are changes in the volume of the river itself. 

 Because the river is fed by the Great Lakes 

 to the West, its changing volume echoes 

 the events in the development of those lakes 

 during and after the melting away of the 

 great ice sheets which covered their basin 

 during the last glacial period. As the ice 

 edge slowly receded, it progressively un- 

 covered the lake basins, shifted their drain- 

 age outlets, and otherwise modified the 

 volume of water and the falls recession. 



When the ice sheet first melted back from 

 the Niagara region, no falls appeared, for 

 the Ontario waters were high and flooded 

 the area, being banked on the north by the 

 ice itself and draining southwest through 

 the Erie basin towards the Mississippi. But 

 when the ice uncovered a lower drainage out- 

 let to tlie east along the Mohawk and Hudson 

 rivers, the Ontario waters sank, leaving the 

 iiigh embankment over which the Niagara 

 waters tumbled. At first, a temporary Lake 

 Tonawanda formed just back of the em- 

 bankment edge, discharging by five spill- 

 ways. Of these, the Lewiston spillway was 

 lowered by erosion most vigorously, so that 

 it soon robbed the others of their waters. 



concentrating tiic whole discharge and gorge 

 cutting to the Niagara River. 



During the ensuing first stage of gorge 

 cutting, only a portion of the Great Lakes 

 drainage came througii Niagara, for lakes 

 Superior and Michigan were cut off by the 

 banking ice and discharged by way of the 

 Mississippi. But the waters from Lake Erie 

 and that portion of Lake Huron from which 

 NIAGARA 



Lockport 

 dolomite 



r^'*^ A. 



tVhiflpool 

 sandstone 



Queenslon 

 shale 



Sectional View of Horseshoe Fall 



After Gilbert 



the ice had melted did give Niagara a volume 

 about 2.5 percent of the present. But be- 

 cause the rock layers slope slightly upward 

 to the north, the fall was nearly twice as 

 high and so cut deeply for some two thou- 

 sand feet upstream. 



America I 



A second stage bl 

 uncovered a low ea ' 

 per Great Lakes a^ 

 Canada. This roll* 

 the Erie waters, le 

 of the present volui 

 and an eighth, the jii 

 persistent, the res 

 narrower and shal 

 the turbulent char 

 stream. 



But tlie land toil 

 heaving upward inji 

 sheets melted oft'^ I 

 Trent outlet so hii 

 send the upper lalj 

 the Erie basin andl 

 But also there was I 

 outlet at Rome, N 

 the Ontario waters ,i 

 thus raised until thi' 

 '■'orsJ-e to the foot ' 

 deep digging thro 

 Whirlpool Sandsto 

 gorge was therefor i 

 not deep, though t 

 a narrow channel t 

 bed. But present 

 uncovered the St. j 

 waters of Lake On 1 

 abandoning the Rl 

 to Niagara its hifrj 

 its gorge deep onci i 



Paiioiamic W iuler Vieu of Niagara Falls from Cuiuidi;iii Side of Goorge 



