13 



the head of the narrow gorge the sandstone, as suggested 

 by the dip, is 70 feet (21-3 m.) or more below the surface. 



After the relatively narrow, shallow sections of the 

 gorge had been made by the small cataracts, the rush 

 of the large-volume river through these sections deepened 

 them still further, not by the action of vertical cataracts, 

 but by the slower and very different process of chute 

 or rapids erosion. The deepening of the Old Narrow 

 gorge north of the University has been substantially 

 completed in this way; that of the gorge of the Whirlpool 

 rapids is now in progress. 



RELATIONS OF THE GREAT LAKES TO NIAGARA HISTORY. 



Having in mind the foregoing outline of the geological 

 conditions under which the gorge was made and of the 

 physical processes involved, it is important to review 

 briefly the relations of the Great Lakes to the history of 

 Niagara river. 



Excepting a few small streams which enter Niagara 

 river above the falls, all the water in the river comes 

 from the Great Lakes above. During two periods the 

 discharge of Lake Erie alone passed over the falls; at 

 other times the full discharge of the four upper lakes. 

 During one period the full four-lake discharge was con- 

 siderably increased. The lakes are simply storage reser- 

 voirs and act as equalizers of flow and Niagara river 

 is merely the overflow of these great reservoirs. On 

 this account the river is characterized by a steadiness 

 and uniformity of volume found in few rivers. It has 

 a slight annual variation of volume, due to spring freshets 

 which raise the level of the lakes and summer droughts 

 which lower their levels, through a total range of about two 

 feet ( • 61 m.) and it has a longer period of variation amount- 

 ing to three or four feet (-91 to 1 21 m.) and corresponding 

 roughly to the eleven year wet and dry periods which vary 

 with the frequency of sun spots. Considerably larger 

 variations are caused by cyclonic storms on Lake Erie and 

 by ice jams in the river itself. A heavy southwesterly 

 gale of unusual duration has been known to raise the level 

 of the water at Buffalo nearly eight feet (2-44 m.) and a 

 similarly severe northeasterly gale has lowered it nearly 

 six feet (1-38 m.) making a total variation of about 14 feet 

 (4-27 m.) in the depth of the water at the head of Niagara 



