14 



river. The great ice jam of February, 1909, held the water 

 back so that the American fall went almost entirely dry 

 and the Horseshoe fall was greatly reduced. Niagara 

 river knows no such thing as the great floods that affect 

 rivers like the Ohio and the Mississippi. The shorter 

 periodic variations (annual, eleven year, etc.) are much 

 too slight in amount and also too short in duration to 

 find any expression in the dimensions of the gorge. 



When carefully studied in detail, the gorge is found 

 to show certain parts that are relatively wide and deep 

 and others that are relative narrow and shallow, and 

 these sections vary in length from 2,000 feet (600 m.) 

 to 2 J miles (3-6 km.) and are therefore much too large 

 and required vastly too long a time in their making to be 

 referred to anything like the brief and small variations 

 suggested above. 



It might be thought that variations of geological 

 structure or lines of weakness in the rocks have been a 

 principal cause of variation in the dimensions of the gorge, 

 but this is surely not so in any important degree. The 

 geological structure throughout the length of the gorge 

 is remarkably uniform. Except the thickening of the 

 capping limestone in going from Queenston to the falls, 

 no variation of structure or lines of weakness are known 

 that would produce a perceptible effect. In the present 

 stage of investigations it does not seem possible to estimate 

 accurately the effect of this variable factor. Its import- 

 ance, however, dwindles to almost nothing in comparison 

 with a certain other variable factor which has surely been 

 the one great cause of variation in the dimensions of the 

 gorge : variation in the volume of the river. 



Beginning at the mouth of the gorge and noting its 

 dimensions, especially its width at the top of the cliffs, 

 any one may easily recognize four clearly defined sections 

 or divisions, not including the whirlpool which lies in 

 a re-excavated portion of the much older, buried St. 

 David gorge. But as a matter of fact there are five 

 sections, the first two occupying the oldest part of the 

 gorge, that between its mouth and Niagara university. 

 The two together are nearly one and a half miles (2 -4 km.) 

 long. The remaining portion of the gorge shows three 

 divisions very distinctly. The first is a wide section, 

 the lower part shallow and the upper part deep, extending 

 from the bend at Niagara university up to the upper side 



