Science, Geology and Physics 



the neighborhood of the present American Fall. The subsequent 1907 

 retreat of the Horseshoe Fall has had the effect of lengthening ' ert 

 the gorge, but the American Fall has not in the same time made 

 an alcove in the side of the gorge. With reference therefore to 

 the question of the age of the gorge, it is the Horseshoe Fall 

 whose rate of recession is important. 



The chief data for the estimation of the rate of recession are 

 the maps of 1842 and 1905, the time interval being sixty-three 

 years. The outlines from these maps are shown in fig. 4. These 

 data, like other statistical data, can be discussed in a variety of 

 ways and made to yield widely divergent results — a fact suffi- 

 ciently illustrated by earlier estimates of the rate of recession 

 based on comparisons of the map of 1842 with that of 1875, 

 1886 or 1890. The following paragraphs therefore set forth 

 somewhat fully the methods here used, with the principal con- 

 siderations on which they are based. 



In the lengthening of the gorge the river does its principal 

 work in that part of the Horseshoe curve where the current is 

 deepest. The agitation of the plunging water is there so powerful 

 as to roll about the fallen blocks of limestone, using them as 

 tools to grind the shale, and at the same time breaking them up 

 and eventually washing them downstream. The scour maintains 

 a deep hollow beneath this part of the fall, a hollow whose depth 

 is greater than the height of the fall. ... At the sides of 

 the channel, especially near the right bank, where the sheet of 

 falling water is comparatively thin, the fallen blocks are not 

 cleared away, but cumber the base of the cliff. . . . As the 

 cataract retreats, it leaves behind it a deep channel, or elongated 

 pool, in which the current is slow. Below the cataract the gorge 

 is widened at top by the falling away of its banks. When the 

 shale is exposed to the air it becomes subject to frost action, and 

 for a time the limestone ledge above continues to be undermined, 

 but a practical limit is reached as soon as the talus of fallen 

 material covers the slopes of shale, and thereafter the change is 

 exceedingly slow. The real lengthening of the gorge is along 



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