RIVER EROSION 169 
away. So the mighty cataract leapsfrom this hard 
shelf of limestone and dashes against the soft shale; 
which it constantly removes. Little by little the 
shale is worn out from beneath the limestone, which the 
weight of the water then breaks away; but the con- 
tinued stretch of this hard 
limestone layer maintains the 
fall always at about the same 
height. 
- During less than one-half 
of a century, the Horseshoe 
Fall has moved back more 
than two hundred feet, and 
there is evidence that Niagara 
Falls have been at nearly all 
parts of the gorge (Fig. 82). 
Every year the falls have 
- moved a short distance up 
stream, until they are now 
Fig. 84. 
seven miles from the starting Multonomah Falls, Oregon, a fall 
point, where they existed of great height (824 feet) which 
bik first ites ne ae of ae has not cut a perceptible valley. 
Brie’ were drained out over the bluff at Lewiston 
(Fig. 83). Even now each year witnesses that same 
slow retreat, and the outline of the falls has changed 
perceptibly since first seen by the white man. 
What is true of Niagara is equally true of many 
