426 G. K. Warren— Valley of the Minnesota and Mississippi. 
tigation, to make more definite this method of finding the 
unknown factor, time. 
Since the Falls of St. Anthony were at the junction of Min- 
nehaha Creek, they have receded six and a quarter miles. 
Minnehaha Falls, since that time, have receded three-fifths of a 
mile. Both streams have cut into the same formation, starting 
with the same height of fall. These relative rates have been 
as about 1 to 10. The proportion of the volumes of the two 
streams, judging by their present drainage areas, is about as 72 
square miles is to 21,600 square miles, or about as 1 to 300. 
That is to say, the recession of the Minnehaha Falls has been 
thirty times faster than it would have been if proportioned to 
the volume. This may be accounted for by the greater atmos- 
heric influence of the smaller falls, which, examination shows, 
eeps ahead of the effect of the water, forming a cave under the 
fall by the dropping down of material which the water then 
washes away. At the greater falls the volume of water almost 
constantly protects the rocks from the action of the atmosphere. 
Hence we must give, as said before, a very considerable influ- 
ence to the operations of the atmosphere in aiding the erosions 
of small streams, and in demolishing cliffs where the water can 
remove the débris. a he 
I attribute a more recent origin of the gorge of the Mississipp1 
from Fort Snelling to the Falls of St. Anthony than to that o 
the Minnesota above the junction. The general map indicates 
that the same force which formed the valley below the junction 
formed that of the Minnesota above. 
ward, made this upper part of the Misstenp pe When we note 
the great extent of the eroded valley of the Minneso 
work, 
Regarding the Mississippi Valley as originating as a whole 
by the action of a stream since the glacial ice oceupied its basin, 
would note that as far down as the island of Rock Island 
there is no decided indication of other than successive changes 
attending such action, and the gradual filling up of the valley 
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