74 The American Geologist. August, 1892 
south and with each other, by Rice creek and Trout brook, con- 
stituting a marked valley which connects the Mississippi above 
the falls with the Mississippi below the falls by a nearly direct 
course. (Compare plate vi.) 
The writer, in former papers, has discussed the recession of 
the falls of St. Anthony,* and has deduced a time-measure for 
their recession from Fort Snelling to Minneapolis, and has shown 
that this also measures the time elapsed since the last glacial 
epoch. This result, approximately 7,800 years, has generally 
been accepted by glacialists, especially in America. It was 
shown that just prior to the last general glaciation the Mississippi 
river at Minneapolis occupied an old valley which diverges from 
the present channel within the limits of the city, at the mouth of 
Bassett's creek, passed southward where now a series of lakes lie 
(Calhoun, Harriet, etc.) and joined the present Minnesota chan- 
nel at some point a short distance above Fort Snelling. There it 
turned to the northeast, at a right angle, and went on to St. Paul, 
and thence, with another right angle, as now, it finally took its 
undeviating course southward. The significance of the first of 
these right angles has been pointed out in that earlier discussion. 
It is the purpose of this paper to point out and discuss the sig- 
nificance of the second. 
In the light of what has been shown for the first it is a reason- 
able expectation to find a similar explanation of the second, 
should similar or identical conditions attend the second. 
Those conditions are: 
1. A great river seeking a channel of discharge, through a 
country uniform in its topography and geological structure, to the 
ocean. 
2. The derangement of the natural, direct and primary course 
of its erosion by the on coming of the conditions of a glacial 
epoch. 
3. The choking up of the (then) existing channel and the 
acceptance by the river on the retirement of the glacial conditions 
of another slightly different channel. 
4. Its entering again upon its old channel at a lower point, 
and the birth at that point of a new waterfall. 
*Fifth annual report of the Geological Survey of Minnesota, 1876, pp. 
156-189. Quart. Jour. Geol. Soc. Lond., Nov., 1878, pp. 885-902. Final 
report of the Geological and Natural History Survey of Minnesota, vol. 
n, pp. 313-341. 
