368 The AhiCrican Geologist . December, 1897 
the east which is continued for a short distance into the nortli- 
ern portion of the area, is chiefly due to the opposition of the 
ice and its accumulated debris coming from the west. The 
whole eifect of postglacial erosion has been to deepen the chosen 
course of the stream. Any variation from it is seen solely in 
the slight shifting from the minor abandoned channels in its 
later development. 
If this be the true explanation of the evidence at hand it 
argues that the remarkable eroeion phenomena, which are so 
noticeable in the gorge in the vicinity of the Dalles, are all of 
late glacial and postglacial age, the larger part of which was 
accomplished at the immediate close of the glacial period 
during the time that the river served as the overflow channel 
for the West Superior glacial Idkes.* 
At this time tlie volume of water discharged was abundantl}'^ 
sufficient to account for all the erosion phenomena which seem 
so superior to the amount now carried b}' the St. Croix river. 
Ohief among these phenomena are the enormous pot holes worn 
in these rocks at St. Croix Falls and Taylor's Falls. 
Summary. The foregoing discussion, dealing with phenom- 
ena observed in this district, has a bearing upon some of the 
questions receiving present attention among northwestern 
glacialists. 
Among these is the question of single sheet as opposed to 
several independent lobes, the latter of which is more agree- 
able to accumulating evidence. 
The marked variability in the bearing of the glacial striae, 
the peculiar character of certain areas of drift accumulation, 
and the change from a surface drift of western character to 
one of eastern origin, suggests that the district is one of the 
critical areas for the study of glacial geolog3\ 
Mr. Upham's determination of the northern limit of the 
blue tillf at about the course of Snake river, and its southern 
limit, through the northern part of Washington county, no 
doubt accords with the limit noted in this district, and is con- 
sistent with the facts presented in this discussion. 
Mr. Upham's attempt to correlate the farthest advance into 
*Geol. of Wisconsin, vol. i, 1883, p. 280 and p. 261. 
Upham, Geol. and Nat. Hist. Survey of Minn.. Annual Reports. 
tGeol. and Nat. Hist. Survey of Minn., Final Report, vol. ii. 1888, p. 
il2. 
