Geolo(jy of the St. Croix Dalles. — Berkey. HOT 
The localities illustrating this erosion are so numerous that 
it is scarcely necessarj^ to do more than call attention to the 
map of the district. The most important example is that of 
the valley now partially occupied by the St. Croix river above 
the Dalles. The stream lies for nearly five miles almost with- 
out a break, between parallel ridges of diabase, standing, on 
an average, one mile apart and reaching an elevation of from 
100 to 300 feet above the adjacent sedimentar}'^ rocks. This 
difference of elevation is believed to be due not so much to 
preglacial land erosion as to the direct action of the ice during 
the glacial period itself. 
The effect of the glacial erosion upon the drainage of this 
portion of the St Croix valley may be traced to a limited ex- 
tent. Almost nothing can be claimed to be added to our 
knowledge of the preglacial St. Croix. The present channel 
is not believed to represent the original location of the river, 
although smaller streams may liave occupied portions of it. 
The rock wall which occupied the site of the present Upper 
Dalles was an effectual dividing ridge, and to the north of this, 
small streams, occupying the present positions of the St. Croix 
river and Rock creek drained northward until some river was 
reached which probably emptied into the Mississippi.* In 
the southern portion of the district all evidences indicate that 
the present St. Croix river gorge is postglacial. 
The inter-glacial St. Croix, up to the time of the invasion of 
the western lobe, at the beginning of the Wisconsin epoch. 
seems to have followed the preglacial drainage lines as under- 
stood above. But with the encroachment from the west a new 
connection with the Mississippi wa:^ made, and it at that time 
followed the extreme front of the invading ice. 
The channel at present occupied by the St. Croix river varies 
but little from that just mentioned. An impracticable early 
course in the vicinity of Dresser Junction was abandoned. 
The river fell back finally with the retreat of the ice to its 
present more favorable course. The chief factor in making 
the present channel the most available and permanent line of 
drainage was the glacial erosion accomplished at this locality'. 
There is little doubt but that the considerable bend toward 
*Mr. Uphain, Report of the Park Com.. Stat.- Park of the Dalles of 
the St. Croix, 1897, p. 45. 
