352 The American Geologist. •^""•^' ^^os. 
plain of the Red river of the North. Probably at that time the ice 
had been melted away from nearly or quite all of the southern 
half of Minnesota. That the retreat of the ice-sheet had uncovered 
the southern third of the St. Croix basin is shown, in Nessel town- 
ship, Chisago county, Minnesota, near Rush City, by an interglacial 
land surface, with wood and peaty matter upon a deposit of modi- 
fied drift that was laid down during the previous retreat of the ice.* 
Above the wood and peat of this place, and above an extensive plain 
of the Buchanan modified drift reaching thence several miles^ east- 
ward, a somewhat uniform mantle of till, 10 to 20 feet deep, was 
spread during the ensuing Illinoian and lowan glacial readvance. 
We thus know that the district including the Dalles and extend- 
ing northward at least to Rush City was uncovered from the ice- 
sheet during the Buchanan stage of the Glacial period. Later the 
increasing snowfall again permitted nearly all of this basin to be 
enveloped by the ice of the Illinoian and lowan stages, reaching on 
the St. Croix river southeasterly to the conspicuous moi'aine belts 
which pass from St. Paul and Minneapolis northeastward to the 
northern half of lake St. Croix and through the southeastern part 
of Chisago county, continuing thence onward in Wisconsin. 
Terraces of sand and gravel, which are found in the St. Croix 
valley 4 to 10 miles north of Taylor's Falls, mostly having a hight 
of about 90 feet above the river, are remnants of valley drift de- 
posited during the Wisconsin stage of the final departure of the ice- 
sheet. These gravel deposits, continuous as one expanse of modi- 
fied drift from the jack pine barrens of northwestern Wisconsin, 
bear testimony that a part of the floods from the dissolving ice then 
passed southward along the present St. Croix, and that the erosion 
of the valley in the vicinity of the Dalles had been mainly accom- 
plished previous to the Wisconsin stage. We are led, therefore, 
to the conclusion that much channeling of the valley here, enlarg- 
ing it along all its course from the Dalles southward to the Apple 
river, an-d eroding the drift bluff, an escarpment of till, which I'ises 
steeply on the west side of the valley at Taylor's Falls and north- 
ward to the hight of 200 to 220 feet above the river, took place 
mostly during the prolonged Buchanan interglacial stage. It was a 
nearly similar history with that of the Minnesota river during the 
same Buchanan time in the re-excavation of its valley, which had 
doubtless become chiefly filled with drift during the principal Kan- 
san stage of glaciation. 
When I wrote the chapter on this district for the Final Report 
of the Minnesota Geological Survey (volume ii, 1888, pages 399-425, 
with map of Chisago, Isanti, and Anoka counties), I believed that 
the preglacial and postglacial courses of the St. Croix were alike; 
but I now attribute the establishment of this great river course and 
valley at the Dalles, and for many miles above and below, to the 
* Geology of Minnesota, vol. ii, 1888, pp. 414-418. 
