Age of the St. Croix Dalles— Upham. 353 
capricious outlines of the retreating ice-front in Buchanan time, 
probably sending a considerable stream across the preglacial water- 
shed and along this course at first because the ice itself was a 
barrier on the lower country westward. The erosion by this stream 
had cut down this section of the valley and the two gorges of the 
Upper and Lower Dalles so far before that lower land was uncov- 
ered from the Ice that the channel so begun still continued as the 
lowest then available for the river, and the erosion apparently ex- 
tended as deep as to the present river level before the renewal of 
ice accumulation. 
The duration of the interglacial stage attended by great de- 
crease of this part of the continental ice-sheet has been estimated 
by Winchell, from his investigation of the drift-filled gorge of the 
Mississippi west of Minneapolis, to have measured about 15,000 
years.* Within that time, preceded and followed by long stages of 
glaciation of this district, the drainage from an embayment of the 
ice boundary, at the junction of glacial currents flowing in Minne- 
sota from the northwest and in Wisconsin from the northeast, 
passed in a large river, the interglacial St. Croix, across the former 
watershed where we now have the gorges of the Dalles. 
Separate preglacial streams flowing from this locality south- 
ward and northwestward during many thousand years of the Ter- 
tiary era, in the now continuous river course, had doubtless per- 
formed the greater part of the valley erosion on each side of the 
old watershed, which itself, we may also believe, was deeply in- 
dented here by a col of the trappean rocks in which the Dalles are 
channeled. The separate valleys leading away from the col, as 
eroded during the very long Tertiary era, may have attained nearly 
the same size which they now have as parts of the present contin- 
uous valley, varying mainly from about a half mile to one mile in 
width and from 75 feet to about 150 feet in depth below the ad- 
joining rock cliffs. 
In the Upper Dalles, at and just south of Taylor's Falls, extend- 
ing about two-thirds of a mile, and again in the Lower Dalles situ- 
ated two miles farther down the river and reaching one-third of a 
mile, immediately above the village of Franconia, Minnesota, the 
rock cliffs of trap, Keweenawan diabase, rise almost or quite per- 
pendicularly on each side of the river, inclosing it at each place by 
a very picturesque gorge. The vertically jointed and castellated 
walls of the Upper Dalles form a gorge from 200 feet to about 500 
feet wide, which turns at a sharp angle in its central part from a 
course nearly due south to another bearing west-southwest. The 
course of the Lower Dalles, about 500 feet wide, is also west-south- 
west, this direction being in each case determined by a principal 
system of parallel and nearly vertical joint planes. 
* Paper before cited in the Akbbican Geologist, (vol. x), esti- 
mating the interglacial stage as 9,750 years; which is corrected to about 
15.000 years in the same volume, p. 302, Nov., 1892. 
