6o Tlic American Geologist. July, i898 
Croix had continued in the direction of the lower part of the Upper 
St. Croix its course would be from the Big bend through Sunrise 
City, Center City and Chisago lake to Prairie hollow where it would 
join the Lower St. Croix without making an abrupt bend as is now the 
case. This distance is 20 miles, — seven miles shorter than its present 
course. The region between the Big bend and Prairie hollow is 
covered with drift averaging over 100 feet in thickness and having a 
rough morainic contour over the greater part of the distance, but 
the following facts seem to warrant the conclusion, that in preglacial 
time the Middle St. Croix followed the course indicated above, from 
the Big bend to Prairie hollow. 
A depression about a mile wide and from fifty to one hundred feet 
deep extends, as a continuation of the Upper St. Croix valley, about 
four miles south of the Big bend. The bed of the Sunrise river in 
the last two miles of its course is below the rock formations on either 
side of the valley. The wells in the vicinity of Sunrise City extend 
about 50 feet below the bed of the Sunrise river and in no case have 
they reached rock bottom. 
At Prairie hollow the rock walls of the gorge of the Lower St. 
Croix continue northward one-fourth of a mile beyond the end of 
the Middle St. Croix gorge. The extension of the gorge is filled 
with drift and near its west side is a drift hill about 300 feet above the 
river. Prairie hollow is the sandy prairie formed at the base of this 
hill. Between the high drift hill and the eastern side of the gorge 
is a depression which extends northward until it is lost in the morainic 
contours of the drift, over three miles from the river. This valley is 
about a mile wide and from 50 to 200 feet below the drift hills on 
either side. For some distance the valley is below the top of the rock 
formations in this vicinity. Well borings show that a buried preglacial 
gorge extends at least three miles north of Prairie hollow. Beyond 
this wells do not extend deep enough to be of assistance. Copious 
springs are abundant at Prairie hollow and issue from the lower layers 
of the drift which fills the preglacial gorge. The water from these 
springs forms two large creeks which empty into the St. Croix. In 
the present gorge below and above Prairie hollow springs are relatively 
very scarce. 
The only evidence of the buried gorge in the intervening territory 
is that afforded indirectly by Chisago lake. This large lake has no 
surface outlet. The lake rises and falls independently of the quantity 
of rainfall, of the season and the condition of the surrounding region. 
The subterranean outlet seems to be permanently located, for when- 
ever the lake falls rapidly the water from all parts of the lake always 
flows toward the same point. It is probable that the springs at Prairie 
hollow are the outlet of the underground stream which discharges 
the water of Chisago lake and whose course is determined by the 
buried preglacial gorge. 
The Middle St. Croix lies in the region occupied at various times, 
separately and together, by the glaciers extending from the Keewatin 
