Correspondence. 59 
feet deep. The postglacial rock erosion is however small compared 
with that during preglacial time. 
The Middle St. Croix. Special attention is called to this part of 
the valley because it seems that here the river was compelled to 
abandon its preglacial course and to form the present gorge which 
again unites with the preglacial valley in the Lower St. Croix. The 
present course of the Middle St. Croix is 2"] miles long and for reasons 
appearing below is divided into three sections. 
Amador Section. At the Big bend the St. Croix valley makes 
an abrupt turn and extends east for four miles and then southeast 
for ten miles to the mouth of Rock creek. This part of the gorge is 
designated the Amador section because it forms the northeast bound- 
ary of Amador township in Chisago county. The Amador section fol- 
lows the course of a small preglacial stream which flowed northwest- 
ward. Glacial erosion widened and deepened the pre:jlacial valley 
The main erosion features however indicate that the gorge is chiefly 
of postglacial erosion. 
The Dalles Section. From Rock creek the St. Croix gorge extends 
south three miles to the Upper Dalles, and then southwest three miles 
to the Lower Dalles and Franconia. The descriptions of this section 
by Mr. Upham and Dr. Berkey show that the gorge here is essentially 
of postglacial origin. 
The Osceola Section. From Franconia the gorge extends south 
three miles to Osceola, Wis., and then southwest four miles to a 
point about one mile north of the bridge of the Minneapolis, Sault 
Ste. Marie and Atlantic railway. At this point, locally known as 
Prairie hollow, the river makes an abrupt bend to the east of south and 
the valley suddenly becomes much wider. The several drift sheets ap- 
pear in place above the rock strata on both sides of the gorge and 
have every appearance of having been continuous at one time. The 
bottom and sides of the gorge show no signs of glaciation. The drift 
found below the top of the rock strata has fallen down from the layers 
of till above. The present gorge, in part follows and cuts across nar- 
row and shallow drift filled gorges whose preglacial streams flowed 
westward. 
The Middle St. Croix river in its distance of 2"] miles falls 78 
feet. The valley bears a strong contrast to that of the Upper and 
Lower St. Croix, in the absence of undisturbed glacial drift, in its clear 
cut features and in its youthful appearance. 
The Lower St. Croix. From Prairie hollow the St. Croix valley 
extends in a southerly direction for 41 miles, to its junction with the 
Mississippi valley at Prescott, Wisconsin. The Lower St. Croix valley 
is entirely due to preglacial erosion. The river has nowhere succeeded 
in cutting entirely through the glacial drift. The Lower St. Croix river 
falls only 13 feet. 
The buried gorge. In tracing the course of the St. Croix river 
on a good map of this region, it is noticeable that if the Middle St. 
