Interglaclal Chronoineter. — W/ncJiell. 79 
Be all that as it iua\', there is one fact whicli tends rather in the 
opposite direction, and indicates that the Minnesota itself cut a 
gorge between St. Paul and Fort Snelling and also further up its 
valley even in pre-glacial time, and that the interglacial Missis- 
sippi found this gorge in existence when it was compelled to aban- 
don permanently its own pre-glacial gorge. That evidence consists 
in the depth of the gorge between St. Paul and Fort Snelling. 
This excavation has to be measured through the whole thickness 
of rock-strata which have been cut, regardless of the accidents of 
the present, for the spreading of the drift has obscured the history 
of the river preceding the ice age. This country became dry land, 
so far as can ])e judged from the latest marine formations (not 
considering the Cretaceous), at the close of the Lower Silurian. 
The whole of the Lower Silurian was probably deposited through-- 
out the area included in this calculation, and at the present time 
there are preserved, in the hills to the south of St. Paul, as well 
as at St. Paul and toward the northwest and northeast, as already 
stated, some of the higher layers of the Trenton, even including 
some distinctivel}' (lalena characters. The thickness of all the 
Trenton beds (shales and thin limestones) is 114 feet. The St. 
Peter is exposed, above the river level, about 75 feet, and the 
gorge, as revealed by deep wells, is cut about 200 feet below the 
water level. The total excavation in the rock therefore is 389 feet. 
As this comports with the general depth of the Mississippi gorge 
below St. Paul, which must be referred to pre-glacial history, 
rather than with any excavation that can be referred to intergla- 
cial time, it points strongly to pre-glacial time for the excavation 
of the valley between St. Paul and Fort Snelling — and hence also 
to the occupancy of the present valley in general by the Minnesota 
since pre-glacial time. 
At any rate, whether the interglacial falls began at St. Paul or 
not, the length of time required for their recession to the mouth 
of Bassetts creek is sufficiently long for the purpose of this paper. 
We will, therefore, assume the shorter recession, and calcu- 
late from the point above Fort Snelling on the Minnesota bluffs at 
which the Mississippi may l)e supposed to have leaped from the 
Trenton limestone to the river below when it was driven from its 
pre-glacial gorge. 
In a direct line the mouth of Nine-Mile creek in Bloomington 
township, which may most reasonably be taken as the point of 
