W. Upham— Minnesota Valley im the Ice Age. 109 
deep well at Mankato, drift was found to extend 65 feet below 
the river. | 
A summary of the glacial history of the Minnesota valley, 
as recorded in its physical and geological features here de- 
‘scribed, is nearly as follows. This channel excavated in the 
Lower Magnesian or Calciferous formations far below the bot- 
tom of the present valley, appears to have been eroded by a 
river during the later Paleozoic and earlier Mesozic ages, be- 
fore the Cretaceous subsidence which carried much of this 
till: During the ensuing inter-glacial. epoch, the drainage of 
this area cut a channel, which, because of the natural slopes of 
the basin determined by pre-glacial erosion, coincides alon 
much of its lower part, where it crosses the nearly horizontal 
Paleozoic formations, with the old valley eroded in these 
Strata before the ice age. The pre-glacial, and probably also 
the interglacial river lay far below the present stream. The 
till of the later glacial epoch appears to have only partially 
blocked up this river-course along the greater part of its ex- 
_ tent, and portions which may have been obstructed were soon 
channeled anew, and this valley from its mouth to New Ulm or 
beyond was filled with modified drift, to the height of its present 
i ion, during the recession of the last ice- 
the Saskatchewan River. As long as streams poured into this a 
ie 
Fae ee 
See EL, 
Swept away, and the channel was excavated to a depth lowers 
ssissippi at Saint Paul. Since the ice-barrier which 
used Lake Agassiz disappeared and that lake was drainet 
