W. Upham—Minnesota Valley in the Ice Age. 105 
period intervened between the great glacial epochs; the earlier 
ice-sheet gradually retreated northward; a lake was formed in 
the Red River Valley by the receding ice-barrier on the north; 
the outflow from this lake, and the drainage of the Minnesota 
basin itself appears to have excavated the valley of the Minne- 
sota River nearly as it now is; and the further recession of the 
ice-sheet probably even allowed the drainage of the Red River 
basin to take its course northward, as nowy to Hudson Bay; 
this being indicated by fossiliferous beds, containing the shells 
and vegetation of swamps, and trunks of trees, underlain and 
overlain by till, within the area that had been covered by this 
interglacial lake and was afterward occupied by Lake Agassiz 
at the close of the last Glacial epoch. 
Since all this, a severely cold climate again prevailed, accu- 
mulating a vast sheet of ice once more upon British America 
and the greater part of Minnesota. Beneath this glacial sheet 
the valley of the Minnesota River was partly refilled with till, 
_ but it evidently remained an important feature in the contour 
of the land surface. Perhaps it had been the pathway, along 
the lower part of its extent, of a sub-glacial river during this 
later epoch of ice. At the final melting of this ice-sheet, its 
waters, discharged in this channel, quickly removed whatever 
8 : 
obstructing deposits of drift it had received, and undermined 
its bluffs, giving them again the steep slopes produced by flu- 
vial erosion. This partial excavation and sculpture anew were 
then immediately, followed, during the retreat of the ice-sheet, 
by the deposition of the modified drift 75 to 150 feet deep, 
remnants of which occur frequently as extensive terraces on 
the sides of this valley, from its mouth to New Ulm, and less 
distinctly beyond. Had not the great valley existed nearly in | 7 
its present form through the last Glacial epoch, it could not — 
have become filled with this modified drift, which must belong 
ex 
