VALLEY OF THE MINNESOTA RIVER. 379 
Redwood valley districts. and near Fort Ridgely. This indi- 
cates that some ancient stream had cut a gorge in the Lower 
Magnesian rocks and had drained northern Minnesota into the 
great Cretaceous Mediteranean. Since no Tertiary deposits 
are found in Minnesota it may be concluded that they, with 
most of the Cretaceous strata, were torn up by the ice-sheet of 
the first glacial epoch. In this way the ancient gorge was 
filled with debris and while this does not consist altogether of 
unmodified drift, it is in some part of such nature. The pres- 
ence of beds of sand and gravel deep in the till indicates that 
streams must have carried on their work during the subsi- 
diary interglacial epochs and doubtless vegetation re-estab 
lished itself during some or all of these interglacial periods, 
for vegetable debris is found in the lower forest beds of the 
till. By this ploughing up before the first great ice-sheet of 
the Quaternary age, the Cretaceous deposits and the Tertiary, 
if any existed, were mingled together into a layer of till from 
265 feet thick,in places,down to somewhat less than a hundred, 
on higher levels. This layer of till persists over most of the 
Minnesota valley to the present time. During the epoch of 
the deposition of this first layer of till the ice-sheet extended 
south to Cincinnati and northern Kentucky, and into Missouri. 
Almost the whole of Minnesota was covered by it. As re- 
cession began, exposing the surface of the country once more, 
the melting ice and snow sought out the gorge of the Minne- 
sota and it served as a drainage-trough for vast quantities of 
water. In this epoch it was the outlet channel of a large 
glacial lake which occupied the valley of the Red river and 
must have been somewhat similar in extent and character to 
the later glacial lake Agassiz. During this period excava- 
tion of the till which had filled the gorge was carried on and 
doubtless a large river occupied the present bed of the Minne- 
sota. 
Later a second principal encroachment of the ice began and 
extended south to Des Moines, Iowa. During its recession it 
piled up the Leaf’ hills moraine which bounds the Minnesota 
valley on the north. As the ice retreated from the morainic 
area the valleys of the Red and Saskatchewan were occupied 
by the glacial lake Agassiz and from the southern boundary of 
the lake its waters were drained through lake Traverse, 
Brown’s Valley and Big Stone lake along the present gorge 
of the Minnesota river. Under the erosive energy of this 
large stream, which filled the gorge from bluff to bluff, 
