VALLEY OF THE MINNESOTA RIVER. 579 



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, 



