580 THE GEOLOGY OF MINNESOTA. 



[Strise. Till. Glacial epochs. 



Glacial and modified drift. Glacial striae are plainly seen on the south- 

 west part of the outcrop of quartzyte that forms the water-fall in section 31, 

 Stately, having a course S. 50 to 55 E., with reference to the true meridi- 

 an; and upon the ledge of gneiss in section 12, T. ill, R. 38, bearing S. 50 

 to 60 E. 



The surface of Brown and Redwood counties is principally till, or the 

 mixture of clay with smaller proportions of sand and gravel and occasional 

 enclosed boulders, which was thus deposited in a mingled unstratified mass 

 by the ice-sheets of the glacial period. Its thickness in these counties is 

 generally from 100 to 200 feet. Within the till are found occasional layers of 

 sand or gravel, which often yield large supplies of water in wells. Many of 

 these veins of modified drift were probably formed by small glacial streams, 

 and they cannot be regarded as marking important divisions of the ice age. 

 It is shown, however, by shells, remains of vegetation and trees, found evi- 

 dently in the place where they were living, underlain and overlain by till, 

 that this very cold period was not one unbroken reign of ice, but that this 

 retreated and re-advanced, or possibly at some times was nearly all melted 

 and then accumulated anew. 



Two principal glacial epochs can be distinguished:* in the first of which all of Minnesota 

 except its southeast corner was deeply covered by the continental ice-sheet, and its border was 

 several hundred miles south of this district, in Nebraska, Kansas, Missouri, and southern Illinois; 

 whereas in the later very severely cold epoch, the ice-fields were of less extent, and terminated 

 from 50 to 300 miles within their earlier limit, covering all the basin of the Minnesota river, but 

 not enveloping a large tract in the southwest corner of Minnesota and leaving uncovered a much 

 larger area than before in the southeast part of the state. Between these glacial epochs the ice- 

 sheet was melted away within the basins of the Minnesota and Red rivers, and probably from the 

 entire state. The greater part of the till appears to have been deposited by this earlier ice-sheet; 

 and during the retreat of the ice this till was overspread in some places, especially along the ave- 

 nues of drainage, by beds of modified drift, or stratified gravel, sand, and clay, washed from the 

 material that had been contained in the ice and now became exposed upon its surface to the mul- 

 titude of rills, rivulets and rivers that were formed by its melting. 



In the ensuing interglacial epoch, this drift-sheet was channeled by water-courses till its 

 valleys were apparently as numerous and deep as those of our present streams. The interglacial 

 drainage sometimes went in a different direction from that now taken by the creeks and rivers; 

 and the valleys then excavated in the drift, though partly refilled with till during the last glacial 

 epoch, are still, in some instances, clearly marked by series of lakes, as described in the report of 

 Martin county (pages 479 to 485). More commonly the interglacial water-courses must have occu- 

 pied nearly the same place with the valleys of the present time; and there seems to be conclusive 

 proof that this was true of the valley of the Minnesota river. A long 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 tin's lake, and the 

 drainage of the Minnesota basin itself, appear to have excavated the valley of the Minnesota river 

 nearly as it now is; and the further recession of the ice-sheet probably even allowed the drainage 



Compare the first annual report , p. 61; the fifth, p. 177; and the reports of Martin and Dakota counties. 



