402 THE ICE AGE IN NORTH AMERICA. 



buildings on these beach ridges as the most dry and sightty 

 spots on their lands, affording opportune for perfectly drained 

 cellars even in the wettest seasons, and also yielding to wells, 

 dug through this sand and gravel, better water than is usually 

 obtainable in wells on the adjacent clay areas. While each 

 of these farmers, in fact every one living in the Red River 

 Valley, recognizes that it is an old lake bed, few probably 

 know that it has become for this reason a district of special 

 interest to geologists, who have traced and mapped its upper 

 shore along a distance of about 800 miles. 



Numerous explorers of this region, from Long and Keat- 

 ing in 1823, to Gen. G. K. Warren in 1868 and Prof. N. H. 

 Winchell in 1872, recognized the lacustrine features of the 

 valley; and the last named geologist first gave what has been 

 generalh r accepted as the true explanation of the existence of 

 the lake, namely, that it was produced in the closing stage 

 of the glacial period by the dam of the continental ice-sheet 

 at the time of its melting away. As the border of the ice-sheet 

 retreated northward along the Red River Valley, drainage 

 from that area could not flow as now freely to the north 

 through Lake Winnipeg and into the ocean at Hudson Bay, 

 but was turned by the ice-barrier to flow south across the 

 lowest place on the water-shed, dividing this basin from that of 

 the Mississippi. This lowest point is found, as before noted, 

 at Brown's Valley on the western boundary of Minnesota, 

 where an ancient water-course, about 125 feet deep and one 

 mile to one and a half miles wide, extends from Lake Traverse, 

 at the head of the Bois des Sioux, a tributary of the Red River, 

 to Big Stone Lake, through which the head stream of the Min- 

 nesota River passes in its course to the Mississippi and the 

 Gulf of Mexico. 



Detailed exploration of the shore lines and area of this 

 lake was begun by the present writer for the Minnesota 

 Geological Survey in the years 1879 to 1881, under the direc- 

 tion of Professor N. H. Winchell, the State geologist. In 



