OTIIKK LAKKS 127 



though Somali, served to produce a remarkable toi)ogra])hy, which lias 

 been described by Mr U})liaiii. Lakes Benton, Shaokatan, and Hen- 

 dricks, the last within the territory of South Dakota, were embraced in 

 the interval between two glacial moraines, and were at one time much 

 extended. On account of the ice-barrier toward the northeast, these 

 valleys were filled by glacial lakes whose outlets were across the outer 

 moraine. Here were excavated conspicuous channels,* which remain to 

 this day and testify to the ])ower of the streams that formerly passed 

 through them. These lakes were each about 1,850 feet above the sea. 



Similarly, lakes Shetek and Heron, farther southeast, were once much 

 enlarged by the obstruction caused by the ice-margin in its retreat 

 northward. Lake Shetek probabl}^ extended for at least 10 miles up the 

 Beaver Creek valley and covered the Des Moines valley southeastwardly 

 to the Great bend of the Des Moines. Lake Heron, a little farther 

 southeast, was blocked in its northern outlet to the Des Moines by the 

 glacier that formed the moraine at Windom. It rose sufficiently to 

 cross the divide toward the south, and took the old abandoned channel 

 southward through Jackson county. 



Glacial Lakes of the Driftless Area 



There remains yet to be mentioned another interesting class of glacial 

 lakes. These were located in the deep gorges of the Mississippi valley,- 

 within the " driftless area " of the state. In order that it may appear 

 in what way these lakes can be included in the category of glacial lakes, 

 it will be well to recall some of the facts which have been ascertained 

 respecting the glacial epoch in Minnesota, so far as it relates to the Mis- 

 sissippi gorge at and below Hastings. 



It is now several years since the writer noted occasional deposits of 

 till, and sometimes northern boulders in the gorge of the Mississippi, 

 well within the " driftless area," when such could not be found in the 

 country farther west. This was observed at Winona and at Red Wing. 

 At the same places, and indeed all along the Mississippi, both within 

 and outside of the " driftless area," the river is bordered by terraces of 

 gravel and sand which are like those of the rivers in drifted regions, the 

 material of which is freely attributed to the wash of the drift brought 

 forward by the glacier. Toward the north, indeed, these gravelly terraces 

 merge into those which are connected with the morainic belts and with 

 the gravel plains of the upland. It has been difficult to understand how 

 such gravel and sand could be so far transported by a gently flowing 

 current from its supposed source. 



* Final Report of the Geological Survey of Minnesota, vol. ii, pp. G03, GO-1. 



