DRIFT PHENOMENA IN WISCONSIN 1 45 



probably flowed in a general northeasterly direction to the Bara- 

 boo, through a valley comparable in size to the preglacial part 

 of che present valley. As the ice advanced, the lower part of 

 this valley was occupied by it, and the creek was compelled to 

 seek a new course. The only course open to it was to the north, 

 just west of the advancing ice, and, shifting westward as fast as 

 the ice advanced, it abandoned altogether its former lower course. 

 Drainage from the ice then carried out and deposited beyond the 

 same, great quantities of gravel and sand, making the overwash 

 plain. This forced the stream still further west, until it finally 

 reached its present position across a sandstone ridge or plain, 

 much higher than its former course. Into this sandstone it has 

 since cut a notable gorge, a good illustration of a postglacial 

 valley. The series of changes shown by this creek is illustrative 

 of the changes undergone by streams in similar situations and 

 relations all along the margin of the ice. 



Damming of the Baraboo. — West of the terminal moraine on 

 either side of the Baraboo River are broad flats, extending at 

 least as far up the stream as Ablemans. At various points, 

 exposures show the material of the flat to be laminated clay of 

 lacustrine type. -The tributaries to the Baraboo in this region 

 are bordered by similar flats composed of similar material. 

 These flats, together with the flats along the main stream, repre- 

 sent the bottom of a glacial lake. The outlines of this lake can- 

 not now be given. At Ablemans there is a body of loess, ^ the 

 upper surface of which corresponds in elevation with the flat of 

 the Baraboo below. 



When the ice lay in the valley of the Baraboo with its edge 

 just west of the city, the stream was effectually dammed. The 

 waters which were held back by the ice dam, reinforced by the 

 drainage from the ice itself, soon developed a lake in the valley 

 of the Baraboo, above the point of obstruction. This body of 

 water likewise extended up the lower course of every tributary, 

 presumably rising until it found the lowest point in the rim of 

 the drainage basin. The location of this point, and therefore its 



'Journal of Geology, Vol. IV, p. 929. 



