PLEISTOCENE HISTORY OF LOWER WISCONSIN RIVER 689 



but the valley was probably again re-excavated to about its maxi- 

 mum depth. 



There are no terraces in the valley which correspond in age to 

 the Ilhnoian drift found at the eastern end of the region, so the 

 assumption is that the outwash from this glacial advance did not 

 fill the valley high enough to be above the present surface of the 

 river. The evidence of five well records^ shows that at one time the 

 valley floor stood, in the main and also the tributary valleys, 30 

 to 50 feet below its present level long enough to accumulate a bed 

 of peat. It is probable that the outwash from the Illinoian invasion 

 filled the valley only to this level, 30 to 50 feet below the present 

 surface. Then ensued a period during which the vegetation accu- 

 mulated on the swampy surface of this outwash. 



During the lowan epoch loess was blown on to the western part 

 of the area, burying the drift with a blanket of eolian material. 



In Wisconsin time not only the moraine at Prairie du Sac, but 

 the valley train in the Wisconsin Valley, was deposited. As the 

 ice withdrew east of the divide near Portage, ponding produced a 

 lake which drained westward down the drift-filled Wisconsin 

 Valley. This was for a time the main drainage for at least a hundred 

 miles of ice-front lying toward the north, consequently a large quan- 

 tity of clear water flowed down the valley. The upper part of the 

 fill was largely cut away, leaving remnants which now constitute 

 the upper terrace in the Wisconsin Valley. The down-cutting 

 river reached grade at the level of the top of the lower terrace. 



After the ice had withdrawn and the glacial lake was drained, 

 the postglacial Wisconsin River cut away large parts of the lower 

 terrace to form its present flood-plain. 



' Well records which show peat 30 to 50 feet below the surface; fair grounds at 

 Richland Center; schoolhouse i\ miles northeast Richland Center; Bear Creek f mile 

 north of junction with Little Bear Creek; Little Bear Creek I mile north of junction 

 with Bear Creek | rmle southwest of Leland. 



