IOWA ACADEMY OP SCIENCES. 79 



the whole process was very recent, and that unless the lake is 

 very young indeed, it must have had an outlet elsewhere. 

 The depth of the lake is only twenty feet, and recent erosion 

 in the little stream and at the point where it leaves the lake 

 shows that had the lake used this outlet long it would now be 

 dry. Within ten years piling has been put in to protect the 

 exit, and if this were removed the level of the lake would to-day 

 fall three feet. 



The Boyer river lies four miles west of Wall lake. Follow- 

 ing the Chicago & North-Western track westward, frequent 

 cuttings show that the region as far as the Boyer river is 

 covered with a drift having the characteristics of the Wiscon- 

 sin, while the surface features are morainic. An excellent 

 exposul-e in a railroad cutting on the west bank shows twenty 

 feet of loess over sand, and the fields which at the time were 

 freshly cultivated, revealed characteristic loess soil. The 

 Boyer river, then, at this point, marks the western limit of the 

 moraine. A study of the map would also suggest that at this 

 point some new factor enters to determine the nature of the 

 stream. From Wall lake to its mouth its course is southwest, 

 iu accordance with the general drainage scheme of tributaries 

 of the Missouri in Iowa. From its source to the vicinity of 

 Wall lake, however, its course is south and southeast. Below 

 Wall lake the railroad commissioners' map traces seven tribu- 

 tary creeks that come from the east, while there is not one for 

 an equal distance to the north. 



Below the point where the extensive hollow described in 

 this paper meets the Boyer river, I am told by Mr. Bain, that 

 there is an unusually strong gravel train following the Boyer 

 river, while above this point on the Boyer river, the train, 

 while perhaps present, is relatively insignificant. 



The logical inferences from these conditions, I think, are as 

 follows: While the Wisconsin ice lasted a glacial river of con- 

 siderable size flowed through the valley now represented by 

 Wall lake and the hollow connected with it to the southwest, 

 and emptied into the Boyer river. With the retreat of the ice 

 the river disappeared, leaving a channel nearly level. Slight 

 inequalities gave rise to a very shallow pond at the upper end 

 of the hollow, the beginning of Wall lake. For the subse- 

 quent development of WalL lake I would accept White's 

 explanation. 



