48 



A. H. PURDUE 



plane through the center. Not infrequently the heads are forked, 

 as shown in Fig. I. The bottoms and slopes are covered with 

 angular residual chert, leaves and branches of trees (Fig. i). 

 If surface water ever flows through the upper parts of these val- 

 leys, it is only after excessive rainfalls. Extended travel over 

 the region has not yet brought to the writer's notice a single case 

 in which .the upper part of one of these valleys has been occupied 



Fig. i. 



by surface water in sufficient quantity to remove the residual 

 chert. 



Unlike the valleys which Professor Marbut describes in Mis- 

 souri, these valleys are narrow and steep-sloped in their upper 

 parts and pass into rather wide, open valleys which owe their 

 forms to both solution and corrasion. The topographic transi- 

 tion from that part of the valley that is due wholly to solution 

 to the part that is due to both solution and corrasion is of course 

 gradual. The point, as the writer conceives it, where the one 

 passes into the other is determined by the groundwater level. 

 Above the point where the valley cuts into the groundwater, the 

 work is done by solution ; below that point a surface stream 



