500 IOWA ACADEMY OF SCIENCE Vol. XXVI, 1919 



come to gather in a part of the run-off which really belonged to 

 the Raccoon system, although perhaps the Boyer never actually 

 tapped any of the feeders of that system. This would seem to ac- 

 count satisfactorily for the deep narrow valley through the ridge 

 and the broader, shallower one to the northeast of it. Then when 

 the Wisconsin glacier overwhelmed the main stream of the upper 

 Boyer system and the moraine obliterated its valley the empty 

 sag remained as a testimonial to former conditions and one of the 

 tributaries became the main stream of the system. 



Note may be made here of the presence in Porter creek valley, 

 north of Boyer, as well as in Otter and Buffalo creek valleys and 

 also in Boyer valley at several points, of gravels which are older 

 than the Wisconsin stage and which therefore show that the present 

 drainage features were established before the Wisconsin ice dis- 

 turbed the pre-existing drainage. These gravels will be described 

 elsewhere in connection with the geology of Crawford county. 



It seems to the writer, then, to summarize, that Boyer valley 

 originated sometime following the retreat of the Kansan glacier 

 from western Iowa and that the river developed the course now oc- 

 cupied across Harrison and Crawford counties, while in Sac county 

 there were two branches, the western of which is now the upper 

 Boyer, while the eastern is represented by the empty sag extending 

 from the river to Wall Lake, and beyond here perhaps by upper In- 

 dian creek. The Wisconsin glacier blotted out the upper part of this 

 eastern branch, leaving the lower part as a partly filled, undrained 

 marsh beyond the glacier's margin. It seems that the sag valley 

 and the river valley as well are too mature to have been the result of 

 Wisconsin and post-Wisconsin erosion alone. Their history goes 

 far back of Wisconsin glaciation through the uncounted years and 

 centuries of the development of the deep-cut topography on the 

 Kansan plain. 



