304 A. G. LEONARD DRAINAGE CHANGES IN NORTH DAKOTA 



probable that they would have persisted and survived the rapid erosion to 

 which they are subjected by the tributaries of the Little Missouri if they 

 had long been in such close proximity to that stream. Had the latter 

 occupied its lower valley longer than postglacial time, it is likely that the 

 Killdeer Mountains would long since have been swept away by erosion. 

 T. T. Quirke, in an unpublished paper, shows that the peculiar topo- 

 graphic features of the Killdeer Mountains are probably caused by the 

 change in the course of the Little Missouri when it abandoned its pre- 

 glacial valley. 



