264 



SCIENCE 



[N. S. Vol. XXXIX. No. 999 



stream. This indicates that the upper part 

 of its present source is younger in time." 



The fact has long been recognized that 

 the Ohio and Missouri rivers mark approxi- 

 mately the southern limit of the great ice 

 sheet of the Pleistocene, as though the lat- 

 ter had exercised a controlling influence in 

 the courses of the former. The Ohio, being 



2 Missouri Geol. Survey, "Vol X., PI. XX., and 

 U. S. G. S. Bull. 158, PI. XXIII. 



more accessible, has been studied by several 

 geologists, but by none more carefully than 

 by Professor W. G. Tight.'' 



There the general pregiacial drainage 

 was toward the northwest and was changed 

 to the southwest. In the case before us we 

 shall find reason to believe the pregiacial 



3 Professional Paper No. 13, U. S. G. S. (1903), 

 "Drainage Modifications in Southeastern Ohio and 

 Vicinity. ' ' 



