SOUTH DAKOTA SCHOOL OF MINES 



35 



south of the South Dakota-Nebraska boundary line and 

 Pleistocene gravels are found in occasional places. 



Figure 8 — Diagram showing the successive and overlapping Tertiary 

 formations of the Rocky Mountain region, with names of the im- 

 portant life zones. Osborn. 1909. 



An important work of investigators has been to further 



subdivide the deposits and to correlate in so far as possible 



the resulting subdivisions. Hayden early attempted a sub- 



