OLIGOCENE OF EUROPE, NORTH AFRICA, AND NORTH AMERICA 211 



By permission of the U.S. Geological Survey. 



Fig. 99. — Oligocene, Lower Miocene and Pleistocene exposures of South Dakota, Nebraska, and 

 eastern Wyoming. After Darton and Thomson. Oblique lines = Titanotherium Zone, Chadron 

 Formation. Dots = Oreodon and Leptauchenia Zones, Brule Formation. Horizontal lines = Prome- 

 rycochoerus and MerycochaTUs Zones, Arikaree Formation. Vertical lines = Pleistocene. Exten- 

 sive Upper Miocene and Pliocene exposures omitted. 



