38 THE WHITE RIVER BADLANDS 



sequence. Volcanic ash occurs at certain horizons and one 

 or two beds in the later formations cover considerable areas. 

 The several geological formations have particular char- 

 acteristics that serve to distinguish them in the field. In 

 view of the importance of these formations the makeup of 

 each is here described in some detail beginning with the 

 Chadron which is the oldest. The others follow in the order 

 of their age. 



OLIGOCENE 



The Chadron Formation 



The Chadron formation, better known by the much 

 older term, the Titanotherium beds, from the name of the 

 large extinct animals, whose bones occur in it so abundantly, 

 receives its name from the town of Chadron in northwestern 

 Nebraska. The formation is best developed and has been 

 most studied in and near the Big Badlands of South Dakota, 

 but is of importance along the northerly facing escarpment 

 of Pine Ridge in South Dakota, Nebraska and Wyoming. 

 Owing to the slight dip of the strata away from the Black 

 Hills, the Pine Ridge outcrop, lying as it does at the base of 

 the high escarpment, passes quickly beneath younger for- 

 mations and leaves only a long narrow east-west band for 

 observation. In and near the Big Badlands the White and 

 Cheyenne rivers and their tributaries have cut deeply into 

 and across the deposits, and there the Chadron is exposed 

 over a large territory. The beds are known to underlie an 

 extensive area of later formations within and beyond the 

 Black Hills region and are well exposed in the valley of 

 North Platte river in western Nebraska, and of South Platte 

 river in northeastern Colorado. 



The formation is made up chiefly of a sandy clay of 

 light greenish-gray color, with generally coarser sandy ma- 

 terials at or near the bottom, including sometimes deposits 

 of gravel or conglomerate several feet thick. The beds im- 

 mediately above the gravels are often of a yellowish, pinkish, 

 reddish, or brownish color, and Mr. Darton states that in 

 northwestern Nebraska, near Adelia, the red color is espe- 

 cially prominent. Aside from this the color in the main is 

 a greenish white, the green showing as a very delicate tinge 

 on weathered slopes, but a distinctly deeper olive green in 

 fresh exposures. The clays sometimes partake of the nature 



