g the merycoidodontid;e 



In the Oligocene a vast and nearly uniform plain was slowly built up eastward from the 

 Rockies, with gentle slopes over which the streams meandered widely. Some of the channels of the 

 great eastward-flowing streams were from 700 feet to a mile in width. Occasionally the rivers over- 

 flowed, either seasonally or periodically, and formed broad, shallow sheets of water, much as does 

 the Nile to-day. These were too ephemeral to support an aquatic fauna. In general we may 

 assume that savannahs were intermingled with grass-covered pampas. 



This slow deposition continued without interruption from early Oligocene to early Miocene, 

 during which time the White River sediments (Chadron and Brule, with a marked erosional uncon- 

 formity between) were spread over an area estimated by Osborn to embrace 97,500 square miles. 

 These deposits vary in general from 200 to 600 feet in thickness, but in Saskatchewan the Chadron 

 alone is about 500 feet thick. The minimum thickness of the lower Oligocene is 30 feet, while in 

 the Big Badlands of South Dakota it averages 1 80 feet. White River deposits now remain in North 

 and South Dakota, eastern Wyoming, northeastern Colorado, Montana, western Nebraska, and 

 Kansas, and in the Province of Saskatchewan. Their origin was chiefly fluviatile (flood-plain, river- 

 channel) and asolian. There were also backwater, lagoon, and shallow-lake conditions. The mate- 

 rials were for the most part products of erosion from the Rocky Mountains and the Black Hills, 

 together with a great amount of material from the many explosive volcanoes of the western region, 

 represented by white bands of almost pure ash near the top of the series. Lenticular masses of lime- 

 stone, varying from a fraction of an inch to a foot or more in thickness, and similar masses of 

 gypsum, representing periods of evaporation, were also occasionally laid down. There were zones 

 of concretions, forming resistant layers, especially in the lower Oligocene. 



Toward the late Oligocene the climate began to be drier but seems still to have been sufficiently 

 moist to prevent the movement of sand hills, although undoubtedly a considerable amount of loess 

 was formed. In the main, this epoch in North America was a period of freedom from major crustal 

 movement, and peneplanation continued until the Rocky Mountains were reduced to monadnocks, 

 probably rising from two to three thousand feet above the general level of the plains. 



Of the fauna as a whole, the lowland forms were browsers, while those of the open country were 

 more or less cursorial in type, such as the horses and the camels. The river-channel sandstones and 

 conglomerates bear the remains of the animals living in forest and river, some of them probably 

 partly aquatic and all of them dependent on permanent water, whereas the clays and fine sandstones 

 have the fauna of the plains. To the first group belonged the titanotheres, entelodonts, peccaries, 

 and the like, while in the second group are the oreodonts, rodents, hyracodonts, tragulines, and 

 so on. 



On the Pacific Coast the Oligocene deposits are less well known and are of considerably less 

 extent, although in the Santa Cruz region of California they have a thickness of four thousand feet. 

 In Oregon there is an intermontane formation known as the John Day. Its thickness is between 

 three and four thousand feet, and its composition is volcanic ash and tuffs, reworked in part, and 

 mainly derived from the Cascade Mountains. This basin was completely surrounded by active vol- 

 canoes. The deposit is probably solian in the main, although there may have been shallow shifting 

 lakes. In the latter part of the period of deposition the streams became more active, as a result of 

 orogenic movements. 



Whether this John Day formation is upper Oligocene or lower Miocene is a mooted question 

 which has led to much discussion. On the basis of the lithology and fauna, the writer considers the 

 lower and middle John Day as upper Oligocene and the upper John Day as lower Miocene. 



There is no sharp line of demarcation in the sedimentary record between the Oligocene and the 

 Miocene, and yet in many localities there are indications of a long period of erosion, especially 

 between the Monroe and the Harrison, and of great environmental changes between these two 

 epochs. The easterly slopes were still present in the Miocene but were steeper in the Great Plains 

 area, and the vast flood plains were cut by numerous streams. 



During the Oligocene we have seen that the general climatic and physiographic conditions were 

 fairly favorable to the fauna and flora. In the succeeding epoch these conditions became progres- 

 sively less favorable, and a drier and cooler climate developed, due partly to an upthrusting in the 



