ENVIRONMENT OF THE TITANOTHEEES 



109 



ally by hundreds, the silicified stumps and partially decayed 

 trunks of trees, weathering out of the fine clays of these deposits. 

 It was noticeable that only the knots and lower stumps had 

 been preserved. Nothing like complete trunks were to be 

 observed, and the entire aspect was that of the remains of a 

 dead and decayed forest on the margin of some streams, where 

 only the less destructible knots and stumps would endure 

 sufficiently long to be finally covered up and preserved. In this 

 same region there were discernible certain strata which seemed 

 to indicate that during the deposition of these beds there has 

 been at several horizons an accumulation of vegetable mold 

 or humus, and on Dry Creek, some 5 miles northeast of Chadron, 

 in Dawes County, Nebr., I observed near the base of the 

 Oreodon beds a stratum of some 2 feet of dark-colored humus, 

 clearly indicating that this region had not been occupied by a 

 great lake while this stratum was being deposited. 



Hatcher concludes that the sandstone, the con- 

 glomerate, and a part of the clay were deposited in 

 river channels and that the lenses of limestone, which 

 are rich in remains of aquatic plants and moUusks, were 

 formed in shallow ponds and lakes that were scattered 

 over the higher tablelands and the broad flood plains, 

 where most of the finer clays were deposited by 

 occasional inundations in the rivers and by wind. 

 These conditions are similar to those now prevailing 

 about the sources of Parana and Paraguay Rivers in 

 central South America. ' There the rainy season 

 extends from October to April, and the heaviest rains 

 fall near its end, when the small rivers from the 

 highlands are flooded and pour their waters over the 

 flood-plain. The water, however, takes a long time to 

 spread over the plain, and it is there highest in July 

 and August and lowest in February. The flood plain 

 of the Paraguay is 1.50 miles wide and broadens up- 

 stream. The flood plains of the upper Paraguay, the 

 Amazon, and the Orinoco are confluent. Here we 

 have a group of regions that are together probably 

 larger than that occupied by the great White River 

 group during Oligocene time — namely, 97,500 square 

 miles. 



RAPID FIUVIATIIE SEDIMENTATION IN THE CYPRESS HILLS, 

 SASKATCHEWAN 



While the conditions thus described existed in the 

 Big Badlands of South Dakota, the streams were 

 much more active at places in areas to the south and 

 north. "That the Cypress Hills Oligocene deposits 

 were the result of rapidly flowing water from the west 

 is evident," observes Lambe (1908.1, p. 7). He con- 

 tinues : 



The thick basal beds of rounded pebbles represent the work 

 of a strong transporting force, such as would be supplied by 

 a turbulent stream of considerable size carrying eastward 

 material from the Rocky Mountains. The sands show false 

 bedding as a result of varying currents. With the accumulation 

 of material eastward, and consequent reduction of the trans- 

 porting force, beds of finer material were deposited at a higher 

 level and probably on extensive areas of overflow. 



The beds that yield the most fossils are composed of 

 a fine conglomerate, which on disintegration has freed 

 the fossils. Associated beds of a rich brown coarse 



sand have also yielded some interesting remains. The 

 vertebrate fauna of this region in Saskatchewan 

 includes about 37 genera and 58 species, comprising 

 among the fishes Amia, Lepidosteus, and catfishes; 

 among the reptiles land tortoises, chelydrids, leather- 

 backs (Trionyx), anguid lizards, palaeophid snakes, 

 and true crocodiles; among the mammals opossums, 

 anthracotheres, elotheres, agriochoerids, camels, tragu- 

 lids (Leptomeryx) , horses (Mesohippus), hyracodonts, 

 true rhinoceroses (aceratheres), titanotheres (several 

 lower Oligocene types), sciurid and ischyromyid 

 rodents, beavers, hares, hyaenodonts, dogs {Oynodictis, 

 DapTiaenus), and cursorial saber-tooths (Dinictis). 



These Saskatchewan beds are not only more than 

 twice as thick as those in South Dakota but they have 

 afforded a truer picture of the highly diversified rep- 

 tilian and mammalian life during the time represented 

 by the Titanotherium zone. The species of titano- 

 theres that they contain belong chiefly to the lower 

 (Chadron A) and middle (Chadron B) levels of the 

 Titanotherium zone of South Dakota. 



SLOW SEDIMENTATION IN SOUTH DAKOTA 



As compared with the 500 feet of fluviatile deposits 

 of the Titanotherium zone in Saskatchewan the bare 

 180 feet of sediments that represent the Titanotherium 

 zone in South Dakota are very misleading as to the 

 length of geologic time they represent. Deposition in 

 South Dakota must have been extremely slow. The 

 finer materials that border the river channels and 

 compose the clays must have accumulated very 

 gradually. That a very long period of geologic time 

 elapsed while these sediments were being laid down is 

 evident also from the great span of evolutionary 

 change indicated by members of each phylum of the 

 titanotheres found in this region. On the lowest 

 levels are found primitive small-horned titanotheres, 

 inferior in size to the smaller existing rhinoceroses; on 

 the highest levels are found gigantic animals, of almost 

 elephantine proportions, armed with great, powerful 

 horns. As a basis for estimating the time required for 

 the deposition of the South Dakota sediments, com- 

 parison may be made with existing conditions along the 

 River Nile, which between Aswan and Cairo is build- 

 ing up its bed at the slow average rate of 10 centimeters, 

 or 0.32 foot, per century. (Lyons, 1906.1, p. 334.) 

 At this rate the deposition of the 180 feet of "Titano- 

 therium beds," if composed entirely of fine clays, would 

 have required some 55,000 years. On the other hand, 

 if we apply Humphreys and Abbot's estimates for the 

 Mississippi River system, namely 0.5 foot in 100 years, 

 about 36,000 years would have been necessary for the 

 deposition of the fine clay materials of the Titano- 

 therium zone. The present author inclines to the 

 opinion that the lower Oligocene evolution of the 

 titanotheres demands a period of not less than 55,000 

 years, which would correspond with the present rate 

 of sedimentation in the flood plain of the Nile. 



