112 



TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA 



The Chadron formation was especially explored by 

 Darton, under whose direction a map showing its gen- 

 eral distribution Csee fig. 8) was prepared. His de- 

 scription of this map may be paraphrased as follows: 



The most extensive outcrops are in the soutliwestern portion 

 of South Dalcota, in a district known as the Big Badlands. 

 These extend along the valley of White River and in the ad- 

 jacent divide between White River and Chej'enne River 

 West of the latter the formation caps many of the divides 

 extending to and up the eastern slopes of the Black Hills. The 

 formation extends eastward to the vicinity of longitude 100°, 

 and it extends westward along the foot of Pine Ridge through 

 Dawes and Sioux Counties in Nebraska and Converse County, 

 Wyo., to the north end of the Laramie Range. The formation 

 probably underlies a large portion of western Nebraska, but it 

 only reaches the surface at the foot of Pine Ridge and along the 

 north Platte Valley west of Scotts Bluff. Isolated outcrops 

 are also reported at Valentine and Lone Pine. The formation 

 appears extensively in northeastern Colorado, on both sides of 

 the valley of South Platte River. There are prominent ex- 

 posures west of Akron, south of which the formation extends 

 across the greater part of Washington County. South of 

 Denver an extensive area caps the divide between the South 

 Platte and the Arkansas Rivers, at the foot of the Rocky 

 Mountains. The deposits in this area have been designated 

 the Monument Creek formation. This consists of two members 

 of which the upper [now called Castle Rock conglomerate; 

 (Richardson, 1912.1)] has recently yielded Titanotherium 

 remains. Other outlying areas of the formation occur in Bates 

 Hole west of the Laramie Range [Wyoming], in Butte County, 

 S. Dak., and in the southeastern corner of Montana. 



An important additional exposure of the Titanothe- 

 rium zone discovered by the American ]\luseum expe- 

 dition of 1909 is at Beaver Divide (Wagonbed Spring), 

 in the southern rim of the Wind River Basin, near 

 Hailey, Wyo. Here a deposit containing a skull of 

 a primitive Oligocene titanothere was found overlying 

 a deposit of upper Eocene age containing Amynodon. 



The thickness of the Chadron formation varies, but 

 in some places it reaches 180 feet. It consists of 



clays, sands, gravels, and sandstones, clay predominating greatly 

 over the other materials. * * * This clay is of pale-greenish 

 color, weathering in typical badland form and often having the 

 peculiar character of fuller's earth. At the base of the formation 

 there is usually a bed of gravel and sand merging upward into 

 sands and sandy clays, which in the Big Badlands and western 

 Nebraska are often of a reddish color. At various horizons 

 through the formation there are beds of sandstone from a few 

 inches to 4 feet in thickness and of local extent. Ordinarily 

 these coarse materials exhibit current bedding and from their 

 character and relations are clearly the products of running 

 water. Beds of volcanic ash occur extensively in the Chadron 

 formation in the Big Badlands and at intervals along the 

 northern front of Pine Ridge (South Dakota). 

 Hatcher observes (1893.1, pp. 206-207): 

 The clays- greatly predominate, consist of very fine particles, 

 and are quite compact. In places they are composed almost 

 entirely of pure kaolin, but they often contain a considerable 

 portion of sand. Near the bottom of the beds the color is often 

 red or variegated, due to the presence in them of small quanti- 

 ties of red oxide of iron; but the prevailing color is a very char- 

 acteristic and delicate greenish white. * * * Owing to 

 the extreme minuteness of the particles forming the clays and 



the absence of sufficient cementing material in them, in most 

 places they readily yield to the action of water and are quite 

 rapidly eroded. The clays of the Titanotherium beds were 

 probably derived from two sources, viz, from the Cretaceous 

 clays and shales and from the kaolinization of granite feldspars. 



The sandstones are never entirely continuous and never more 

 than a few feet thick. They present everj- degree of compact- 

 ness, from loose beds of sand to the most solid sandstones. 

 They are composed of quartz, feldspar, and mica and are evi- 

 dently of granite origin. When solidified the cementing sub- 

 stance is carbonate of lime. 



The conglomerates, like the sandstones, are not constant, are 

 of very limited extent, never more than a few feet thick. They 

 are usually quite hard, being firmly held together by carbonate 

 of lime. A section of the beds taken at any point and showing 

 the relative position and thickness of the sandstones, clays, and 

 conglomerates is of little [stratigraphic] value, since these vary 

 much at different and quite adjacent localities. 



These descriptions by Darton and Hatcher reveal 

 a wide contrast between the composition of the 

 Titanotherium-heen-ing beds and that of the upper 

 Eocene deposits of the mountain-basin region. 



COMPARISONS OF BASINS IN WESTERN UNITED STATES WITH THE FIOOD 

 PLAIN OF THE NUE 



The flood-plain deposition of the Nile, which has 

 been very carefully studied, also throws light on the 

 mode of formation of parts of the Chadron formation. 

 The following passages are taken from "The physi- 

 ography of the River Nile and its basin," by Capt. 

 H. G. Lyons (1906.1, pp. 241, 311, 334): 



When rivers already loaded with sediment emerge from their 

 mountain valleys of high slope on to a level plain under arid 

 climatic conditions where the water table is at somie distance 

 from the surface their water sinks in almost at once instead of 

 flowing on the surface and therefore deposits its load of sediment 

 as an alluvial fan. This fan is built up most rapidly at its 

 head, and as the floods of successive years come down new tem- 

 porary channels are formed which divide and reunite, forming 

 a network of channels, each b}' deposit building up banks for 

 itself, which are probably cut through in the next season of the 

 summer rainfall. 



While it is the finer silt which is deposited in the irrigation 

 basins, on the shelving banks of the river, and on such parts of 

 the flood plain as are annually flooded, it is the bottom load 

 which is deposited in the bed of the river itself, and this con- 

 sists of the coarser sand which the current can not carry so 

 readily as the finer material. If the Nile mud is treated by 

 levigation so as to remove the finest particles of clay and sand 

 the residue is a fine whitish-gray sand, such as is seen forming 

 sand banks in the Nile wherever the conformation of the river 

 is such that the velocity of the fiood current is reduced at that 

 point. 



In this Aswan-Cairo reach of the Nile, then, we have to do 

 with a river which is fiowing with a low slope through an alluvial 

 plain which it has formed and which, if uncontroUed, it annually 

 floods, depositing on the flood plains part of its load of silt as 

 the velocity of flood water is diminished. 



The Nile between Aswan and Cairo follows a depression in 

 which it has gradually deposited a considerable thickness of 

 alluvial mud, and now it meanders on the flood plain which it 

 has formed. In earlier times side channels followed the lower 

 margins of the valleys, and lagoons and swamps existed in the 

 same part of the valley. 



