362 



THE AGE OF MAMMALS 



and cross-bedding \^^th the clays; these indicate the existence of old river 

 channels taking a nearly east and west direction, or approximately the 

 same as that of the streams draining the country at the present time. 

 Some of them are traceable for long distances. It is in these peculiar beds 

 of sandy clays that all the fossils of this region occur. 



Still more recent than these are new and fresh river channel forma- 

 tions (Fig. 167) which also cut their way into the Middle and Lower Mio- 

 cene and constitute the famous 'Blanco Formation' of Cummins and Cope. 

 These beds occupy a comparatively narrow valley or basin formed for 

 their deposition by ancient erosion of the older Lower Miocene (Panhandle 

 beds); they are traceable southeastward for fifteen or twenty miles to 

 the edge of the plains; there is total absence of any proof for the theory 



Blanco 



(-middle Pliocene) 



± 



*ROCK creek" 

 (Kquuszone, lower Pleistocene) 



CLARENDON 

 (upper Miocene) 



By permission of the American Museum of Natural History 



Fig. 167. — Diagrammatic section of the Llano Estacado region of Texas, showing the 

 intrusion of Miocene, Pliocene, and Pleistocene river channels in an older Middle Miocene 

 Formation. After J. W. Gidley. 



of lake origin of these beds, and many evidences of river or stream deposi- 

 tion. Occasional deposits of Fuller's or diatomaceous earth are accounted 

 for by the supposition that there were in this ancient valley occasional pools 

 filled with clear water partially isolated from the main stream. The ani- 

 mals which were preserved here include the armored and gravigrade eden- 

 tates, the short-jawed mastodons, and advanced types of horses and camels. 



A third period of river or flood-plain formation traversing the same 

 Lower Miocene substratum occurred during the Pleistocene epoch, lading 

 down the broad bands of the 'Rock Creek Formation,' also of fluviatile, 

 alluvial, and seolian origin, composed of cross-bedded sands, gravels, and 

 clays. The wind, carrying large quantities of fine dust and sand on the 

 surrounding plains, may have played an important part in forming these 

 deposits. The mammals represented consist wholly of land forms, and 

 some of the bones show weather-checking; they contain the characteristic 

 Lower Pleistocene forms, Equus, Elephas imperator, and Platygnnus. 



This geologic succession in Texas may be summarized as follows: 



Lower Pleistocene, Rock Creek Formation 



Middle Pliocene, Blanco Formation 



Upper Miocene or Lower Pliocene, Clarendon 



Formation 

 Middle and Lower Miocene, Panhandle For- 

 mation 



Equus and Elephas imperator Zone 

 Ghjptntherium Zone 

 Procamelus Zone 



Merycochcerus Zone 



