360 



THE AGE OF MAMMALS 



Middle Pliocene 



Blanco Formation of Texas, Ghjptotherium Zone 



The Blanco Formation of Texas is decidedly distinct and more recent 

 in its mammalian life than that of the Republican River, of the Rattlesnake, 

 or of the Snake Creek beds, just described. It is provisionally regarded as 

 of Middle Pliocene age. Its most distinctive characters are the appearance 



of short-jawed masto- 

 dons with few grinding 

 teeth approaching the 

 Stegodon type, and of 

 South American 

 armored edentates, or 

 glyptodons. 



Ccenozoic beds of 

 Texas. — Before de- 

 scribing the fauna of 

 this very important 

 formation, it is desir- 

 able to outline the 

 characters of the 

 Caenozoic deposits of 

 northwestern Texas as 

 successively studied 

 by Cummins (1891) and Gidley (1899, 1900, 1901). As in Nebraska, 

 South Dakota, Montana, and Oregon, we obtain vistas of the Caenozoic 

 depositions in this southwestern portion of the United States which afford 

 vivid pictures of the life succession. The credit for prior exploration 

 belongs to Mr. W. F. Cummins of the Texas Geological Survey, whose early 

 collections were submitted to Cope for determination. Credit for the more 

 mature determination of the age of these beds and the fauna which they 

 contain belongs to the American Museum expeditions conducted by Mr. 

 Gidley. The following is a summary of the conclusions reached by the 

 latter author.^ 



Since the close of the Triassic there has been no great disturbance or 

 change of level in the region of the Staked Plains (see map), hence the strata 

 of the Triassic which underly this hilly region are for the most part nearly 

 horizontal, and the country at the beginning of the Miocene epoch was 

 comparatively level. After a long period of erosion in which the Cretaceous 

 deposits were removed during Lower or Middle Miocene times, flood plain 

 and lacustrine conditions prevailed and the 'Panhandle Formation' (Fig. 

 167) was widely spread over the vast area now occupied by the Staked 



' Gidley, J. W., The Freshwater Tertiary of Northwestern Texas. American Museum 

 Expeditions, 1899-1901. Bull. Amer. Mus. Nat. Hist. Vol. XIX, Nov. 21, 1903, pp. 617-635. 



Fig. 165. — Upper view of the shield and armored tail of 

 Glyptotherium as exposed in the Blanco horizon of Texas. 

 Photograph by American Museum of Natural History, 1900. 



