32 p. B. KING 



formation of late Miocene and Pliocene age occurs in great thickness 

 in fault troughs that extend southward near the present course of 

 the Rio Grande. 



These later Tertiary deposits were contemporaneous with erup- 

 tions in the volcanic fields of the eastern Cordillera, and near them 

 contain water-borne volcanic gravels and air-borne volcanic ash. 

 The remainder of the deposits were derived from erosion of the 

 rocks of the ranges. Those in the Great Plains contain fragments 

 derived from the Rocky Mountains on the west; those of the 

 Bidahochi formation, fragments from at least as far as the San Juan 

 Mountains on the northeast. 



Except in fault troughs, the preserved deposits of middle and 

 upper Tertiary times are relatively thin at any locality — a thousand 

 feet thick or less, rather than much more than a thousand feet as 

 with the Paleocene and Eocene deposits. Their average texture is 

 considerably coarser than the latter, not only cobbly or bouldery 

 near the mountains, but also with lenses and layers of gravel far out 

 in the plains country. Nevertheless, they appear not to have resulted 

 from renewed folding of the region, as they overlap the edges of the 

 ranges without disturbance, and in places nearly bury a rough 

 topography of earlier rocks with complex structure. 



Fossil plants and mammals in the middle and upper Tertiary 

 deposits record not only a spread of grasslands at the expense of 

 forests, an accentuation of Eocene tendencies, but also an increasing 

 regional altitude and aridity. By late Miocene time many of the 

 earlier browsing herbivore mammals had disappeared; those that 

 survived, such as the horses, had a dentition adapted to feeding on 

 harsh grasses. Indications of a semi-arid regime appear first in the 

 Oligocene floras and faunas and increase to a climax during late 

 Miocene and Pliocene time when the climate seems to have been 

 much like that in the present Great Plains. 



The deposits themselves are compatible with this inferred environ- 

 ment. In the late nineteenth century it was supposed that the Great 

 Plains deposits had been laid down on the floors of a succession of 

 great lakes, hence that they were originally horizontal and later 

 were tilted regionally eastward. Critical study by many later geolo- 

 gists has made clear that they were largely of stream origin, with 

 local ponds at most and with finer deposits perhaps brought in by 

 the wind. The coarse deposits that were formed in the channels of 



