2S0 The American Geologist. November, 1905 
Pecos and Rio Grande) "How in the broad valleys between 
lines of block mountains," And in another place refers to 
the long basin plains of the Pecos and states that the Pecos 
has cut down to a depth of 2,500 feet below the level of the' 
old plain. It would seem that Dr. Keyes has failed to rec- 
ognize the fact that the Rio Pecos derives most of its wa1,er 
supply from the eastern side of the ranges of the Rocky 
mountains ; that the drainage of the river corresponds very 
-closely with the strike of the Cretaceous beds, the Pecos 
itself being a very asymmetric river, having all of its tribu- 
taries of any consequence on its western side. And when 
it is borne in mind that the river is flowing in its southern 
-course through southeastern New Mexico along the out- 
crop of a great bed of gypsum and that the Cretaceous ter- 
ranes of the Llano Estacado are dipping to the eastward it 
will be seen that in the development of the Pecos valley the 
axial stream has been migrating slowly eastward down the 
dip of the strata against the edge of a hard stratum. On 
the westetrn side of the Pecos the surface of the region con- 
forms very closely with a very hard limestone element of 
the Cretaceous series which rises rapidly toward the west 
nearly to the crests of the bordering mountains, while on 
the eastern side of the river there is a sharp escarpment of a 
few hundred feet from the upper edge of which extends the 
great plain of the Llano Estacado, which slopes gradually 
to the southeast, the surface of which is strewn with the 
'Tertiary gravels. The Rio Pecos, therefore has no moun- 
tains bordering the eastern side of its valley and there are 
no extensive detrital plains in any way comparable to those 
of the Rio Grande along the course of the Pecos outside of 
the mountain valleys at its head waters, except the great 
frontal apron of Tertiary mountain wash just referred to. 
If we are to assume that two thousand feet of sedi- 
ments have been removed from the Llano Estacado then it 
might be proper to say that the Pecos is flowing 2,500 feet 
below the aggradation surface of the Cretaceous terranes; 
otherwise we must consider that the major portion of the 
valley of the Pecos is scarcely more than a good sized drain- 
age ditch along the line of strike of the hard beds of the 
Cretaceous formation which underlies the : Staked Plains 
region. 
