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combined thickness of 10,000 feet. Where the Long arroyo 
traverses the median sandstone it cuts a deep canyon into this in- 
durated formation giving to the entire waterway its title. 
To the lower part of this sandstone terrane Holmes* gave the 
title of the Pinon Mesa group. The sandstones together with other 
beds above and below have been also termed Wasatch beds. 
Below, the Canyon Largo formation is sharply demarcated. Its 
massive basal sandstone of bright yellow hue contrasts greatly 
with the somber colored marls of the Puerco terrane. This basal 
member of the formation is a conspicuous topographic feature 
wherever exposed, and rises over the more yielding marls beneath 
as a pronounced escarpment or perpendicular facade. These 
characteristics everywhere in the field serve to locate the bottom 
of the formation. 
Above, the Canyon Largo section is not so readily defined as the 
base. The transition from prevailing sandstones to prevailing 
marls is not so abrupt as it is from the latter to the marls at the 
base. The sandstones are prominent beds for a distance of about 
700 feet from the bottom of the formation. The vertebrate re- 
mains appear to be the most important feature in drawing the ex- 
act line of separation between the Canyon Largo and the Chaco 
terranes. Cope’s reference to the shark bed near the top of the 
formation also seems important in this connection. 
The Chaco marls, or uppermost member of the Chaman series, 
is a formation that is quite distinct from the other parts of the 
Eocene section, and is easily recognizable in the field. These 
marls cover the broad Chaco mesa. The southern escarpment of 
the Chaco mesa is a prominent feature of the landscape of the 
region and is composed of the sandstones of the Canyon Largo 
formation. 
These marls constitute a part of the section which has been called 
the upper Wasatch group; and also thfe Green River shales. 
The Chaco marls occupy the central part of the great Eocene 
plateau of northwestern New Mexico. The geologic structure of 
the plain being that of a broad, shallow basin the youngest de- 
posits of which are preserved in the middle. All around general 
planation has removed these beds and older and older and older 
strata appear as the distance from the center of the basin increases. 
The area occupied by the marls of the Chaco formation lies 
chiefly in northwestern Sandoval county, the southwestern part 
*U. S. Geol. Sur. Terr., Ninth Ann. Kept., p. 249, 1877. 
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