TERTIARY TERRANES OF NEW MEXICO. 
BY CHARLES R. KEYES. 
Deposits of Tertiary age are extensively developed in New Mex- 
ico. The greatest thicknesses are found in the northwestern and 
central parts of the state, where their maximum measurements 
reach no less than 3,000 feet. Many minor areas are also known. 
These deposits recline usually in marked unconformity upon the 
beveled edges of the Cretaceous and older strata. They are geo- 
logically among the most important deposits of the region. 
Tertiary deposits in New Mexico indicate a marked change in the 
physical conditions of the region as compared with those conditions 
which prevailed during former geologic periods. In place of open- 
sea depositions, as was generally the case among all earlier forma- 
tions, varied conditions existed. In some localities mari- 
time conditions prevailed ; in other places another set of 
conditions occurred, and some of these deposits have been regarded 
as having originated in large lakes. In still other localities exten- 
sive continental deposits were formed and were of fluviatile and 
eolian origins. The last mentioned class of deposits have a special 
interest at this time since it is being discovered that many so-called 
lake beds are not of lacustrine origin but are river formations. 
The first mentioned classes need no particular description at this 
time but the conditions under which fluviatile deposits are laid 
find exceptionally favorable expression. This consideration applies 
particularly to the Llano Estacado formations and many other de- 
posits having as yet no special geographic designations. This phase 
of the subject also refers particularly to most of the Quaternary 
deposits which cover the bolson plains spreading out from the 
piedmont areas. 
The general geologic section of the New Mexican Tertiary de- 
posits may be considered as follows: 
Age 
Series 
Formations 
Thickness 
Pliocene 
Pecosian 
Llano Estacado sands 
200 
Miocene 
Arriban 
Santa Fe marls 
500 
Eocene 
Chaman 
Chaco marls 
Canyon Largo sandstones 
1,000 
700 
Nacimientan 
Torrejon sands 
Puerco marls 
300 
500 
(223) 
