126 
UTAH PENEPLAINS 
during this epoch. Recent erosion destroys nearly all traces of 
the planation. 
Around the southern end of the Rocky Mountains, in north¬ 
eastern New Mexico, the missing testimony is found. In the Mesa 
de Maya coal field, where Laramie sediments were, from earliest 
exploratory days, supposed to be upwards of 4,000 feet thick, it 
is discovered as long ago as 1904 ^ that the true Laramian series 
is indeed wholly absent. The lower part of the section is now 
known to be Montanan in age; and the upper portion Tertic. A 
well-marked unconformity plane persists midway in the section. 
The hiatus which it represents appears to encompass all Laramie 
time. Consequently no rocks of true Laramian equivalency exist 
in this district. 
Laramie planation seems to have removed more than one-half 
of the Cretacic rocks from the Rocky Mountain region. This fact 
probably explains why the uplift is now so completely denuded of 
sedimentaries, it being impossible to ascribe the tremendous ero¬ 
sive effects entirely to recent erosion. Laramie planation is thus 
fully as extensive as any of the other great erosion efforts now 
known to have occurred since Paleozoic times. 
In Wyoming also the earliest Tertic beds rest upon folded and 
beveled Cretacic strata. 
Until quite lately evidences of a great Miocene peneplanation 
profoundly affecting the entire Rocky Mountain region were 
known only at a single point. Recent erosion almost completely 
obliterated all traces of its existence. The isolated, circumscribed 
area of Mesa de Maya, in northeast New Mexico, was not so very 
long ago the only surviving vestige of this once vast plain. Heavy 
lava flows preserved to us last remnants of this old plains-surface. 
When the flat summit of this lofty mesa shall have disappeared, 
as it will ere so very long, the last traces of this important base- 
level will be gone from the belt lying immediately east of the 
Rockies. There will then remain only subdued mounds to sug¬ 
gest the former possible position of a higher plains level. The 
very existence of this plain would then be entirely hypothetical, 
and its height above the newer and lower plains surface a matter 
of mere conjecture. 
2 Eng. and Mining Jour. Vol. LXXVII, p. 670, 1904. 
