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IOWA ACADEMY OP SCIENCE 
which in Tertiary times was one of vast extent and formed the 
general upland surface of this part of the North American con- 
tinent. 
The Mesa de Maya plain stands 3,000 feet above another well 
defined plain which is much better preserved than itself, and at 
the present time is of much wider extent. This last mentioned 
plain is called the Ocat'e plain — from the great Ocate volcano that 
was built up on this level and over which it poured out its basaltic 
lavas for hundreds of square miles, thus preserving the plain 
from the ravages of time. 
Ocate mesa or plain lies 500 feet above still another very exten- 
sive tableland — the Las Vegas plain, which is now the main up- 
land surface of most of New Mexico, western Texas, Oklahoma and 
Kansas, and southeastern Colorado. Beneath the level of the last 
mentioned plain the present waterways have corraded canyons 
and valleys 2,000 to 3,000 feet still lower. There are besides these 
principal peneplains many other distinct levels at which general 
degradation temporarily stopped at one time or another. 
Of the old Tertiary peneplanation surface of once so vast extent . 
little now remains outside of the Raton region. When the flatness 
of a few of these mountain tops disappears and this will be ere 
long, the last vestiges of this important and interesting plain will 
be gone. There will be then only a long row of what would be 
called monadnocks to suggest the existence of a former higher 
level. The very existence of this plain would then be hypothetical 
and its height above the Ocate level a matter of conjecture. 
The great interest which now attaches to the Mesa de Maya plain 
is that it is a part of the former great plain still handed down to 
us on account of the great basalt flows which were poured out over 
its surface. In place of monadnocks or nothing at all to suggest 
its former existence as a great peneplain a fragment of that great 
plain itself remains. Its real significance as an exact measure for 
late Tertiary can not be overlooked. 
In this region of northeastern New Mexico there are, then, four 
important physiographic levels, 3,000, 500, and 2,000 feet apart. 
By them it is possible to correlate several important episodes dur- 
ing Tertiary and Quaternary times. The Las Vegas plain probably 
marks the beginning of Quaternary time in the Rocky Mountain 
region. With this measure of erosion may be compared the meas- 
ures of deposition. Above the level of the Mesa de Maya plain 
there rises, so far as we know, only the central Archaezoic peaks 
of the southern Rockies. 
