120 
UTAH PENEPLAINS 
in time, or to correlate in the field the different erosion-levels 
among the multitude of mesas. To this circumstance more than 
any other is attributable the common misinterpretation of desert 
landscape features. 
Notwithstanding the fact that they are now so profoundly 
warped four great peneplains are readily distinguishable through¬ 
out a vast region. Of these four major planation surfaces, which 
spread over the area now occupied by the Rockies since the close 
of Paleozoic times, a Mid Tertic, or Miocene, savannah seems to 
be represented in last lingering traces by the summital flats of the 
High Plateaux. When these few all but vanquished remnants 
finally wither away, as they shortly will, no positive evidences of 
this once broad peneplain remains. 
In the High Plateaux of Utah lie, also, the key to the genesis 
and structure of the much discussed Great Basin ranges. 
Usually genetically associated with the desert ranges the lofty 
Utah mesas belong not to the Great Basin province, but to an 
entirely different orogenic type, the Colorado dome. Upon the 
north flank of the latter they recline as remnantal patches of great 
sections of weak deposits which elsewhere have been completely 
stripped off the indurated Paleozoics that constitute the founda¬ 
tion of the broad arch. The High Plateaux appear to owe their 
belated preservation to Tertic extravasation of lavas that serve 
as protecting caps for the unconsolidated strata beneath. Were it 
not for these remnantal tablelands long and exciting chapters in 
the history of the Cordilleran region would be lost beyond possible 
rescue. 
Like so many of the neighboring Great Basin ranges the High 
Plateaux stand 11,000 to 12,000 feet above sea-level, and 5,000 
to 6,000 feet above the valley-floors between — the present general 
plains-surface. Comprising as they do, a dozen or more lofty 
eminences, they have flat-topped summits of such limited extent 
that they now are really narrow mountain ridges not essentially 
distinct from the other desert ranges. They differ from the ranges 
of the Great Basin lying to the west only in the circumstance that 
the remnantal lava sheets which surmount them still preserve 
traces of the original high plains-surface, whereas the Great Basin 
blocks not so protected comprising the other desert ranges do not. 
