UTAH PENEPLAINS 
133 
Important unconformity planes are also indicated at several 
horizons where notable formational hiatuses are now known to 
exist. The erosion surface at the base of the Jurassic section is 
doubtless very much more extensive than is generally supposed. 
Locally considerable portions of Triassic sediments are unrepre¬ 
sented. Early Cretacic beds are entirely missing. Although, as 
on the east side of the Cordillera, the great Mid Cretacic Dakotan 
sandstone reclines in marked unconformity upon the bevelled 
edges of the older strata little note is usually taken of the fact. By 
both Powell and Dutton particular stress is placed upon the signi¬ 
ficance of the unconformity at the bottom of the Tertic column. 
But its expanse is far greater than even these writers suspected. 
It turns out to be really one of the great erosion planes of the 
Rocky Mountain province. 
Although casual notice is taken of the terrepleins capped by 
lava composing the High Plateaux no suggestion is made of their 
being possible planation effects. According to Dutton Miocene 
time marks the beginning of regional denudation on a large scale. 
The present intermontane plains are really quite complex in 
their origin. The general plains-surface probably lies 400 to 500 
feet above; and is only sparingly represented by certain isolated 
plateau mesas. The entire region is now undergoing rapid lower¬ 
ing through desert processes of deflation wholly unknown to the 
early workers in the West. The time in the present cycle during 
which the general depletion of this sort has been going on seems 
to be closely paralleled by the excavation of the inner gorge of 
the Grand Canyon. 
Notwithstanding the , fact that Mid Mesozoic sediments are 
well represented in the High Plateaux region their base rests upon 
a well-marked erosion surface. Both Early Jurassic and Late 
Triassic sections are wanting. The partial representation of the 
period by sediments appears to be a marginal episode of the con¬ 
tinental interior conditions when land persisted throughout the 
period. In the Utah region the great erosional interval was in¬ 
terrupted. 
The great Dakotan sandstone comes down the west flank of the 
Cordillera almost as steeply as it does on the Rocky Mountain 
front. Very low down, towards the Colorado-Utah line, this 
