136 
UTAH PENEPLAINS 
Great Plains remains to be determined. Careful examination of 
the succession as displayed on the Rocky Mountain front will 
doubtless reveal such horizon. 
Application of the term Laramian Hiatus to the erosion interval 
is thus as appropriate in the west as it is in the east. Whereas in 
the Raton region a similar discordance in sedimentation is so 
obscure that it was for more than half a century entirely over¬ 
looked, in the Utah held the older strata were strongly folded and 
planed off before the younger sediments were deposited. A mile 
of Cretacic beds were beveled off together with the Jurassic shales 
and sandstones. 
Realizing that as this planation took place the ancestral Rockies 
were completely razed to the level of the sea and that the pene¬ 
plain then stretched unbrokenly across the tract where the primitive 
Cordillera previously stood, this basal erosion plain of Tertic times 
is best paralleled with the Raton peneplain of the east side of the 
Rocky Mountains of today. In the San Juan Mountains of south¬ 
western Colorado the conditions recently recognized by Atwood 
appear to be in full agreement. 
Many of the High Plateaux are heavily capped by old lavas, 
remnantal patches of former extensive surface flows. On the 
Sevier Plateau, for instance, the successive lava sheets attain a 
total thickness of more than 4000 feet. The ancient erosion 
surfaces upon which the lavas were outpoured in the different 
plateaux have not all the same elevation, indicating, as in more 
recent times, that the lava-streams were extravasated at widely 
different dates. 
The strata of the Plateaux substructure being mainly shales and 
soft sandstones are commonly assigned to Eocene age, but though 
still Tertic they are probably very much older than any known 
Eocene beds of other parts of the world. 
In the correlation of principal planation levels of the High 
Plateaux region there appear to be presented in the post-Paleozoic 
section five distinct, wide-spread and notable horizons of uncon¬ 
formity which represent erosion intervals of vast duration and in¬ 
tensity. Their stratigraphic position coincides exactly with 
similar erosion plains in the rock-column on the east flank of the 
Rocky Mountain uplift. These levels are also traceable around 
