NATURAL BRIDGES OF UTAH 
215 
automobiles. The Iron County bridges are situated far up on 
the western flanks of Markagunt Plateau; while those of San 
Juan County occupy entrenched positions in the top of a vast 
platform. 
The general geological features of southern Utah have to be 
viewed in their relations to those of the whole state. The Wasatch 
and the Uinta mountains of northern Utah were brought into 
existence by the Laramide revolution of Late Cretacic times; 
and that the High Plateau country to the southward was elevated 
toward the close of Tertic times. Moreover, the two areas not 
only differ in point of age, but more particularly in point of 
structure. The formations of the mountainous section are almost 
everywhere highly folded and contorted; while those of the 
Plateau country occupy nearly horizontal positions. Topograph¬ 
ically the northern area is sharp and serrate, characterized by 
' V-shaped canyons, here and there modified by the effects of 
Pleistocene glaciation. At the south the typical uplands are flat- 
topped and the incisions are of the steep-sided box-canyon type. 
The late Captain Dutton calls attention to the stupendous 
amount of erosion which has taken place over the Colorado 
Dome country since the time of its uplift. According to this 
explanation the thousands of square miles comprising the Col¬ 
orado platform proper have been stripped of all the formations 
from the Tertic to the Carbonic Periods, comprising an aggregate 
average thickness of close to ten thousand feet, and that the 
higher plateaus at the north and north-east are simply peripheral 
remnants of the one-time almost infinitely larger mass. 
Whether Dutton was fully justified in his far-reaching con¬ 
clusions may forever lack final evidence, yet it cannot be doubted 
that at least large vertical sections along the northern margin 
have been denuded much as he suggested. Proof of this is seen 
in the almost numberless outliners, the great stairway of with¬ 
drawing cliffs, and the beheaded streams on top of the residual 
