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Updated: May 17, 2025
At sometime in the later part of the Cretaceous period the first breaks took place, changing the structure of the range from symmetrical to monoclinal and outlining the present form of the Sierra Nevada." Along the eastern margin a system of fractures was thus outlined which toward the close of the Tertiary was to be still further emphasized.
It is wholly within the greater depths of the Grand Canyon, east and southeast of the Kaibab Plateau. The intercanyon valleys of this portion of the Grand Canyon extend back from three to seven miles west of the river, and are eroded in the crest of the Monoclinal fold that forms the eastern margin of the Kaibab Plateau."
The highest part of the plateau is a crust block ninety miles long and thirty-five miles in maximum width, which has been hoisted to nine thousand three hundred feet above, sea level. On the east it descends four thousand feet by a monoclinal fold, which passes into a fault towards the north. On the west it breaks down by a succession of terraces faced by fault scarps.
This uneven floor could be accounted for either by the profound warping of a valley of erosion or by the unequal depression of the floor of a rift valley. But that the trough is a true valley of fracture is proved by the fact that on either side it is bounded by fault scarps and monoclinal folds. The keystone of the arch has subsided.
The whole region has been subjected to many displacements, both flexures of the monoclinal type and faults. Some of these flexures attain a length of over 80 miles and a displacement of 3,000 feet, and the faults reach even a greater magnitude.
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