OROGRAPHIC MOVEMENT IN BONNEVILLE AREA 363 



The main argument for tlie existence of an outlet for Lake Bonneville 

 at the Eed Eock Pass north of Cache Bay is hased on present elevations. 

 Analysis of the measnrahle diastrophic movements that have taken place 

 since the highest stage of water really precludes all possibility of the 

 lake's ever having overflowed its rim. The later Bonneville terraces have 

 not ])een ol)served nearer than 10 or 12 miles of the head of the postu- 

 lated outlet. Above this point the projected level of highest water lies 

 355 feet. Of this interval about 100 feet are taken up by the depressive 

 warping. Another 125 to 150 feet, which represents probably not one- 

 half the actual displacement value, is taken up by measurable faulting 

 of very recent date. A diastrophic oscillation of 200 to 250 feet more 

 than suffices for all the manifold drainao^ changes of the region as far 

 north as Yellowstone Park. 



LATEST UPRISING OF COLORADO DOME 



In the course of his historical account of the Grand Can3^on, Button 

 makes much of the revived regional elevation iu late Tertiary or early 

 Quaternary times. The magnitude of this movement is indicated in the 

 cutting of the inner gorge of the canyon. It is estimated to be l)etween 

 3,000 and 4,000 feet in the central part of the broad dome, gradually 

 diminishing outwardly in all directions to nothing. That the effects of 

 this crustal uprising reached well into the Bonneville basin is clearly 

 shown l)y the fact that even the highest shorelines at the extreme south 

 are sharply warped; so that, as has been already mentioned, a difference 

 in elevation within a distance of a few miles is more than 300 feet. The 

 amount of pre-lacustrine movement in the vicinity must have l)een even 

 greater. It is quite possible that a part of the warping, especially in the 

 south, is to be accredited to Wasatch diastrophism; or it is more than 

 likely that the pre-lacustrine movement belongs entirely to the Colorado 

 Dome disturbance, and that the post-lacustrine movement is to be ascribed 

 chiefly to Wasatch orogeny. This possible deviation is a matter of mere 

 detail and does not vitally affect the main issue. At any rate the upris- 

 ing of the Colorado dome was sufficiently effective at this distance from 

 its center to form the southern barrier of the lake basin at the time of 

 the water^s greatest expansion and highest stage, when its level was 

 nearly 1,000 feet above that of the present Great Salt Lake. 



If there once were a through-ffowing river traversing the ancient 

 Bonneville A^alley, the local uprising must have been rapid enough to 

 outdo the most vigorous corrading powers of that stream. Finally, a 

 point was reached when the watercourse ceased to reach the Colorado 

 River and its waters became impounded behind the barrier formed. Dut- 



