368 C. K. KEYKS OROGRAPHIC ORIGIN OF LAKE BONNEVILLE 



old c-ourse out on its new sui'J'acc, (iiialJy to (liscliar<i\' into the Columbia 

 Eiver through the valley of oue of the lattei-'s minor tri])ularies whieh it 

 greatly and rapidly enlarged. It is clearly imj)osed on the country it 

 traverses. It is an imposed ri>'er. 



Contemporaneity of ni:i(miboi{]N(! Physical Evi^xts 



tertiary strata 7a' bonneville ba8tn 



As yet the subject of geological age of the formations represented in 

 the Bonneville region has not been critically investigated, nor has it re- 

 ceived the careful attention that it merits. In near-by districts Mid 

 Tertiary beds are known to be widely distributed. Under the title of 

 !N"eocene, Gilbert^ ^ notes that in places these formations extend over the 

 rim of the lake basin and are independent of tlie old vsliorelines. Al- 

 though this fact severely limits tlie remoteness of the lake epoch, it does 

 not iiecessaj'ily su])port the assumption of its contemporaneity with the 

 Glacial epoch. Between Mid Tertiary and Mid Quaternary, or Pleisto- 

 cene, the time is long. It i)ermits an anti(]uity of the lake that is not 

 usually considered. 



■LNNER GORGE OF GRAND CANYON 



Two notable epochs of uprising mark the uplifting of the Colorado 

 dome. During the earlier one of these the larger streams managed to 

 maintain their original courses. In tlie later epoch, when the present 

 inner gorge of the canyon was chiefly formed, the progress ap]:)ears to 

 have been so rapid that some streams were not able to cut channels apace 

 "with the upward movement. The connnencement of the inner gorge 

 Dutton^- is inclined to place in Mid Tertiary times. 



It is manifest that if the regional uplifting produced a barrier which 

 any of the through-flowing streams could not conquer, the river waters 

 would soon become dammed and a lake would result. The beginnings of 

 Lake Bonneville must ha\e thus soon found expression i]i late Tertiary 

 times pr(jbably. 



SIMILARITY OF OLD LAKE TERRACES 



Of the several shore terraces which Lake Bonneville displays those of 

 the Provo stage, al)out GOO feet aljove the present level of Great Salt 

 Lake and 350 feet below tJie liighest stage of the impounded waters, are 

 the most conspicuous and best preserved. Compared with the Provo ter- 

 races, the highest Bonneville terraces are poorly preserved. Over con- 



Vfon. U. S. (k'ol. Survey, vol. i. 18!)0, p. 214. 

 Mon. U. S. (iool. Survey, vol. ii, 1SS5, p. 227. 



