12 INTRODUCTORY. 



THE EOCENE OK LACUSTRINE AGE. 



The early Tertiary history of the Plateaxi Province is much clearer 

 than its history during prior epochs. The shore of the great Eocene lake 

 which covered its expanse and received its sediments can he defined with 

 tolerable accuracj 7 throughout those portions of it which lay within the 

 area constituting the field of this survey. Its northern and the greater part 

 of its western shore line has been traced from the Uintas to the Colorado, 

 and most of the way coincides with the boundary already described as 

 separating the Plateau Province from the Great Basin. South of the High 

 Plateaus, however-, the Eocene lacustrine beds stretch westward beyond this 

 boundary, and are found among the southern Basin Ranges. We know, 

 too, the origin of a large portion of its sediment. Much of it came from the 

 Great Basin, and probabl}^ still more from the degradation of the Wasatch, 

 the Uintas, and the mountains of Western Colorado, which girt about its 

 northern half. The southern shore line is not at present known, and there 

 is much uncertainty at present as to the exact course of its southeastern 

 coast. From what is known, however, we may wonder at the vast dimen- 

 sions of such a lake, which must have had an area more than twice that of 

 Lake Superior, and may even have exceeded that of the five great Canadian 

 lakes combined. Still more astonishing is the vastness of the mass of strata 

 thrown down upon its bottom. Around the flanks of the Uintas and South- 

 ern Wasatch the thickness of the Eocene beds exceeds 5,000 feet, though 

 they attenuate as we recede from the mountains, but never fall below 2,000 

 feet so far as yet observed. And where this minimum is observed there is 

 good evidence that the deposition had terminated long before it ceased 

 elsewhere, and that the series was never completed. 



The deposition ended in the southern and southwestern part of the 

 lake area much earlier than in the northern part. Around the southern 

 portions of the High Plateaus no later beds than the Bitter Creek (which 

 constitute the lower one-third of the local Eocene) were deposited so far 

 as known at present. The inference is that about that time the southern 

 and southwestern portions of the lake began to dry up, while to the north- 

 ward around the Uintas the lacustrine condition persisted for a much longer 



