| OCR Text |
Show 12 INTRODUCTORY. THE EOCENE OR LACUSTRINE AGE. The early Tertiary history of the Plateau 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 be defined with tolerable accuracy 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 probably 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 dimensions 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 Southern 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 (whicli 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 northward around the Uintas the lacustrine condition persisted for a much longer |
