| OCR Text |
Show UNCONFORMITY OF CRETACEOUS AND TERTIARY. 11 • fossils of purely fresh-water types. The junction of the two series is unconformable, and is often highly so. This unconformity is seen in many localities on both sides of the Uintas, along the eastern slopes of the Wasatch, and becomes even more strongly pronounced to the southwest-ward. During the course of this work, localities will be mentioned where it is conspicuously displayed, the Upper Cretaceous (Laramie) beds being flexed at a high angle, the flexures planed off by erosion, and the overlying series resting across the beveled edges, or even upon the Jurassic beds below. It was at this unconformity that Professor Powell drew the dividing horizon between the Tertiary and Cretaceous. Quite independently of any physical break, Professor Meek had chosen the division at the same horizon upon the evidence of invertebrate fossils, though that evidence was regarded by him as being too meager and the species too few and indecisive to justify an unqualified opinion.* Professor Marsh also reached a similar conclusion much more decisively from mammalian fossils from beds just above the unconformity which he referred approximately to the horizon of the London clay or the base of the Eocene, f The physical break which separates these divisions of time is of wider distribution and more emphatic than was supposed when first detected, for the Upper Cretaceous (=Lara-mie) beds are often greatly flexed and eroded beneath the Tertiary, and these occurrences are frequent throughout the province. Very often, and probably in most of the exposures distant from the mountains, the contact is apparently conformable, for the obvious reason that neither series has been sensibly disturbed from original horizontality, or the disturbances have been of late occurrence, involving both series alike. The separation in such cases then becomes a purely lithological one, or sometimes none can be detected. The fossils do not indicate any break, since the base of the Tertiary and the summit of the Cretaceous are lignitic, and furnish only brackish-water mollusca, which are indecisive and have a very great vertical range in nearly all the species. * Invertebrate Palaeontology (1876), Dr. F. V. Haydey's Survey, pp. xlvii et seq. tExpl. 40th Parallel, C. King, vol. ii, p. 329. |
