POST-CRETACEOUS HISTORY OF WESTERN WYOMING loi 



10. Loomis, F. B. "Origin of the Wasatch Deposits," Am. Jour. Set., 4th 

 Ser., XXIII (1907), 356-64. 



11. Veatch, A. C, and Schultz, A. R. "Geology of Southwestern Wyoming," 

 U.S. Geol. Survey, Prof. Paper 56, 1907. 



12. Sinclair, W. J., and Granger, W. "Eocene and Oligocene of the Wind 

 River and Bighorn Basins," Bull. Am. Mus. Nat. Hist., XXX (1911), 

 83-117. 



li- . "Notes on the Tertiary Deposits of the Bighorn Basin," ibid., 



XXXI (1912), 57-67. 



14. Sinclair, W. J. "Some Glacial Deposits East of Cody, Wyoming, and 

 Their Relations to the Pleistocene Erosional History of the Rocky Moun- 

 tain Rdgion," Bull. Geol. Soc. Am., XXIII (1912), 731 ff. 



15. Umpleby, J. B. "An Old Erosion Surface in Idaho," Jour. Geol., XX 

 (191 2), 139-47. Also review by E. Blackwelder, ibid., XX (191 2), 410-14, 

 and reply in ibid., XXI (1913), 224-31. 



POST-CRETACEOUS GEOLOGIC HISTORY 



Antecedent conditions. — In order to understand the effects pro- 

 duced by the various events and changes after the Cretaceous 

 period, the reader should recall the general character of events before 

 that time and particularly the conditions which had been brought 

 about at the close of the Mesozoic era. These will be outlined with- 

 out evidence or argument. 



The oldest rocks of the district, generally referred to the Archean 

 system, were folded and metamorphosed, and subsequently worn 

 down to a peneplain before the middle of the Cambrian period. 

 Beginning with the Cambrian and extending on through all of 

 both Paleozoic and Mesozoic eras, sediments were spread over this 

 peneplained surface until it had been buried to a depth of many 

 thousand feet. Most of these sediments were deposited in the sea, 

 but noteworthy portions were laid down upon land. Occasionally 

 there were short episodes in which the region was subject to erosion, 

 but these periods were not accompanied by orogenic disturbances 

 and served to reduce the strata but little. The variety of sedi- 

 mentary rocks thus produced is shown in the accompanying colum- 

 nar section, and the importance of the varying erodability of the 

 different beds will be better appreciated when the discussion of the 

 modern physiographic forms is undertaken. 



Since the latest and thickest of these deposits are partly marine 

 and extend more or less uniformly over the entire district — except 



