466 N. H. BARTON PALEOZOIC AND MESOZOIC OF "WYOMING 



the time, and there were wide, bare wash-slopes and mud-flats, as is indi- 

 cated by the frequent mud-cracks, ripple-marks, and impressions of 

 various kinds on many of the layers throiighout the formation. The 

 nearly general red tint of the deposits doubtless was the original color, for 

 it is present not only throughout the extent of the formation, biit also in 

 most beds through its entire thielmess, as is shown by deep borings. At 

 various times accumulation of sand and clay was interrupted by chemical 

 precipitation of comparatively pure gypsum in lieds ranging in thickness 

 from a few inches to more than 67 feet and usually free from mechanical 

 sediment. It is apparent tliat this gypsum is the product of evaporation 

 while mechanical sedimentation was temporarily suspended — a condition 

 indicative of greatly diminished rainfall ; otherwise it is difficult to under- 

 stand its nearly general purit}". The Chugwater red beds have been sup- 

 posed to represent the Triassic, but in part at least they are probably 

 Permian. Their deposition appears to have been followed by extensive 

 u])lift without local structural deformation, but with general planation 

 and occasional channeling which represents a portion of Triassic time of 

 unknown duration. It was succeeded by the deposition of later Jurassic 

 sediments. 



JURASSIC TO CRETACEOUS SEAS 



The absence of Jurassic sediments in the central and southern portions 

 of the Eocky Mountain province indicates that this region either was a 

 land area through oi;t Jurassic time or that any Jurassic deposits laid 

 down were removed by late Jurassic or early Cretaceous iiplift and ero- 

 sion. The southern margin of the area of known Jurassic deposition 

 passed across the middle portion of the Laramie basin, but it extended 

 farther south along the front range of the Eocky mountains to a point in 

 Colorado a few miles south of the Wyoming boundary. That this sub- 

 mergence was marine is indicated by the fossils. 



The area of Jurassic deposition extended far to the north through 

 "Wyoming and Montana, to the northeast beyond the Black hills, and 

 probably a hundred miles or more east of the Laramie mountains. The 

 iiuiterials are fine-grained, especially in the upper beds, which are mostly 

 shales, and therefore indicate the absence of strong currents. 



During the long portion of Mesozoic time which followed the marine 

 Jurassic there were deposits of various kinds, but generally uniform over 

 wide areas, gathered in a great series, beginning with such as are charac- 

 teristic of shallow waters along a coastal plain, passing into sediments 

 from deep marine waters, and changing toward the end to fresh-water 



