MOOK, STUDY OF THE MORRISON FORMATION 169 



siderable extent, but not completed. If the process is not interfered with 

 by diastrophism, or other violent disturbance, it is probable that oxidation 

 will continue and red color will be produced more and more, up to the 

 limits set by the nature of the material. Pure quartz sands or kaolinic 

 clays without iron cannot be oxidized to hematitic red beds under ordi- 

 nary conditions. The iron will have to be introduced from outside. 

 This probably accounts for the absence of red color in many of the very 

 fine green clays. 



Interpketation of the Moreison" Formation" 



Prom the foregoing facts recorded concerning the Morrison formatioi,!, 

 from the conditions which are known to prevail on modern flood-plains, 

 alluvial fans and deltas, and from previous knowledge regarding the dis- 

 tribution of land and sea in western N"orth America in Mesozoic times, 

 an attempt will be made to interpret the Morrison formation and to trace 

 the history of parts of western North America during middle Mesozoic 

 time. 



At the close of the Triassic period certain areas in western North 

 America were elevated. This is shown by the presence of folding in 

 Triassic rocks which are overlain unconformably by later beds and by 

 discouformable contact with Jurassic beds. Large areas near the Pacific 

 Coast were greatly affected and, as the effects are -^'isible in eastern Colo- 

 rado and New Mexico, the elevation was probably widespread. Erosion 

 progressed over the greater part, at least, of the western United States 

 until a peneplain was developed. Over this peneplain the sea advanced 

 in late Jurassic time, as shoAvn by Logan (1900, 10), coming from the 

 Pacific through Alaska and western Canada, and extending south into 

 the United States and covering practically the same areas that are occu- 

 pied by the states of Montana, Wyoming and Utah, with very slight 

 extensions into other states. The beds deposited at this time, or at any 

 rate part of them, now constitute the Sundance formation. Li areas 

 where these marine deposits were not laid do"\\Ti, such as most of western 

 Colorado, continental sediments were laid down. These continental sedi- 

 ments may be represented by parts of the La Plata sandstone. Beds with 

 Sundance fossils overlie the La Plata in some areas, however, so it is not 

 possible to correlate these formations directly. 



As the deposits immediately overlying the Simdance are of continental 

 origin in every area which has been described, possibly excepting the 

 Unkpapa sandstone of the Black Hills region, and it is probable that this 

 also is continental, it is evident that the sea withdrew from the Eocky 



