310 AV. T. LEE MORRISON A CRETACEOUS FORMATION 



The contrast between the Morrison and the overlying sandstone has 

 been pointed to 1)}' some geologists as evidence of a time break, and it 

 ndght be interpreted as such if it were not negatived b}- other evidence. 

 With few exceptions, the Morrison is present wherever the Dakota occurs 

 in the Eocky Mountain region, and these exceptions are found near the 

 areas over wliich the ^Forrison prolial)ly did not extend. Had any great 

 h:'ngth (if time intcrNciied lictwoon tlicsc roi'iiiiil imis, the soft IxmIs of tlie 

 Morrison would ]ii'ol);ilil\ have l)et'ii crodiMl ;i\\ay in some places; but jio 

 such place has yet been described. Be tbis as ii may, there is no escape 

 from the conclusion that the iMorrison, as a formation, is structurally 

 much more closely related to the overlying formations of Cretaceous age 

 than it is to underlying formations. This was recognized long ago by 

 Emmons (6, page 23), when he said: '^'From the point of view of the 

 stratigrapher, the assignment of the Morrison beds to the Lower Cre- 

 taceous rather than to the Upper Jurassic is much more desirable . . . 

 because it places the physical break whose effects are recognized over the 

 whole continent between these two great time divisions rather than in 

 the midst of one of them." 



The same idea was recently expressed by Ulrich (1, page 615), when, 

 in considering a question somewhat similar to the ])resent one, he con- 

 cludes : "Let us then be reasonal)le and practical and accept with proper 

 valuation these diastrophic boundaries which nature has most clearly and 

 widely indicated.'^ 



PHYSIOGRAPHIC CONDITIONS 



Some fifteen years ago, when I began a study of the Morrison forma- 

 tion, it was generally spoken of as a lake deposit. It was difficult to 

 conceive of a formation so wide-spread and so uniformly thin as having 

 been formed in a lake, and several lines of evidence indicated that the 

 formation is best explained as a series of fluviatile deposits. This hy- 

 pothesis seems to harmonize with all the facts that have been gathered. 

 It is evident to those who are familiar with the Morrison that the physio- 

 graphic conditions under which it was formed were very unusual. In 

 fact, it is doubtful if an area could be found at the present time that 

 would even illustrate them. It is cjuite impossible to adequately present 

 in a short paper the physiographic data affecting the Morrison problem, 

 but a brief review may be useful. 



The great sandstone formations of Carboniferous age that surround the 

 Eocky Mountains and the presence in them of coarse conglomerate indi- 

 cate that extensive highlands existed there in Carboniferous time. The 

 Triassic formations also are conglomeratic in some places; but such 



