MORRISON AND SUNDANCE FORMATIONS 249 



Or, as Lee says, the strata are "uniformly variable/^ Mook states that 

 the coarsest material and the thickest beds of quartz sands and arkoses 

 occur in the western areas and in most places in the basal beds, though 

 arkosic sandstones are sometimes present in the middle zone. The fine 

 sediments, mainly of clays, are found throughout and comprise the 

 largest and most typical element in the formation. 

 The Morrison, according to Lee,^ 



"extends from Montana to New Mexico and from the Black Hills and easte]ii 

 New Mexico westward to Utah. ... It was probably deposited over flood- 

 plains on a nearly flat surface [as first held by Hatcher], in lagoons and tem- 

 porary lakes, and in marshes along sluggish streams," 



Mook* concludes that the original area o'f distribution of the Morrison 



"probably amounted to four or five hundred thousand square miles and per- 

 haps more." "The Morrison is essentially a broad alluvial plain, formed of 

 coalescing alluvial fans, and possibly a true delta in the southeastern areas." 

 "The thickness is much greater in the western areas than in the eastern and 

 there is a thinning out eastward, which is very gradual considering the dis- 

 tances involved." 



The same writer states that in the extreme west of Colorado and in 

 Utah the thickness varies from 400 to over 1,000 feet; in the Owl Creek 

 and Bighorn Mountains of Wyoming it is usually between 200 and 150 

 feet; near Great Falls, Montana, about 100 feet; in central Colorado 

 between 350 and 450 feet; in eastern Wyoming and Colorado about 200 

 feet; in east-central New Mexico between 200 and 400 feet; and in tlie 

 Black Hills of South Dakota less than 100 feet. 



Lee's ideas of the physiographic environment of the Morrison are bonio 

 out by LulFs conclusions^ that 



"the habitat of the Sauropoda may best be visualized by imagining conditions 

 such as now exist in tropical America, more especially over the coastal plain 

 of the lower Amazon : low-lying lands, but little above sealevel, with sluggish 

 bayous separated by numerous islands clothed in a dense tropical vegetation. 

 In these fastnesses the creatures would be comparatively safe from their car- 

 nivorous enemies, while in the quiet waters they would find support for their 

 huge bodies both against the burden imposed by gravity and the warning pangs 

 of hunger." 



' W. T. Lee : Reasons for regarding the Morrison an introductorj' Cretaceous forma- 

 tion. Bull. Geol. Soc. Am., vol. 26, 1915, pp. 308-300. 



* C. C. Mook: A study of the Morrison formation. Ann. N. Y. Acad. Sci.. vol. 27. 

 1916, pp. 157, 43, 158, 113-115. 



^ R. S. Lull : Sauropoda and Stegosauria of the Morrison compared with those of 

 Europe and eastern Africa. Bull. Geol. Soc. Am., vol. 26, 1915, pp. 824-325, 



; XX— Bull. Geol. Soc. Am., Vol. 29, 1917 



