250 C. SCHUCHERT MORRISON AND TENDAGURU FORISIATIONS 



RELATION TO ADJACENT FORMATIONS 



The entire Morrison is a fresh-water deposit overlying the marine 

 Sundance formation, and the opinion was general, nntil very recently, 

 that the two are conformable with one another. x4t first sight this con- 

 tinuity appears to be trne, and especially so in Wyoming. The Morrison 

 is, however, a transgressing fresh-water formation over the marine Sun- 

 dance and the latter, too, is a transgressing deposit over a planed floor. 

 The apparently conformable contact between them in Wyoming must, 

 therefore, in reality be a broken one and of the disconformable type. In 

 the field there is rarely any difficulty in distinguishing the two forma- 

 tions, for the Sundance is a fairly regularly bedded series of identical 

 materials over wide areas, is usually replete with marine fossils, espe- 

 cially belemnites, and with skeletons of ichthyosaurs (Baptanodon) and 

 plesiosaurs ; while the Morrison is composed of dissimilar sediments from 

 place to place and the fossils are always land animals, chiefly land plants, 

 dinosaurs, and fresh-water mollusks. The difficulty lies in ascertaining 

 the actual line of contact between the two formations of similar shales 

 and in the further condition that the marine fossils become scarce toward 

 the top of the Sundance, while the dinosaur bones are usually absent in 

 the lowest beds of the Morrison. When the Morrison begins with a more 

 or less thick and unfossiliferous sandstone and reposes on green shales 

 that have belemnites, oysters, or Camptonecies, it seems best to hold that 

 the sandstone is at the base of the overspreading fresh-water formation. 

 These sandstones are, however, very irregular in distribution, and when 

 they are of no great thickness and thin-bedded, then dependence for the 

 separation of the two formations remains with the fossils. They are 

 always the criterion for discerning discontinuity in conformable forma- 

 tions, and it is by no means easy on physical evidence alone to discern 

 a break or diseonformity in such a sequence. The writer's experience 

 in Wyoming is that the Sundance is always fossiliferous throughout, 

 though the fossils are common only locally and mostly so in the calcareous 

 concretions that are more apt to abound lielow the middle of the forma- 

 tion. 



In a conversation with Dr. W. T. Lee, the latter pointed out that as the 

 Morrison is more often not sharply distinguished by geologists from the 

 underlying Sundance, it may well be that the basal sandstones and other 

 earliest deposits now included in the Morrison will in the final analysis 

 prove not to belong to it. Such basal deposits are now included in the 

 Morrison because it is not vet known what else to do with them. 



