HaTCHKR : TlIK JlRASSIC DiNOSAUR Df.posits. 331 



Dinosaur horizon in wliich remains of dinosaurs belonging to Jurassic 

 types are ijuite abundant. I have thought best to assign a thickness 

 of 450 feet to the Jurassic at this locality while limiting the Dakota 

 to the uppermost 200 feet of the sandstone and shale series. 



The Dakota may be seen as a rather thick, heavily bedded light 

 brown or white sandstone capi)ing the small detached tables which rise 

 above the summits of the bluffs on the west side of the creek at the 

 southern end of Garden Park. These sandstones are also conspicuous 

 in the cafion below the Park, where they form the uppermost part 

 of the canon walls and usually present a bold face with sheer es- 

 carpments often from fifty to one hundred feet in height. In places 

 they are very rich in the impressions of fossil leaves, but so far as at 

 present known they are remarkably destitute of all remains of animal 

 life, though a careful and continued search will doubtless yet bring to 

 light remains of the terrestrial animal life that must have lived in the 

 immediate region during their deposition. At the mouth of the canon 

 the rocks of the Dakota are inclined at a considerable angle and dis- 

 appear beneath the surface. The Dakota has been generally assumed 

 to be of fresh water origin, though the evidence in favor of this view 

 has been of a negative rather than a positive nature. For the most 

 part the materials of which these beds are composed would seem to 

 have been deposited in fresh water, but there has lately been discovered 

 some very strong evidence in favor of the marine origin of at least a 

 portion of the series. This will be referred to later when we come to 

 discuss the fossil deposits of the underlying series of sandstones and 

 shales. 



Tlie Benton Shales. — The close of the period which witnessed the 

 deposition of the Dakota sandstones was accompanied by a marked 

 change in the physical conditions that prevailed over this region, as is 

 abundantly emphasized by the nature of the materials composing the 

 rocks of the succeeding formation as well as by the character of the 

 fossils which they contain. For a long period during the Jurassic and 

 early Cretaceous the surface of this region was maintained at an ele- 

 vation for the greater portion of the time at least slightly above sea 

 level and the sandstones and shales were laid down along the shores of 

 adjacent seas, over the bottoms of smaller bodies of water or along 

 the courses and over the flood plains of running streams. That some 

 such conditions as the above attended the deposition of the materials 

 constituting the rocks of the Dakota and Jurassic formations is abun- 

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