ABT. 18 ON A NEW SKELETON OF DIPLODOCUS GILMORE 6 



stone. The whole geological section, beginning with the Triassic 

 and extending upward successively through the marine Sundance. 

 Beckwith (Morrison), Dakota, Mowry shales, and Frontier forma- 

 tion, is steeply tilted with a strong dip to the south. The dip reaches 

 an angle of 60° or more above the horizontal. 



Although fossil bones have been found at several other levels, 

 nowhere are they so abundant or so well preserved as in the sand- 

 stones previously menti<med. The outcropping ledge formed by this 

 layer of fossil-bearing sandstone, which weathers brown, can be 

 easily traced for a mile or more both east and west from the quarry, 

 and fossil bones are evident everywhere. 



In the quarry there is a veritable Noah's Ark of the animals of 

 this period. Here was found the largest of the giant sauropodous 

 dinosaurs closely mingled with remains of the smaller but powerful 

 carnivorous forms and those of the slow and heavy-armored 

 /Sfegosaurus, as well as the lightest and most birdlike dinosaurs. 

 Intermingled with these are occasionally found turtle shells, croco- 

 dile remains, arid fossil wood. 



Some of the skeletons are essentially complete, with most of the 

 bones properly articulated, but more frequently only a third or a 

 fourth of a skeleton remains, such as a complete tail, a section of the 

 back, a neck, or a complete limb or foot. Some few of the bones are 

 badly crushed, but on the whole they are quite free from distortion. 



The character of the sediments appears to represent the area of an 

 old river bar, which in its shallow waters arrested the more or less 

 decomposed carcasses collected from many points upstream as they 

 drifted down toward it. Thus were brought together the animals of 

 a whole region — a fact which vastly enhances the interest of this 

 deposit. The final part of the story necessitates a rapid covering of 

 the stranded carcasses by sand and other river sediments in order 

 that the bones of the skeletons should become fixed in their relative 

 positions before decomposition of the ligamentary attachments 

 allowed them to shift out of position. That many of the larger 

 skeletons were not completely covered immediately is shown by the 

 fact that the bones of the lower side remain undisturbed while those 

 of the upper or exposed side often show much displacement of parts. 

 That this scattering of the bones was due to current action is indi- 

 cated by the fact that wherever shifting has taken place they will 

 invariably be found to the eastward of the main portion of the 

 skeleton. In other words, the direction of the current was from the 

 west toward the east. Current action is further indicated by the 

 character of the sediments in which the bones are embedded; that is, 

 by the strong cross-bedding and the assorting of the fine and coarse 

 materials of which the sandstone is composed. 



