458 SINCLAIR— THE " TURTLE-OREODON LAYER." 



Oreodon beds capping flat-topped buttes south of the town of Scenic 

 and abounding in the tests of ostracod crustaceans, even in com- 

 pletely silicified portions of the rock, they are interpreted as deposits 

 formed in shallow pools on the surface of the level plain on which 

 the Oreodon beds were later deposited, as discussed below. Some 

 have been completely replaced by silica, showing white or brown 

 chert bands. Titanothere bones in place have been seen at several 

 localities but a few feet below these white lenses. Above, the " red 

 layer " is usually overlain by a zone of greenish nodules and sandy 

 lenses in a green clay about 17 feet thick where measured in the 

 eastern part of the basin of Indian Creek. The difference in lithol- 

 ogy and color and the extreme poverty in fossils of this green 

 noduliferous zone, in striking contrast with their abundance but a 

 foot or so below, makes the identification of the upper limit of the 

 horizon in question an easy matter, and the collector is always eager 

 to return to it after his excursion into the barren layer above. 



In Pennington County the " turtle-oreodon layer " is usually a 

 harsh-feeling pinkish-gray clay with greenish banding and mottling, 

 containing in the uppermost six or seven feet one or more zones of 

 calcareous concretions (PI. VII., Fig. i), frequently inclosing turtles 

 or Oreodon and other skulls and partial skeletons. The pinkish 

 tinge is much more apparent when the beds are wet after a rain. 

 They abound in rolled clay pellets best seen in the nodules which 

 are merely local hardenings of the clay without concentric structure. 

 Possibly the inclosed fossils have at times controlled the accretion of 

 calcareous material, but very many contain no fossils nor any visible 

 central nucleus. They vary in diameter from a fraction of an inch 

 to several feet. Their surface may be stained a rusty brown and is 

 pitted with depressions left by the weathering out of the clay pellets 

 just referred to. Sometimes a calcified root tracery is visible on the 

 surface of a concretion. Occasionally, as observed at several places 

 in the basin of Indian Creek, they fuse into a solid sheet of rusty, 

 clayey limestone a few inches to a foot or more thick, from which 

 fossils are absent, but with return to the condition of separate 

 nodules fossils reappear. 



In Pennington County the nodules are locally cut out, either alto- 



