THE " TURTLE-OREODON LAYER" OR "RED LAYER," 

 A CONTRIBUTION TO THE STRATIGRAPHY OF THE 

 WHITE RIVER OLIGOCENE (RESULTS OF THE 

 PRINCETON UNIVERSITY 1920 EXPEDITION TO 

 SOUTH DAKOTA). 



(Plate VII.) 



Investigation aided by a Grant from the Marsh Fund of the 

 National Academy of Sciences. 



By WILLIAM J. SINCLAIR. 

 (Read April 22, 1921.) 



Perhaps the most abundantly fossihferous horizon in the White 

 River Ohgocene badlands of South Dakota is a nodular clay band 

 at the base of the Brule formation (Oreodon beds), vi^hich, from 

 the abundant remains of turtles and oreodons, the rusty color of the 

 fossils in general, and the pinkish-gray tone of the matrix, has long 

 been known to collectors as the " turtle-oreodon layer " or " red 

 layer." It is remarkably persistent throughout a region miles in 

 extent and, although known and collected from for the past seventy 

 years, is still a never-failing source of splendidly preserved material. 

 In thickness it varies considerably, values ranging from 29.5 to 39 

 feet having been obtained within a few miles of each other in Pen- 

 nington County in the Big Badlands, where the " red layer " is most 

 typically developed, and of 43 feet at Cedar Pass near Interior in 

 Jackson County. There is considerable facial difference between 

 these two areas, necessitating their separate description. 



In the Big Badlands, throughout the basins of Indian Creek, 

 Spring Creek, Bear Creek, Jones Creek and Cain Creek, in Penning- 

 ton County, the base of the " turtle-oreodon layer " lies immediately 

 above a white more or less completely silicified limestone in discon- 

 tinuous lenses, forming, wherever present, excellent horizon mark- 

 ers. From analogy with a similar material found in the middle 



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