460 SINCLAIR— THE " TURTLE-OREODON LAYER." 



vast extent of country, with everywhere the persistence of a rusty 

 nodular zone at its top, and its development over what appears to 

 have been a base-leveled surface (Fig. 2, PI. VII.). 



Before discussing the origin of the " red layer " a novel feature 

 should be mentioned, namely, the presence at several horizons in the 

 upper Titanotherium and lower Oreodon beds in the basin of Indian 

 Creek of calcareous crusts and nodules formed by the functioning 

 of blue-green algas, similar to the algal balls described by Professor 

 H. Justin Roddy from Little Conestoga Creek, Lancaster County, 

 Pennsylvania.^ One of these algal reefs occurs in the upper part 

 of the " turtle-oreodon layer," probably in Sec. 6, T. 4 S., R. 13 E., 

 Black Hills Mer., in the most easterly part of Indian Creek basin 

 about half a mile west of the narrow mesa known locally as Hart 

 Mountain. It is exposed along the south side of a minor badland 

 draw and occurs both in the pinkish-gray clays of the " turtle- 

 oreodon layer " and along the margin of a channel-filling of green 

 micaceous sandstone interrupting the upper part of the brown nodu- 

 lar zone. Laterally, the reef passes into the sandstone, some of the 

 algal masses rising as miniature islands through the sand. Most of 

 the algal growth here is in the form of a crust, only a foot or so of 

 its width remaining in place, the rest being scattered as talus blocks 

 over the underlying clays. At least three zones of rusty nodules are 

 present in the general vicinity of this reef. The nodules decrease in 

 number as the south edge of the algal crust is approached and almost 

 disappear. The reef first appears as a thin seam in the pinkish-gray 

 clays and rapidly thickens up to a maximum of two inches or so of 

 typically concentrically-banded, Stromatopora-Uke algal crust and 

 balls. About 50 feet north of the south edge of the algal crust it 

 abuts on the green channel sandstone already mentioned. The 

 whole deposit has a length from north to south of 225-250 paces and 

 is scattered as talus over a width of clay slope from 25 to 50 feet 

 beyond its outcrop. Rusty nodules of the type inclosing rolled mud 

 pellets are much less abundant throughout the area occupied by the 

 algal reef, because cut out by the channel with which it is associ- 

 ated. A few project from their clay matrix below the level of the 



1 Proc. Am. Phil. Soc, Vol. LIV., 1915, pp. 246-258, two figures. 



