FORMATIONS OF THE FRONT RANGE 431 



In the Arkansas valley region it varies but little from 410 feet. The 

 three formations which Mr G. K. Gilbert has differentiated as Graneros 

 shale, Greenhorn limestone, and Carlile shale in the Arkansas valley 

 have been found to be easily traceable all along the Front range through 

 Colorado. 



GRANEROS SHALE 



This formation is a fissile, dark colored shale, generally uniform in com- 

 position and usually about 200 feet thick, though the amount increases 

 considerably west of Denver. In this region prominent zones of iron 

 concretions occupy a thickness of from 40 to 50 feet at a horizon about 

 200 feet above the base of the formation. East of Lyons, on Little 

 Thompson creek, the Graneros formation has a thickness of 525 feet or 

 more, with the following components : 



Feet 



Shales, dark above, lighter below 125 



Fossiliferous limestone J 



Dark shales 225 



Sandstone, partly thin bedded ... 15 



Dark shales, with few thin beds of sandstone 160 + 



GREENHORN LIMESTONE 



This widespread formation consists of impure shaly limestones 25 to 

 40 feet thick, in layers from 3 to 12 inches thick, separated by several 

 inches of shale, as shown in figure 1, plate 27. Figure 2 in this plate 

 illustrates the similarity of the formation in the Black Hills region. The 

 deposit is of light gray color when fresh and relatively soft when moist, 

 but it hardens on weathering, usually becoming sufficiently hard to 

 constitute a low but distinct ridge. 



The limestone is characterized by containing large numbers of Inoce- 

 ramus labiatus, which occasionally occur in other beds but are extremely 

 abundant at this horizon. The formation outcrops in a wide area in the 

 Arkansas valley in Colorado and Kansas. It is 75 feet thick and rises 

 prominently in the ridge behind the house at the Gateway to the Garden 

 of the Gods and outcrops frequently in the Denver region and northward. 

 East of Lyons it appears between the Dakota and Niobrara ridges with 

 a thickness of 25 feet, consisting of limestone layers 4 to 10 inches thick 

 separated by a few inches of dark shales and filled with Inoceramus 

 labiatus. West of La Porte it is 20 feet thick and presents its usual char- 

 acteristics and fossil. 



CARLILE SHALE 



This shale in its typical development about Pueblo and along the 

 Arkansas valley is from 175 to 200 feet thick, west of Colorado City it is 



