STANTON: UPPER CRETACEOUS STRATIGRAPHY 57 



idea that there are no important or persistent sandstones between 

 the Dakota and the Fox Hills, while the fact is that in the area 

 of their map this interval includes two thick coal-bearing form- 

 ations in which sandstones are the dominant feature. 



Evidently at least one new standard section, with modified 

 nomenclature, was needed and this was furnished in southwestern 

 Colorado. Holmes and others on the Hayden Survey nearly 

 forty years ago had shown that the stratigraphic development 

 differed considerably from that of the sections in the upper Mis- 

 souri region and in Colorado east of the mountains. In the 

 course of the areal work under the direction of Cross, the grouping 

 and nomenclature of Holmes' section were modified and pub- 

 lished in the La Plata and Telluride folios^ from which the fol- 

 lowing descriptions are condensed (Section No. 2, p. 58) : 



Lewis shale. More or less sandy gray or drab shale with thin lenses 

 or concretions of impure limestone. Thickness in Durango quadrangle, 

 2000 feet. 



Mesaverde formation. Alternating sandstones and shales with occa- 

 sional marls or thin limestones and a number of coal beds. Lower 250 

 feet form a transition from Mancos shale followed by heavy sandstone 

 125 feet thick. At top a massive sandstone 25 feet thick. Total thick- 

 ness, 1000 feet. 



Mancos shale. Soft, dark-gray or almost black carbonaceous clay 

 shale containing thin lenses or concretions of impure limestone. Thick- 

 ness, 1200 feet. 



Dakota sandstone. Gray or rusty brown quartzose sandstone with 

 variable conglomerate. 100 to 300 feet. 



The names Lewis, Mesaverde and Mancos have since been 

 applied with varying success and acceptability thruout western 

 Colorado, northwestern New Mexico, eastern Utah, and south- 

 ern Wyoming. As described in the different areas each of these 

 formations varies greatly in thickness and considerably in lith- 

 ologic character. For the purpose of showing the nature of this 

 variation let us take the Mancos shale which over a large area is 

 limited above and below by the easily recognized Mesaverde 

 formation and the Dakota sandstone, respectively. In the type 

 section just west of the La Plata quadrangle the Mancos con- 

 sists of 1200 feet of dark gray shale containing some lenses or 

 concretions of impure limestone, but in the Telluride quadrangle 



^ Geologic atlas of the United States, Folio 60 and 57. 



