FOX HILLS AND LOWER MEDICINE BOW 7 



Lance age of the IVIedicine Bow formation has previously been discussed by Hares ^ 

 and substantiated by Dobbin and Reeside.^ 



NORTHWESTERN COLORADO 



The plant remains from this region were obtained from the succession of sand- 

 stones, shales, and coal beds occupying the same stratigraphic position above the 

 Lewis shale as do the sedinients of the Medicine Bow formation in southcrn Wyo- 

 ming. These sediments have hitherto^been referred to the Laramic or "Laramie" 

 formation. In view of their closer geographic position to the type locahty of the 

 Medicine Bow formation than to that of the Laramie formation, as now restricted, 

 it seems more appropriate to refer them to the former formation. Such a reference 

 is furthcr substantiated by the fact that the outcrop of these strata can be traccd 

 with only shght breaks ahi^ost continuously from the area in northwestern Colorado 

 to the western part of the Hanna basin in southern Wyoming. 



Attention in this region was centered about the area surrounding Craig, 

 Colorado, because of the previous reports of plant fossils in this region and because 

 of their occurrence in sedimcnts conformably overlying beds which have yielded a 

 Fox Hills fauna, including the index ammonite Sphenodiscus ^ which has been used 

 throughout my work as a reference zone. 



The first important contribution to the geology of this region was the report on 

 the Yampa coal field by Fenneman and Gale.^ The sediments conformably 

 overlying the Lewis shale were here referred to the Laramie formation on the basis 

 of stratigraphic position. A more comprehensive report by Gale on the coal fields 

 of northwestern Colorado and northeastern Utah was pubhshed in 1910.^ The 

 name Laramie formation was still retained for the strata above the Lewis shale on 

 the basis of a few species of invertebrates and plants. In 1925, Hancock, ^ who was 

 followed by Sears,' expressed doubt regarding the reference of these same beds to the 

 Laramie formation by placing the name in quotation marks. Additional marine 

 invertebrates, including the index species Sphenodiscus lenticularis (?), were ob- 

 tained by Hancock from the lower massive sandstones of his "Laramie" formation, 

 indicating the Fox Hills age of these beds. In the sections wliich we measured a 

 few miles west of Craig, additional Fox Hills species were coUected from the massive 

 sandstones above the Lewis shale. The same criteria for the recognition of the Fox 

 Hills formation as were appUed in southern Wyoming (see p. 6) were found to 

 obtain in this region. 



AU of the plant remains secured from this region were collected from horizons 

 in the lower Medicine Bow formation, lying within a few hundred feet above the 

 massive sandstones here referred to the Fox Hills formation. No attempt was made 

 to determine the upper limit of the Medicine Bow formation, although it clearly 



' Harea. C. J., Waehington Acad. Sci. Jour., vol. 5, 328-330. 1915. 



2 Dobbin, C. E., and Reeeide, J. B., Jr.. U. S. Geol. Survey Prof. Paper 15S-B. 23, 1929. 



'Hancock, E. T., U. S. Geol. Survey BuU. 757, 22, 1925; Dobbin. C. E., and Reeside, J. B.. Jr., op.cit., 23. 



« Fenneman, N. M., and Gale, H. S., U. S. Geol. Survey BuU. 297, 1906. 



' Gale, H. S., U. S. Geol. Survey BuU. 415, 1910. 



« Hancock, E. T., op. cit., 21. 



'Sears, J. D., U. S. Geol. Survey BuU. 751, 269-326, 1925. 



