FOX HILLS AND LOWER MEDICINE BOW 37 



youngest mariiie zone of the Upper Cretaceous system of southern Wyoming and 

 northwestern Colorado. 



In southern Wyoming the plant remains were coUected from 4 localities in the 

 Hanna and Carbon basins. Here the plant horizons Hc within the lower third of 

 the Medicine Bow formation. Measured sections indicate the exact positions of 

 the plant-bearing beds with reference to the marine Sphenodiscus zone of Fox Hills 

 age. Evidence is prcsented for the recognition of thc Fox Hills formation, on both 

 lithologic and palseontologic grounds, in the strata formerly referred to the upper 

 Lewis or basal Mcdicine Bow formations. 



In northwestern Colorado the plant remains were obtained from 2 locahties 

 near Craig from bcds prcviously referrcd to the "Laramie" formation. The basal 

 sandy beds of this formation have yielded marine invertebrates, including the 

 diagnostic Fox Hills ammonite, Sphenodiscus. On the basis of stratigraphic, Utho- 

 logic, and palseontologic evidence the section is divided into the Medicine Bow 

 formation above and the Fox HiUs formation below, the latter lying conformably 

 above the Lewis shale as in southern Wyoming. 



The plant remains from the 6 localities are treated as a unit, referred to as 

 the lower Medicine Bow flora. Of the 64 recognizable forms 47 are referred to 

 species previously described elsewhere, 11 are described as new species, and 6 are 

 recorded as not specifically determinable. It is recognized that many of the generic 

 references are of doubtful botanic validity. Although more plausible generic alter- 

 natives are suggested for most of the species, few changes in nomenclature have 

 been made because of the lack of corroborative proof at the present time. In some 

 species the large suites of specimens obtained have made possible new descriptions 

 and re\asions of synonymous forms. From a quantitative viewpoint 11 species 

 are shown to be dominants on the basis of abundance of individual leaves and 

 occurrence at the largest number of localities. 



The structural characters of the dicotyledonous leaves in the flora indicate 

 warm temperate to subtropical conditions of growth, more nearly approaching the 

 latter. SimUar climatic impUcations are shown by the modern distribution of 

 famiUes and genera whose systematic positions appear certain, and of generic 

 alternatives suggested by simUarities between the fossU leaves and those of living 

 genera. 



The age of the lower Medicine Bow formation is rather definitely estabUshed 

 by its stratigraphic position, its relation to the Sphenodiscus zone and to Triceratops- 

 bearing beds, and its associated fresh-water molluscs. The Laramie and lower 

 Lance (HeU Creek member) floras are shown to be most nearly simUar to that of 

 the lower Medicine Bow and are considered essentially contemporaneous. Tliis 

 conclusion is supported by the relation of both the Laramie and the lower Lance 

 formations to the Sphenodiscus zone and Triceratops-hearing beds. From a pre- 

 liminary study of the Lance floras it seems evident that the Hell Creek flora is 

 essentiaUy of Laramie-lowcr Medicine Bow aspect (Upper Cretaceous) and the 

 upper Lance (TuIIock and Ludlow members) flora is of Fort Union (Paleocene) 

 character. The latter reference is in harmony with the evidence of the vertebrate 



