y 42 Journal of Agricultural Research voi.vi.No. 19 



GEOLOGY OF ROCK CREEK REGION 



The geological formation 1 belongs to the lower part of the Montana 

 oroup of the Cretaceous, characterized by sandstones and carbonaceous 

 shales with local coal deposits. The depth of this deposit is said to 

 average over 305 meters, and there is no evidence that Rock Creek has 

 more than scratched the surface, so the soil of the region is of nearly 

 uniform character. Sandstones beneath the coal contain various fossils 

 of particular interest, because about 50 per cent belong to genera repre- 

 sented there to-day, as cottonwood (Populus spp.), alder (Alnus spp.), 

 birch (Belula spp.), willow (Salix spp.), and others. The rather soft 

 surface shales and sandstones are occasionally exposed, but more fre- 

 quently are covered with a fine gravel wash, somewhat disguised by 

 reason of the growth of short grasses and xerophytic shrubs. These 

 remarks, of course, apply only to the land above the stream. Rock 

 Creek, like all the streams in the Laramie Basin, flows through a valley 

 varying in width and filled with alluvial deposits, in many places 50 to 60 

 feet deep. The character of the upland soils is also typical of a great 

 deal of the Laramie Basin, so that from an agricultural standpoint the 

 results of this study are applicable to a much larger region than that 

 in which the actual observations were made. 



CLIMATE OF ROCK CREEK REGION 



The climate of Rock Creek is essentially that of the Laramie Basin. 

 Meteorological records kept at the State University at Laramie since 

 1 891 show an average rainfall of about 10 inches and a mean annual 

 temperature of about 40 F. The monthly averages for 15 years are 

 given in Tables I and II. 



Table I.- — Monthly means of precipitation {in inches) at Laramie, Wyo., 1891-IQ05 



January o. 23 May 1. 47 September o. 92 



February 34 June 1. 24 October 79 



March 83 July 1.40 November 22 



April 1. 14 August 99 December 33 



Table II. — Monthly means of temperature (°F.) at Laramie, Wyo., 1891-1905 



January 21.6 May 47.4 September 51.8 



February 20. 3 June 56. 6 October 42. 1 



March 28. 4 July 62. 3 November 31. 1 



April 37. 3 August 61. 9 December 21. 8 



The higher temperatures are of short duration, and the maximum 

 rarely reaches 90 F. All the nights are cool. The flora is thus, of ne- 

 cessity, composed of plants that have become adapted not only to the 

 low average temperature and the aridity of the plains but also to the 



1 Darton, N. H., and Siebenthal, C E. Geology and mineral resources of the Laramie Basin, Wyo. 

 A preliminary report. U. S. Geol. Survey Bui. 364, 81 p., 2 pi. 1909. 



