PLANT COMMUNITIES OF SANDY SOIL 315 



railway station at Georgetown. In the area of study the can- 

 yon side shows alternating gullies and ridges, the gullies having 

 the looser and more sandy soil. Ridges are often of solid rock 

 or rock fragments with only a thin coating of sand or gravelly 

 material. Storm waters bring down large amounts of sand, 

 forming in places wide fans. At a point a few hundred feet away 

 from one of these sand fans the same storm may make a tempo- 

 rary stream that carries gravel and rock fragments. Succeeding 

 storms do not always carry the same kind of material to the same 

 place, as storm waters are constantly making new channels down 

 the canyon walls. Thus a sand fan may come to cover up a 

 rock-wash fan or the opposite condition may occur. Because of 

 the steep slope there is little opportunity for accumulation of 

 humus . 



Georgetown, although at an altitude of 8500 feet is in the 

 Foothilll Zone (3) or Submontane Zone (7), as is shown by the 

 presence of plants that do not extend into the montane area: 

 Eriocoma, Bouteloua gracilis, Opuntia, Pediocactus, Anogra al- 

 bicaalis, Artemisia brittonii. The climate is cool, however, with 

 a mean July temperature of about 62° F. and a mean annual 

 temperature of 42°. The coolness of summer may be best ap- 

 preciated from the fact that only the most hardy garden vege- 

 tables can be grown, such as potatoes, beets, lettuce, radishes, 

 and peas. It is too cold for tomatoes, beans or maize, or for 

 apples, plums, and cherries. 



The annual rainfall is about 17 inches. April and May are 

 generally wet, June and July quite dry. There is considerable 

 wind, especially in winter and spring. Snow drifts to a consid- 

 erable extent and much of the ground is exposed a large part of 

 the winter. 



II. FLORA 



From the preceding sketch of physiography and climate it is 

 apparent that the flora of our sandy area must be meager, and 

 of xerophytic type. The most striking feature of the vegetation 

 is a community of xerophytic shrubs. About 12 species are 

 present all told. In referring to these and to other plants the 

 nomenclature of Rydberg (7) will be followed. 



