44 
The Colorado Experiment Station 
western Dolores and Montezuma Counties is largely dependent 
upon the stored moisture of winter snowfall. 
Greasewood Shrub-Steppe (Fig. 14).—This type of vegetation 
is widely distributed throughout the state, both east and west of 
the Continental Divide. It is not indicative of any particular set of 
climatic factors, but is called forth by local soil conditions. Grease- 
wood is almost always an indicator of a high water-table, and con¬ 
sequently of a soil quite rich in “alkali.” 
The most extensive stretch of greasewood in Colorado is that 
in the San Luis Valley, where there is an annual rainfall of less 
than 10 inches. Throughout some parts of this area, the level of 
ground water is kept high and within the reach of the roots of the 
shrubs by natural seepage conditions in the Valley. San Luis Val¬ 
ley is a natural basin, with drainage from the mountains on three 
sides, and although the surface layers of soil are kept dry by ex¬ 
cessive evaporation, both summer and winter, and by the scarcity 
Pig. 14.—Typical growth of greasewood (S 'arcobatus vermiculatus ) 
of rainfall, the subsoil often has a high water content. The level 
of the water-table has been elevated by the system of subirriga¬ 
tion long practiced in the Valley, and conditions have been made 
very favorable for the development of a greasewood type of vege¬ 
tation. In fact, this type of vegetation has widened its limits in 
the Valley, encroaching upon the rabbitbrush and sagebrush types 
of vegetation. So-called “seeped” land in the San Luis Valley in¬ 
variably runs to an almost pure stand of greasewood within twenty 
to twenty-five years. 
Although, as has been said, the presence of a greasewood type 
of vegetation is not a dependable index of climatic factors, here¬ 
with follows a table giving temperature relations of the very ex¬ 
tensive shrub-steppe of the San Luis Valley. From this table it is 
seen that the mean summer temperature (60.9°) is suitable for the 
maturing of the small cereals, except millets and sorghums, of po- 
