IQipl 



VESTAL— PHYTOGEOGRAPIIY OF COLORADO 



159 



parts than can be obtained by the Hnear arrangement. Geog- 

 raphers will note that topographic areas rather than land-forms 

 are used as the units of area of physical conditions (habitats), 

 since land-forms, such as mesas and ravines, may include several 

 topographic areas presenting quite diverse environmental con- 

 ditions. Moreover, a single topographic area, even if physically 

 uniform, may allow the growth within it of several more or less 

 distinct vegetation-types. 



;-*^-.T7.^^. 



Fig. 3. — Maturely dissected foothills near Boulder Creek: pine-sprinkled, rather 

 than forested, surface mostlv co\-ercd with drv grassland. 



A brief statement concerning mountain parks may be made. 

 These are small plains or flat valleys shut in on all sides by hills. 

 They are not well developed in the foothills as compared with the 

 montane zone. They are mostly formed where one of the principal 

 eastward flowing streams is joined by tributaries from valleys 

 opening into the park. There is a single outlet. Many of the 

 montane parks in the Front Range contain the terminal moraines 

 of former valley glaciers from above, and their topography is in 

 large measure the work of ice. The slight gradient causes many 



