30 ILLINOIS STATE ACADEMY OF SCIENCE 



formed. On an average the valley floors have an eleva- 

 tion of 4000 feet above sea level, the passes 6000-7000 

 feet and the monntain summits 9000-10,000. There are 

 only six mountains in the park above 10,000 feet and of 

 these the highest, Mt. Cleveland, is 10,500. 



PLANT COMMUNITIES OF THE VALLEYS 



The dominant plant formation of the region is the 

 Eocky Mountain conifer forest which is found in the 

 river valleys and on the lower slopes of the east side of 

 the divide. The forest of the central valley and of the 

 west slopes is similar but has an admixture of many of 

 the species of the Pacific Coast. 



The vegetation of the Great Plains adjoining the east 

 front is of the grassland or prairie type. This consists 

 chiefly of several species of grass with some herbaceous 

 dicotyls, including a prickly pear cactus, and the 

 shrubby sage-brush. In the river valleys, cottonwoods 

 and willows fringe the streams and in smaller valleys 

 and gullies thickets of shrubby willows, alder, and aspens 

 are generally found. On the rolling hills adjoining the 

 east front, aspen thickets are found and on the summits 

 of the hills there are occasional stands of scattered 

 stunted conifers. 



In the ponds and marshy spots scattered over the 

 prairie and along the stream valleys are characteristic 

 aquatic communities. In most localities are the usual 

 swamp communities including cattails, arrowhead, water- 

 plantain, water smart-weed, and also such unusual forms 

 as the pincushion plant, the owl-clover, and several 

 genera of the evening-primrose family. Many of the de- 

 pressions are strongly alkaline with such halophytic 

 plants as salt-grass, shad scale, and the alkali buttercup. 



The conifer forest (Fig. 1) is found at its best in the 

 river and lake plains in the deep valleys of the east front. 

 Here the trees grow in close stand to a height of 60 or 70 

 feet, and there is a dense mesophytic undergrowth of 

 shrubs and ground plants. The chief trees are the lodge- 

 pole pine, the Englemann spruce, the Douglas fir or false 

 hemlock and the alpine fir. The low trees and shrubs in- 

 clude the mountain ash, mountain maple, the service-ber- 



