SUMMARY 171 



forest species extend into the plains eastward over the higher 

 elevations of land and along the margins of the valleys, 



2. The altitude of most of this region lies above 3,000 

 feet. From the eastern boundary of Montana at about 2,000 

 feet the land rises gradually to the summit of the divide at 

 from 6,000 to 10,000 feet and drops more abruptly toward the 

 west to altitudes of 1,800 feet or less. The region is one of 

 greatly diversified topography with many high and some very 

 rugged mountain chains and intermountain valleys varying in 

 all widths up to 20 miles or over. The general course of the 

 ranges is from northwest to southeast. Geologically the surface 

 formations are largely of the Cretaceous and the Tertiary, with 

 some of more ancient origin. Most of the area has been glaci- 

 ated and extensive fields are covered with gravels. Mountain 

 glaciers still remain in some of the northern ranges. 



3. The climate is relatively dry, the mean annual precipi- 

 tation varying from 10 to 24 inches in most places with occasional 

 exceptions in the direction of the greater numbers. The rain- 

 bearing winds of the northern Rocky Mountains are from the 

 west and southwest. The mountain chains which lie across their 

 path receive heavier precipitation on their western slopes, re- 

 sulting in heavier forests upon the western side at elevations 

 mostly above 4,000 feet and correspondingly drier eastern slopes 

 and intermountain valleys with sparser woods or prairies. From 

 the standpoint of temperature the region is one of long winters 

 and short summers, a, condition accentuated with the increase in 

 altitude. In different places and at different altitudes tempera- 

 ture of -20 to -50 degrees are frequent but not continuous and 

 the winters are not especially severe. The effect of the lower 

 temperature is seen mainly in the shortening of .the growing 

 season and the relative frequency of summer frosts. Low rela- 

 tive humidity and a high rate of evaporation in summer and the 

 seasonal distribution of rainfall add their influence in deter- 

 mining the character of the vegetation. 



4. The sources of the vegetation of the region are mainly 

 two. The Atlantic flora and the Pacific flora meet in the 

 northern Rocky Mountains and overlap. Many trees, shrubs, 

 and herbs have entered from the west, either across the wide 

 basin of eastern Oregon and Washington at some earlier time 



