326 - BOTANICAL GAZETTE [November 



to 7^"^ by larger and smaller canons running from east to west, in 

 which flow the streams. Extending north and south from these 

 main canons are numerous smaller canons and gulches, so that the 

 entire region is very irregularly furrowed in all directions. The 

 foothill region is 6 to 8^°^ wide and at its summit reaches an altitude 

 of about 2250"^, although some of its individual peaks may reach a 



height of 2700"". 



From the western end of the foothills to the base of the main range 

 is a distance of about 16^"". This is the plateau region, ascending 

 gradually from 2250"" to 3000 "" at the base of the range. While 

 the surface of this region is much broken, it is less so than either the 

 foothills to the east of it or the main range to the west, and its general 

 contour is comparatively even. In its eastern part, however, where 

 the streams lie far below the general level, are found the deepest and 

 most rugged parts of the canons. In its western portion are many 

 beautiful little parks along the stream courses (fig. j) which afford 

 good crops of hay and excellent pasturage. Above 3000"^ the country 

 again becomes much broken, the steep and rocky slopes encountered 

 above timber-line culminating at the crest of the range in vast chasms 

 and inaccessible pinnacles of rock. Above 2400"^ the mountains 

 have been extensively glaciated. The ice sheet at the time of its 

 greatest extent covered all but the highest ridges above 2700"^, 

 while tongues of ice in some places extended down to nearly 2400"^. 

 As evidence of this condition we have today the vast morainal 

 deposits, the chains of little glacial lakes (fig. 4), and the existence 

 of a small glacier on the eastern slope of the Arapahoe Peaks some 

 32^"" west of Boulder. The highest summits in Boulder County are 

 Mt. x\udubon and the Arapahoe Peaks, with approximate altitudes 

 of 4050"^. 



The principal streams are North, ^Middle, and South Boulder 

 Creeks, Jim Creek, the North and South St. Vrains, and Left Hand 



^ 



Creek. All of these rise at the base of the range and flow eastward to 

 join finally the North Platte River. 



The dimate of the region is semi-arid. At Boulder, situated 

 directly at the base of the mountains, the average rainfall for nine 

 years has been 42.7^"^. In the higher mountains there is a much 

 heavier annual rainfall than this, the difference between Boulder 



