220 UNIVERSITY OF COLORADO STUDIES 



canyons, flow in a general easterly and northeasterly direction out to 

 the plains, in valleys not well marked, nearly all uniting with the St. 

 Vrain before reaching the county line. 



In the valleys and throughout the irrigated areas there are numerous 

 trees and shrubs, many of them exotic. Ponds and lakes abound, some 

 natural, others artificial. To the westward the plains pass abruptly 

 into the foothills, from which the mountains rise less abruptly to the 

 crest of the range. The mountain area is an ancient granite plateau 1 

 deeply dissected by canyons cut by the streams flowing outward to the 

 plains, the outlines being much more rounded than where similar gorges 

 are cut through horizontal stratified rocks. 



The mountains are covered by coniferous forests, in some places 

 quite dense, coming down to the bottoms of the canyons, where they are 

 interspersed with mountain maple, birch, alder, willow, narrow-leafed 

 cottonwood, aspen and other deciduous trees and shrubs. 2 The cotton- 

 woods and aspens are the only deciduous trees reaching much size and 

 only the latter are found up the mountain slopes and on the divides 

 between canyons. The coniferous forests are irregular, with many open, 

 treeless, meadow or park-like areas. 



Glacial lakes interrupt the courses of some of the streams above 10,000 

 feet. Lakes and ponds, probably mostly of different origin, abound on 

 some of 'the divides down to 8,000 feet or lower. Above timberline, 

 which varies from 10,500 to 11,500 feet, are open divides and rocky 

 gorges, with perennial banks of snow and ice in places favorable for 

 their preservation, including one well-developed glacier. Vegetation of 

 some kind, with attendant insects, extends to the rocky, serrated crest 

 of the range, affording food for birds even on the highest points. Con- 

 sequently some species are found on the range which are never seen in 

 the valley, while some of the valley species do not reach the range. 



Owing to the differences in altitude, temperature, etc., the study of 

 bird migrations is more complicated than in more level regions, there 

 being a sort of vertical migration as well as the regular northward and 



1 Fenneman, N. M., " Geology of the Boulder District, Colorado," U. S. Geol. Surv., Bull. No. 265, 1905. 



3 Young, Robert T., "Forest Formations of Boulder County," Botanical Gazette, Vol. XL VI, pp. 321- 

 352, 1907. Ramalev, Francis, "Woody Plants of Boulder County," University of Colorado Studies, Vol. V, 

 pp. 47-63, i9°7- 



