GEOLOGY AND PETROGRAPHY OF THE SUGAR- 

 LOAF DISTRICT, BOULDER COUNTY, 

 COLORADO 1 



By R. D. Crawford 

 I. Topography 



The Sugarloaf district is included in the region the topography of 

 which has been well described by Dr. Fenneman as follows : 



The belt between the restricted mountain ridge on the west and the foothills on 

 the east is 10 to 12 miles wide. Its generalized eastward slope is about 250 feet per 

 mile, or from one-fourth to one-half that of the range proper. This strip is cut by 

 narrow valleys to a depth of over 1,000 feet. The interstream areas are generally 

 steep and narrow hills without flat tops, though some ridges continue for several 

 miles with approximately level and unbroken crests. From any high point almost 

 all the summits appear to be in the same plane. The range proper on the west and 

 isolated lumps on the plateau rise with conspicuous abruptness, Sugarloaf Mountain, 

 the top of which is 1,000 feet above the general level, being a prominent example. 

 The topography at once suggests an imperfectly base-leveled surface that has been 

 deeply cut by streams whose erosive power has been increased by recent elevation. 3 



More in detail, the area described in this paper is drained by Four- 

 mile and Lefthand creeks with numerous short tributaries. The two 

 main streams flow easterly through the district where each has cut a deep 

 canyon with steep walls. The sides of Lefthand Canyon rise 1,300 

 feet within half a mile, and while the north side of Fourmile Canyon 

 is somewhat less abrupt, on the south side within three-fourths of a 

 mile Sugarloaf Mountain rises 1,800 feet above the bed of the stream. 

 Between the valleys the surface possesses less relief; so gentle are some 

 of the slopes that sufficient soil has accumulated to support a moderate 

 growth of pine trees, and in several places cultivated land produces fair 

 crops of grain and alfalfa. 



Several of the prominent peaks and ridges have resulted from the 

 high resistance of dike rocks. While pegmatite does not occupy the 

 highest points as do the eruptives, it has been an important factor in 



1 1 am indebted to Professor R. D. George for assistance in many microscopic determinations in the 

 preparation of this paper. 



' Fenneman, N. M , Bulletin 265, U. S. G. S., p. 11. 



97 



