510 WILLIS T. LEE 



of Boulder is built upon the ground recently abandoned by the 

 stream. A north-south section through the city (see map, Fig. 

 i), cutting directly across this recently formed grade, shows a 

 slope of the surface to the south of nearly fifty feet to the mile. 

 The tops of the mesas and the surface of the mesa-terrace have 

 an inclination somewhat greater than this but in the opposite 

 direction. It is near the city of Boulder that the most conspicu- 

 ous slopes are found bordering the mesa- terrace. The transition 

 slope has been cut away and bluffs formed fifty feet high in 



Fig. i. — Diagrammatic section from the mesa (A) south of Boulder City, to the 

 northern border of the present valley bottom, illustrating the assigned origin of the 

 lateral inclinations of the mesa tops, mesa-terrace and valley bottom. y4=mesa; 

 Z?=mesa-terrace ; C=valley bottom on which the city stands ; i?=Boulder Creek ; 

 JF— Fort Pierre shale. The dotted lines represent the migrations of Boulder Creek 

 together with the gradual lowering of its bed, the two processes combined producing 

 the northward inclination of the mesa tops and the mesa-terrace, and the southward 

 inclination of the valley bottom. 



places. Should this process be continued to the extent of reduc- 

 ing the mesa-terrace to the lateral proportions of the mesas, the 

 transition slopes would all disappear, and the bounding slopes of 

 the mesa-terrace would present essentially the same aspect as 

 those of the mesas. The probable action of the stream in pro- 

 ducing the northern inclinations of the mesa-tops and the mesa- 

 terrace together with their bounding slopes, and the southern 

 inclination of the valley bottom by means of lateral migrations 

 is illustrated in Fig. 4. 



It is quite impossible to say what influence, if any, surface 

 movement has had in changing the inclinations of the surfaces 

 or determining special features. It seems, however, unneces- 

 sary to appeal to such movement, since the phenomena may be 



