DEBRIS-COVERED MESAS OF BOULDER, COL. 509 



as they more effectually lowered their channels a new grade was 

 produced in the mesa zone, represented now by the mesa-terrace. 

 At the same time this new grade was covered with material 

 formed by a mingling of the old mesa-tops debris with the 

 fresher material from the mountains. 



The process of forming the grade and covering it with debris 

 in this manner may be studied in minutest detail at the present 

 day in Boulder Creek Valley. As the present streams are cutting 

 away the mesa-terrace and mingling its fragmental material with 



Section of formations shown in Fig. 2. 

 Fig. 3. 

 E — Fragmental debris. B — Red Beds. 



D — Fort Pierre Shale. A — Crystallines. 



C — Dakota. 



the fresh material from the mountains, so the more ancient 

 streams undermined the sheet of debris on the mesa-tops and 

 mingled it with that of the then flood plains. 



The production of the abrupt slopes forming the sides of the 

 mesas, and to a less extent bounding the mesa-terrace on the one 

 hand, and on the other hand the lateral inclination of the sur- 

 faces of both, is well illustrated in Boulder Creek Valley. Before 

 issuing from the crystalline area Boulder Creek has a high gradi- 

 ent and little of its energy is spent in cutting sidewise. Where 

 it passes to the shale formations at Boulder, its gradient becomes 

 lower and much of its energy is spent in cutting laterally. 

 The result is a comparatively slow lowering of the bed of the 

 stream and a comparatively swift migration laterally in the shale 

 region. The migration is at present toward the south. The city 



