508 WILLIS T. LEE 



some remnants south of this mesa. The mesa-terrace has a cor- 

 responding inclination. 



From the foregoing facts the conditions of formation may be 

 inferred : 



i. At the time the mountains began to assume their present 

 elevation they were faced, if not covered, with the sedi- 

 mentary formations whose truncated edges are now exposed 

 along the foothills. At an early stage of erosion the young 

 streams carried away the shales, since they lay uppermost, and 

 formed the grade upon which the sandstone debris was depos- 

 ited. The streams found little of a coarse or enduring nature 

 with which to form a deposit and thus prevent the cutting down 

 of the shales to a low gradient until they had cut back into the 

 Red Beds. 



2. When the erosion began to work effectively upon these, 

 much coarse and enduring material was loosened and carried 

 down the high gradient of the mountain flank, but the streams 

 were unable to carry all of the coarse parts of it< across the low- 

 ered gradient of the shale tract, and hence deposited it as the 

 mantle of the present mesa tops. 



3. As the streams by headward extension reached back into 

 the crystalline area, they derived a less relative amount of 

 material from the Red Beds and more from the crystalline area. 

 Besides, by reaching backward, their gradients had been reduced 

 and they reacquired some eroding power in the mesa zone, and 

 hence cut into it and formed the lower surface — the mesa- 

 terrace — on which is found relatively more crystalline material 

 and relatively less of that from the Red Beds. The granitic 

 material of this mantle being relatively old is much decomposed, 



4. The later deposits represent the process carried farther, 

 giving a still larger proportion of crystalline debris, and this, 

 being young, is undecomposed. 



At first Bear Creek was an important stream, as shown by the 

 wide gap between Green Mountain and South Boulder Peak. 

 But, for reasons unexplained, Boulder Creek and South Boulder 

 Creek, on either hand, gained in importance at its expense, and 



