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GUIDEBOOK OF THE WESTEEIT UNITED STATES. 



14,179 feet) on the right. The lower slopes are more or less covered 

 with timber, which becomes scanty as the height increases, until 

 finally even the stunted balsams disappear (see PI. XLV, A, p. 92) 

 and at the summits there is nothing but wind-swept rock. The slopes 

 vary in color according to the light, at times being rich red or bright 



yellow in the strong sun- 



c — ^.^ — ^^/f^'/Z/Z/Z/A light and at others deep 



purple or a steelj' blue. 

 The color of the lower 

 sloj)es depends largely on 

 the vegetation, but that of 

 the upper slopes depends 

 on distance and light. In 

 spring and summer the 



FiGDKE 41.-Formatiou of a gravel terrace. shrubs and treeS prCSOUt 



many shades of green and j-ellow, but they are most brilliant in Sep- 

 tember, when the first frost touches them and tinges them with red 



or gold. 



The railroad follows the valley up to the village of Poncha, where 

 the road to Marshall Pass turns to the south (left), but a branch 

 Poncha. keeps straight ahead to the mining town of ]M6n- 



Eievation T,4so feet, arch, 15 miles distant, where it ends. From Monarch 

 Population (in- the principal highway between Salida and the Gun- 



spring^s) 323.*^ ^ uisou Valley is an automobile road across the range. 

 Denver 220 miles. The Marshall Pass line turns to the south in a 



broad curve and begins to climb the range. For half a mile it cuts 



the material would fill this pond and 

 form a plain that would stretch en- 

 tirely across the valley. The result is 

 shown by section A, in figure 41, in 

 which the valley is represented as 

 filled with gravel and sand, forming a 

 plain a-b, over which the stream flows 

 at c, far above bedrock. If the stream 

 then succeeds in cutting through the 

 dam of lava it quickly trenches the 

 sand and gravel laid down in the pond, 

 except the parts that lie at some dis- 

 tance back from the middle of the 

 channel. The result is shown by sec- 

 tion B, in figure 41, in which the 

 stream has cut the trench d-f-e, leav- 

 ing d and e as terraces on the sides of 

 the valley, composed of sand and 

 gravel which the water has deposited. 

 Most of the terraces in the mountains 

 have had such an origin, except that 



the ponding has generally been due 

 not to lava flows but to the sinking 

 of the crust of the earth, which 

 would have the same effect as a lava 

 flow. In some places it may have been 

 due to a decrease in the volume of 

 water flowing in the stream, and al- 

 though at first thought this may not 

 seem to be comparable to the lava flow 

 in its effects, a careful study will show 

 that the carrying power of a stream 

 is directly affected by its volume and 

 grade, so that if its volume or its 

 grade is reduced its carrying power 

 will be reducetl — it will not be able to 

 sweep along the boulders that it had 

 before handled readily. A stream thus 

 reduced in volume or grade silts up its 

 bed, and if later its flow or grade is in- 

 creased it cuts away all this material 

 except the remnants that form terraces. 



