WEATHERING 



41 



Fig. 18. — Differential weathering. The limestone has 

 been dissolved, leaving the quartz veins projecting. 



Widening of Val- 

 leys. — Valleys are 



widened by the work 



of streams (p. 81), 



but a large part of 



the width of their 



upper portions is due 



to the work of the 



weather, which first 



disintegrates the 



rocks, after which 



rain, hillside creep 



(p. 31), and other agents bring the weathered material within reach 



of the stream which carries it away. 



Rock Mantle and Soil. — We have seen that everywhere on the 



earth's surface the rock is being broken to pieces by one or more of 



the agents of weathering. This results in the accumulation of a 



mantle of rock waste which 

 in the process of time would 

 cover the lands to a great 

 depth if it were not removed. 

 The thickness of the mantle 

 rock varies greatly. In tropi- 

 cal regions the solid rock may 

 not be encountered even at 

 a depth of 150 feet, and in 

 Washington, D.C, granite 

 can be excavated with pick 

 and shovel at a depth of 80 

 feet. In the Valley of Vir- 

 ginia and in the Blue Grass 

 regions of Kentucky a thick 

 layer of soil, representing the 

 insoluble portion of many 

 feet of limestone, covers the 

 underlying rock. Under 

 normal conditions (Fig. 21) 

 Fig. 19. — The more rapid weathering of t fr e SQl \ J s thickest on the 



a weak bed of limestone in the cliff has , 11 r 



formed the shelf. Helderberg Mountains, crests and at the baseS of 



neat Albany, New York. hills, and thinnest on the 



