LIFE HISTORY OF A MOUNTAIN 



333 



Sand carried by the wind is usually 

 raised from the ground, and has an up- 

 ward trend when striking the rock ; hence 

 the hollows it drills out are often arched 

 upwards. (See illustration page 248.) 



The resulting form of a moun- 

 tain, however, does not, as a 

 rule, depend on the action of one • 

 agent, either of uplift or decay, 

 but rather on the cumulative 

 and selective effects of many. 



A most important factor in 

 the control of land forms is the 

 kind of material on which the 

 agents work. 



Limestone, for instance, 

 usually weathers into broad up- 

 lands. The silence and loneli- 

 ness of the hills are never so 

 keenly felt as on these bare and 

 treeless heights. The very sound 

 of one's footstep is deadened 

 on treading the soft and vel- 

 vety grass which covers the 

 scanty soil in places where the 

 solid rock does not come to 

 the surface. Even the vallej's 

 are often waterless and silent. 

 A broad \aew over the upland 

 gives no sign of the deep gorge- 

 like hollows, and in traversing 

 the country we are not aware 

 of their existence until we stand 

 on the very brink of their pre- 

 cipitous slopes. And 3'et these 

 valleys show features which can 

 only be explained by water 

 action. If we come across a 

 streamlet on a limestone moor 

 and follow it down-stream, we find in 

 many cases it disappears suddenly into 

 a " swallow hole," and flows under- 

 ground through joints and caves. The 

 valleys represent thecourse of the drain- 

 age before it was diverted below the 

 surface. 



Chalk is chemically and, in its origin, 

 closely akin to limestone, but it is soft 

 and pulverulent, it is not strongly jointed, 

 and usually weathers into smooth, rolling 

 downs. 



Sandstone varies so much in texture 

 and powers of resistance that we some- 

 times find it in the form of great massive 

 hills like the Brecknocks, and at other 

 times it wears into low flat plains like the 



plains of Cheshire and Shropshire and the 

 Vale of York. 



The Pennine Chain is in large measure 

 composed of sandstones alternating with 

 shales ; the former give rise to l(jng 



IVLATTERHORN WITH ZMUTT GLACIER AND STREAM 

 BELOW, 



" The fragmental material is carried away and the rocks are left 

 exposed to fresh attacks." 



scarped hills, and the latter are reduced 

 to deep and fertile valleys. 



Slate hills have a tendency to assume 

 a conical form, and where precipices of 

 bare rock stand out on their flanks they 

 show sharp, saw-like edges. 



Under normal con^litions the above 

 rocks would be laid down in layers arranged 

 horizontally. But we have seen that in 

 mountain-building they become crumjiled. 

 contorted and fractured by internal forces, 

 and the work of denudation is profoundly 

 influenced by these conditions. 



Internal forces also manifest themselves 

 in the i)rotluction of great reservoirs of 

 molten rock behnv the surface. When 

 these communicate with the external 



