310 LAND SCULPTURE 



been worn down to a low- lying, featureless surface, with only 

 occasional low protuberances rising above the general level. 

 Such a surface is called a peneplain, and represents what is usu- 

 ally the final stage of a cycle of denudation. Here and there an 

 isolated peak may remain high enough to deserve- the name of 

 mountain, which owes its preservation to the exceptionally resist- 

 ant nature of the rocks of which it is composed, or to its excep- 

 tionally favourable position with reference to the drainage lines. 

 A renewed upheaval of the peneplain will begin another cycle 

 of denudation, revivifying and rejuvenating all the destructive 

 agencies, and valleys and hills will be carved out of the approxi- 

 mately level surface. In a peneplain dissected by the revived 

 streams the sky line of the ridges is notably even, and all the 

 heights rise to nearly the same level. Differences of level are, 

 however, frequently produced by a warping process, which may 

 accompany the upheaval, raising some portions of the peneplain 

 to greater heights than others. Excellent examples of reelevated 

 and subsequently dissected peneplains are the uplands of southern 

 New England and the highlands of New Jersey. 



One agent of subaerial denudation has such a characteristic 

 and peculiar method of work that it requires a few words of 

 separate consideration. This agent is the glacier. In Chapter 

 VI these peculiarities were described, and it will suffice here to 

 recall them briefly. A glacier, in those parts of its course where 

 deposition does not occur, sweeps away whatever previous accu- 

 mulations of soil and loose debris it may encounter, laying bare the 

 rock. The rocks, hard and soft, are ground down, marked with 

 long parallel scorings, grooved, and polished. The valley in which 

 an Alpine glacier flows was made in the first instance by the 

 atmosphere and running water, but the moving ice has modified 

 it in several particulars. A glacial valley is widened, losing the 

 V-shape and taking on more the shape of a U. The longitudinal 

 slope of the valley is less continuous than in one made entirely by 

 water, being broken by ridges or escarpments of polished rock, 

 behind which are depressions. These depressions may be con- 

 verted into lakes when the glacier has retreated, but it is still a 



