594 



PHYSICAL GEOGRAPHY. 



swiftly to the broader and gently descending expanses. Cases of the 

 former kind are in Wilton, Lyndeborough, and Mont Vernon. The 

 Souhegan and its tributaries have cut channels two or three hundred 

 feet deep out of a plateau. The most of the farms are upon the high 

 ground. Every river valley in the state illustrates the type of river 

 erosion peculiar to the rainy districts, or the broader instances just re- 

 ferred to, and it is not needful to specify examples. I do not know 

 that the erosion has been more thorough in those districts said to re- 

 ceive the greatest annual fall of rain. 



The action of glaciers like those in Switzerland has not been of great 

 importance in shaping our valleys, since so much more important results 

 have been produced by the "Drift." The local glaciers scoop out valleys; 

 they leave behind moraines, either irregular mounds or small ridges 

 athwart the streams, since cut through. Frequently the sides of the 

 valleys have been left vertical, with ice-markings upon the walls. These 

 mural surfaces are never extensive. 



Ocean action is peculiar. Rocks exposed to the waves usually present 

 a precipitous front, since the wearing away takes place only at the base 

 of the cliff. When the ledge or bank of earth has been undermined, the 

 top falls off, and thus a precipitous front is always exposed ocean-wards. 

 In studying the landscape back from the shore, these precipitous cliffs 

 may be seen where there has been a submergence in recent times, their 

 bases all occupying the same level. Ancient sea-beaches usually accom- 

 pany the former shore-line thus indicated. 



The most important sculptor has been the ice of the Drift period. 

 Thousands of facts will describe minutely all the phenomena of this 

 sort hereafter; but the style of markings left by them may be readily 

 recognized everywhere. I have for good reasons made a broad distinc- 

 tion between the Drift and local glaciers, the latter having exerted very 

 feeble influences as compared with the former. Every mountain and 

 rocky hill in the state, except the upper five-hundred feet of the Mt. 

 Washington cone, show the markings of this mighty rock-breaker ; and 

 therefore its influence has been more potent in giving the present shapes 

 to our scenery than that of all the other agents combined. 



The distinguishing mark of ice-action in the Drift period is the round- 

 ing and smoothing of the ledges. Invariably our rocks have been rounded 



