l80 SURFACE GEOLOGY. 



There is reason to believe that examples similar to the above exist by 

 the hundred. They are not usually visible, except where excavations 

 have been made. 



Planishing and Embossment. 



Should the earth be entirely removed from the ledges, the majority of 

 them, save where subsequent erosion has obliterated the marks, would 

 show upon their exposed sides a planing down and rounding. These 

 ledges are not planed down flat, like a floor, but are rounded, retaining 

 essentially their original forms. It is a common sight, when travelling 

 along the deeper valleys, to observe, above the line of the modified drift, 

 numerous rounded, dome-shaped ledges. A close scrutiny will disclose 

 the fact that these have been worn the most upon the northern side, 

 while the southern escarpment may be rough and uneven. Hence it is 

 obvious from which direction the force proceeded which planed down the 

 ledges. The sides most worn are those which have been struck. We 

 often speak of the struck or sfoss and the lee sides of these rounded ledges. 



The embossed ledges are often grouped in considerable numbers, 

 looking as if there were an assemblage of haystacks closely crowded 

 together. Precisely similar phenonena occur in the glaciated region of 



Fig. 50.— Embossed Rocks on Mt. Monadnock. 

 the Alps, where De Saussure applied to them the name of roc/ics mo7i- 

 tonnes. An example of them, as seen in New Hampshire, is given in 



