SOME INSTANCES OF MODERATE GLACIAL EROSION 165 



Larger valleys. — In the first place, the major valleys all antedate 

 the ice-advance, for they extend at all angles to the direction of ice- 

 motion, and have all the characteristics of subaerially formed valleys. 



Angular cliffs. — A second evidence is furnished by angular cliffs, 

 especially on lee slopes. These vary in elevation from an inch or 

 two to a score of feet or more, being due either to the influence of 

 joints or of dikes. They are seen in all parts of the Cape, the smaller 

 instances being revealed where quarrying operations have recently 

 stripped rock-surfaces bare. 



Some of the smaller cliffs may be the result of plucking, but the 

 larger ones are certainly not of this origin. The smaller cliffs are 

 often associated with incompletely developed roches moutonnees sur- 



Fig. 3. — To show the relation of roches moutonnees surfaces and minor cliffs to 

 the concentric jointing (dotted lines) of the Cape Ann granite. 



faces, which conform closely in outline to the concentric joints of 

 the granite (Fig. 3). Viewed from the stoss side, these surfaces are 

 typical roches moutonnees, but from the lee side they appear angular, 

 with small cliffs extending from one roches moutonnees down to the 

 next lower joint plane. 



Dikes. — In a number of places there are small valleys where dikes 

 cross the granite, and these occur at all elevations, even above the 

 level at which the sea formerly stood. They extend at all angles to 

 the direction of ice-motion, and, being occupied by drift, are not of 

 postglacial origin. Every evidence proves these valleys to be due 

 to greater decay of the dike than of the inclosing granite; and that 

 the ice should not have eroded the granite down to the level of this 

 dike decay is proof of its slight erosive power at this point. 



At the Rockport granite quarry one of these decayed dike valleys 

 extends up to the edge of the quarry, and, while the residually decayed 

 surface material is gone, there is evidence of some decay in the dike 

 even to the bottom of the quarry. This decay consists of the develop- 



