MUSEUM OF COMPARATIVE ZOOLOGY. 193 



on its western side, we see that the general motion was accompanied by 

 local and subordinate movements which were of a very complicated na- 

 ture. In the lower part of the large exposure just referred to, the 

 scratches, which are peculiarly well preserved over the entire surface, 

 have the general direction proper to the normal movement of the ice in 

 this held ; but within the space of a few feet they turn abruptly in the 

 manner indicated in Plate IV. The change in the direction of the 

 movement of the ice, which is indicated by the scratches in the distance 

 of about five feet, is as much as eighty degrees of arc. Unfortunately, 

 at the time of the observation the mass was broken away at the point 

 wliere the turn was made, but from the remains of the surface it is clear 

 tliat the total change in direction was accomplished within the distance 

 of ten and probably within less than five feet. It is difficult to account 

 for this sudden change in the course of the ice movement. The geueral 

 inclination of the surface on which it occurred is slight, the hill rising 

 on the eastward with a slope of not more than fifteen degrees, and to a 

 height of only about thirty feet above the place where the turn occurs. 

 The decline of the surface toward the valley on the westward appears to 

 have been equally gradual. 



It may be remarked, in passing, that sudden turnings of the ice where 

 it came in contact with slight obstacles appear to be much more com- 

 mon in Southern New England than in the more northern parts of the 

 continent. This may possibly be explained by the fact that the glacier 

 during the last stages of its movement in this southern district con- 

 tinued to advance even after it became relatively thin, and that in this 

 condition it was more influenced by local and slight irregularities than 

 was the case in the regions where it had a profounder depth. It is 

 not at all likely that the steadfast flow which conveyed the materials 

 of the great boulder train belongs altogether to the same part of the Ice 

 Period as these, the last formed scratches on Iron Hill. 



The surfaces of the hill which remain as they were left by the ice are 

 generally smooth. They exhibit, as is the case usually with massive rocks, 

 a breadth of level or gently curved faces on which are incised the shallow 

 depressions of the glacial scratches. Here and there we find cavities out 

 of which large fragments have been plucked by the dragging action of 

 the ice. \yherever these depressions left by the violently separated 

 fragments exist, we observe that the angles of the depression have been 

 more or less rounded by erosion subsequent to their removal. They 

 occur in all states of obliteration, some retaining their originally sharp 

 outlines, others being so far effaced as to be almost indistinguishable. 



