288 SURFACE GEOLOGY, 



A special exploration has shown that these hills are also finely developed, 

 being more numerous but somewhat less massive, in. Merrimack, Hills- 

 borough, and Cheshire counties, and in many parts of central Massachu- 

 setts. They vary in size from a few hundred feet to a third or a half 

 mile in length, with usually about half or two thirds as great v/idth. 

 Their height, corresponding to their area, varies from forty or fifty feet 

 to one hundred and fifty or two hundred feet. But whatever may be the 

 size of these hills, they are singularly alike in outline and form, usually 

 having steep sides, with gently-sloping, rounded tops, and presenting a 

 very smooth and regular contour. From this resemblance in shape to 

 an elliptical convex lens, Prof. Hitchcock has called them lenticular hills, 

 to distinguish these deposits of glacial drift from its broadly flattened or 

 undulating sheets, which are common throughout the state. 



The lenticular hills have a well defined trend, which shows a very 

 notable parallelism with the striation of the rocks. Next to the coast it 

 is prevailingly north-west to south-east, while farther inland it has very 

 few exceptions from a nearly north and south course. In addition to the 

 occurrence of the glacial drift in lenticular hills, it is frequently amassed 

 in slopes of similar lenticular form. These have their position almost 

 invariably upon either the south or north side of the ledgy hills against 

 which they rest, showing a considerable deflection towards the south- 

 east and north-west in the east part of the state. It cannot be doubted 

 that the trend of the lenticular hills, and the direction taken by these 

 slopes, have been determined by the glacial current, which produced the 

 striae with which they are parallel. 



Slopes of till accumulated on the lee side of projecting ledges have 

 been described by European glacialists, the hill and the detritus sheltered 

 behind it being commonly known as "crag and tail." The greater por- 

 tion of these slopes which have been noted in New Hampshire are shel- 

 tered in this way; but about a third of them lie upon the northern side, 

 which was exposed to the ice-current. In rare cases these slopes have 

 gathered upon both north and south sides alike, blending together and 

 assuming the form of a lenticular hill of glacial drift, but having expos- 

 ures of ledge at the top. In many true lenticular hills outcrops of solid 



Natural History by Prof. N. S. Shaler (vol. xiii, pp. 196-203), and by Prof. C. H. Hitchcock (vol. xi.v, pp. 

 63-67). They seem to resemble the "drums" or "sowbacks" of the till in Scotland, mentioned in Geikie's 

 Great Ice Age. 



