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 width. 
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 /enticular 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. xix, pp. 
63-67). They seem to resemble the “ drums”’ or ‘‘ sowbacks”’ of the till in Scotland, mentioned in Geikie’s 
Great Ice Age. 
