1 68 SURFACE GEOLOGY, 



ridge. On either side of it, nortli and south, prominences project 20 feet higher, malc- 

 ing them 91 feet above the base assumed at 0, and 132 feet above the river, or 182 feet 

 above the sea. 



Branches not adequately indicated on the map run off at various points and form 

 enclosed basins, which have no outlet except as channels have been cut through the 

 loose material of the ridges, either by natural or artificial means. Quite an extensive 

 body of water was included, till long after the settlement of the town, in an enclosure 

 between b and c. It has been drained, partly by a channel of its own formation and 

 partly by artificial means, and is now occupied by a muck swamp, which is 20 or 30 

 feet deejj. A trigonometrical section of the west ridge, near the point c, gives the 

 height above this swamp 61 feet, with a base of 250 feet. The rate of descent from 

 the apex at this point to the base is therefore one foot in two. 



A few rods east of the point there are irregular remnants of ridges of the same 

 general character with the others, running south-east across the Shawshin river and 

 Boston & Maine Railroad. East of this railroad it is apparently pushed into a great 

 number of irregular prominences enclosing numerous bowl-shaped basins, one of which, 

 of oblong shape and about fifteen feet deep, is at the very summit, the rim of which 

 rises to a height of about 100 feet above the river. A mile south, at Pomp's pond, and 

 partially connected by intervening ridges, is a similar cluster of rounded hills and en- 

 closed basins, surmounted by a sharp peak of still greater height. 



We should also observe that clusters, or ganglions, of such irregular ridges, encircling 

 bowl-like reservoirs and rising into sharp peaks, occur at frequent intervals along the 

 whole belt of the formation we are describing. Frequently, as a ridge is suddenly 

 pushed up into a pinnacle, it will put out a spur, returning to itself and forming a 

 closed basin at or near its top. 



These ridges are ordinarily composed of sand, gravel, and pebbles, the latter from a 

 few inches to two or three feet through, sometimes irregularly stratified, the coarse 

 material being as likely to abound near the top as at the bottom ; at other times, 10 or 

 15 feet or more in thickness will give no signs of stratification whatever. The top of 

 the ridge is usually just wide enough for a foot-path ; and pebbles a foot or two in 

 diameter dot its course at frequent intervals. Usually, also, the base of the ridge is 

 partially hid by subsequent accumulation of stratified sand and fine gravel, or by peat- 

 bogs. 



Another point of importance is, that the fi-agments of rock in the ridges are nearly 

 all somewhat rounded and apparently water-worn, though it is evident that they have 

 not all been subjected to the same amount of attrition. I have searched in vain among 

 the ddbris of the formation for scratched stones, though striated stones are found in 

 abundance near the surface in the immediate vicinity. Furthermore, the pebbles are 

 not of local origin. Merrimack slate abounds, as does a porphyritic gneiss, whose po- 

 sition is well determined in central New Hampshire. In Topsfield, a portion of the 

 pebbles are cleariy from ledges only a few miles to the north-west. 



The formation does not lie at a uniform altitude above the sea, but rises over hills 



