252 SURFACE GEOLOGY. 



Stream and producing a pond. What circumstances destroyed this bar- 

 rier are not known, but the former existence of the pond attracts univer- 

 sal belief. Another case is in the edge of Lyman below Young's pond. 

 A new road follows the stream above the saw-mill, and the gravel hills 

 on both sides for a quarter of a mile look as if they had been cut apart 

 within the memory of the inhabitants. An old shore line, marking the 

 former height of the pond, is still quite conspicuous. As one passes the 

 south end of Station pond in Springfield, he is at a loss to see why the 

 water does not immediately make an opening for itself towards New 

 London. Upper Beech pond in Wolfeborough is an example of an exist- 

 ing pond kept in by a deposit of loose gravel (p. 128). There are many 

 instances of ponds in gravel plains kept in by loose materials when the 

 amount of water is not sufficient to lead to a sudden removal of the bar- 

 rier. Such are indicated on the general map by the absence of any outlet. 



Our largest lake, — Winnipiseogee, — must be a rock basin, although no 

 ledges appear at its outlet (p. 121). The land falls off abruptly for about 

 1400 feet at the Weirs, in slopes say 600 feet long on the north-west, and 

 800 on the south-east of the middle of the channel, the amount of the fall 

 being nearly 80 feet. As one looks at this from the east it resembles a 

 gorge excavated by running water, and it would not be strange if the 

 till had once been continuous across the present outlet. In that case 

 the outlet would have been from Alton bay towards the Cochecho river. 

 The present channel has been deepened about a yard by the Lake com- 

 pany. About 25 feet of till has been removed by erosion at the outlet 

 of Newfound lake. 



The majority of our ponds have ledges at their outlets. Those who 

 are interested in the subject can ascertain whether a given example has 

 been excavated from the ledges by observing the nature of the outlet. 

 It is very often the case that no ledge is visible about a pond, save where 

 the water discharges itself. If any dams are constructed upon loose 

 gravel or sand, it will be well for those interested in the integrity of the 

 works to examine them from time to time. 



Dispersion of Boulders. 



The second part of our subject relates to the removal of fragments of 

 stone from their original ledges, and to any peculiarities that may be 



