GLACIAL DRIFT. 251 
a synclinal basin; second, whether it might result from the removal of a 
soluble mineral like limestone. Running water excavates rocks only 
where a descending current can act; hence it could never remove any- 
thing from beneath its level. Ordinary lake ice cannot excavate beyond 
a few inches below water level. Nor could the ocean, if it were present, 
excavate any deeper. There is no other agent left for us to employ, 
except the glacier. When this starts, having a strong force to urge it 
onwards, it will plough out rocks in its descent, and then move up hill. 
This sort of denudation is fitted to excavate lake basins. 
There are numerous examples of these basins excavated out of the 
rock for lakes and ponds in our state. I purposed to make a catalogue 
of them, to be presented here, specifying all whose outlet flowed over 
ledges, and those where till or modified drift constitute the barrier. I 
found, on studying the subject, that a barrier of lower till would keep in 
the water as well as a ledge, so that the enumeration of ponds kept in by 
barriers of rock or till made little practical difference as to the stability 
of the water. Furthermore, a lake may be essentially kept in by a rock 
barrier, while the drift spreads an additional obstacle to the flowage out- 
wards; and, as scarcely any examples could be found of ponds kept in 
by barriers of loose materials, it may not be necessary to present a for- 
mal catalogue of the nature of outlets, citing, instead, a few familiar 
examples. 
Geological treatises make mention of Runaway pond in Glover, Vt., 
where a sandy barrier led to the sudden outrush of the water, after an 
attempt made to increase the size of the outlet by excavation. Similar 
floods have occurred recently by the removal of artificial dams, as in the 
well-known Mill River disaster near Northampton, Mass. We have a 
record of a similar flood on the Mascomy river, occasioned by the burst- 
ing of the original barrier of the lake at East Lebanon. The loose ma- 
terial was not abundant, so that little damage resulted. The Northern 
Railroad company have since renewed the barrier with better material, 
It is an interesting circumstance to recall the ancient barrier of till about 
a quarter of a mile below the present outlet (p. 216), whose removal in 
pre-historic times may have caused a great freshet. In Warren, the 
signs of the bursting of a barrier in ancient times are very distinct. A 
gravel ridge like a kame crossed the valley of Black brook, damming the 
