A broad belt of limestone extends along the eastern side of the
granite ridge of Shelter Rock and in preglacial time formed a
broad-bottomed valley whose master stream had reached old age. When
the glacier came it hampered the drainage by scooping out the rock
bottom of the valley in places and by dropping deposits at the mouth
of Beaver Brook valley, thus forming Beaver Brook Swamp or "The Flat,"
as it is called (fig. 6).
Among the deposits at the southern end of Beaver Brook Swamp is
considerable stratified drift in the form of smoothly rounded hills or
kames, which are situated both on the border of the valley and in the
swamp. Till containing medium-sized boulders of granodiorite-gneiss
occurs along the road which borders the east side of the densely
wooded swamp.
Along the northeastern border of the swamp is a flat-topped terrace of
till, perhaps a lateral moraine, through which a small stream heading
to the north has cut a V-shaped ravine. A lobe of fine till extends
into the valley from the northeast and narrows the outlet.
Between the railroad and highway, which cross the northern end of the
swamp, is an irregular wooded eminence of rock, partly concealed by a
veneer of drift. Between this knoll and Shelter Rock are heavy
deposits of sand in the form of a short, broad terrace with lobes
which point into the Still River valley. A similar terrace is found to
the northwest on the opposite side of the valley.
At the northern end of Shelter Rock along the blind road leading to
the summit is a peninsula-like body of drift which contains huge
granite boulders mixed here and there with pockets of sand and gravel.
Stratified drift was found at the foot of the hill, and till overlying
it higher up. The more usual arrangement is boulder clay overlain by
modified drift, the first being laid down by the ice itself, the
second being deposited by streams from the melting glacier in its
retreat. Huge boulders, many ten feet or more in diameter, are strewn
over the northern slope of Shelter Rock.