f
drainage is, it would have been still more so had Rocky River when
ponded overflowed at the head of its western instead of its eastern
fork, taken its way past Sherman into the Housatonic near
Gaylordsville, and discharging at this point lost the advantage of the
fall of the Housatonic between Gaylordsville and Boardman.

In glaciated regions an area of swamp land may be taken as an
indication of interference by the glacier with the natural run-off.
The swamp in which Wood Creek joins the upper fork of Rocky River
(fig. 1), was formerly a lake due to a dam built across the lower end
of a river valley. Although the ponded water extended only a short
distance up the steeper side valleys, it extended several miles up the
main stream. The whole area of this glacial lake, except two small
ponds and the narrow channels through which the river now flows, has
been converted into a peat-filled bog having a depth of from 8 to 45
feet.[8]

At the termination of the swampy area on the eastern branch of Rocky
River no indication is found of a dam such as would be required for so
extensive a ponding of the waters. Here the valley is very narrow, and
though the river bed is encumbered with heavy boulders, rock outcrops
are so numerous as to preclude the idea of a drift cover raising the
water level. This is just the condition to be expected if Rocky River
reached its present outlet by overtopping a low col at the head of its
former eastern branch.

The southern end of the Neversink Pond valley is the only other place
whose level is so low that drift deposits could have interfered with
the Rocky River drainage. The moraine at the head of this valley,
crossing the country some two miles north of the city of Danbury and
binding together two prominent north-and-south ridges, was evidently
the barrier which choked the Rocky River valley near its mouth and
turned back the preglacial river.

When Rocky River was thus ponded its lowest outlet was found to be at
the head of its eastern fork. Here the waters spilled over the old
divide and took possession of the channel of a small stream draining
into the Housatonic. Accordingly Rocky River should be found cutting
its bed where it crosses the former divide. It seems reasonable to
regard the gorge half-way between Jerusalem bridge and Housatonic
River as approximately the position of the preglacial divide and to
consider the small flat area to the north of Jerusalem bridge as a
flood pla