The preglacial history of Rocky River as outlined assumes that before
the glacier covered this part of Connecticut the present lower course
of Rocky River was separated from the rest of the system by a divide
situated somewhere between the present mouth of the river and the
mouth of Wood Creek. It remains to be shown by what process Rocky
River was cut off from its southern outlet into Still River and forced
up its eastern branch and over the col into a tributary of the
Housatonic. Though the preglacial course of Rocky River appears to be
more natural than the present one, it is really a longer course to the
Housatonic; the older route being 32 miles, whereas the present course
is 19 miles. This fact explains, in part, why the glacier had little
difficulty in altering the preglacial drainage, and how the change so
effected became permanent. Eccentric as the resulting system of
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 plain on softer rock, worn down as low as the outcrops of more
resistant rock occurring farther down the valley will permit. The
reversal of the river may account for the sudden transition from a
flat-bottomed valley to a rocky gorge; and for the abrupt change in
the profile, bringing the steepest part of the river near its mouth.
The increased volume of water flowing through the channel since
glacial time has plainly cut down the bed of the ravine between
Jerusalem and the river's mouth, but the channel is still far from
being graded.
[Footnote 8: Report of soundings made in 1907 by T. T. Giffen.]