The conclusion that the Still-Umpog was not reversed by a glacial dam
does not preclude the possibility that this valley has been occupied
by a south-flowing stream. It is probable that in an early stage in
the development of the drainage, the streams of the Danbury region
reached Long Island Sound by way of the Still-Umpog-Saugatuck valley.
Along this route, as described under the heading "The Still-Saugatuck
Divide," is a fairly broad continuous valley at a higher level than
the beds of the present rivers. A south-flowing river, as shown in
fig. 9, brings all the drainage between Danbury and the Housatonic
into normal relations.
This early relationship of the streams was disturbed by the reversal
of the waters of the ancient Still in the natural development of a
subsequent drainage. The Housatonic lowered the northern end of the
limestone belt, in the region between New Milford and Stillriver
village, faster than the smaller south-flowing stream was able to
erode its bed. Eventually a small tributary of the Housatonic captured
the headwaters of the south-flowing river, and by the time the latter
had been reversed as far south as the present divide at Umpog Swamp,
it is probable that the advantage gained by the more rapid erosion of
the Housatonic was offset by the Saugatuck's shorter course to the
sea. As a result the divide between Still and Saugatuck Rivers at
Umpog Swamp had become practically stationary before the advent of the
glacier.
The complex history of Still River is not fully shown in the stream
profile, for the latter is nearly normal, except in the rock basins in
the valley of the Umpog. This is due to the fact that changes in the
course of the Still, caused by the development of a subsequent
drainage through differential erosion, were made so long ago that
evidence of them has been largely destroyed.
The foregoing conclusion practically eliminates hypothesis IV--that
the Still developed from the beginning as a subsequent stream in the
direction in which it now flows. This hypothesis holds good only for
the short portion of the lower course of the present river, that is,
the part representing the short tributary of the Housatonic which
captured and reversed the original Still.