A reversal of Still River may be explained by glacial scouring which
caused the northern end of the valley to become lower than the present
divides at West Redding and Mill Plain. The evidence of such scour
should be an overdeepened, U-shaped main valley and ungraded
tributaries.
The northern part of Still River valley has not the typical U form
which results from glacial erosion. As contrasted with the U-shaped
glacial valley and the V-shaped valley of normal stream erosion, it
might be called rectangular so sharply does the flat valley floor
terminate against the steep hillsides. The floor is too smooth and
flat and the tributary valleys too closely adjusted to the variant
hardness of the rocks to be the work of such a rough instrument as the
glacier. A level so nearly perfect as that of the flood plain is the
natural result of erosion of soft rock down to a baselevel, whereas
glacial scouring tends to produce a surface with low rounded hills and
hollows.
Overdeepening would be expected, because glaciers erode without
reference to existing baselevels. That a river valley should be cut
out by ice just enough to leave it graded with respect to the main
valley would be an unusual coincidence. This is what is found where
the Still River valley joins the Housatonic, and it indicates normal
stream erosion. Also, if the limestone of the northern Still River
valley were gouged out by the glacier, the action would in all
probability have been continuous in the limestone belt to the north
of the Housatonic, and where the belt of soft rock crosses the
Housatonic the river bed would be overdeepened. Although the valley of
the Housatonic near New Milford is very flat, as is natural where a
river crosses a belt of weak rock, the outcrops are sufficiently
numerous to show that it has not been overdeepened. The limestone area
along the East Aspetuck is largely overlain by till, but here again
the presence of rock in place shows that the valley has not been
overdeepened. Moreover, limestone boulders in the southern part of
Still River valley are not as abundant as they should be under the
hypothesis that the northern part had been gouged out extensively.
That the northern part of the Still River valley was not deeply
carved by ice is shown also by the character of the tributary streams.
The three small brooks on the west side of the valley, near Beaver
Brook Mountain, were examined to see if their grades indicated an
over-deepening of the main valley. These streams, however, and others
so far as could be determined, were found to have normal profiles;
that is, their grades become increasingly flatter toward their mouths.
The streams are cutting through the till cover and are not building
alluvial cones where they join the lowland. All their features, in
fact, are characteristic of normal stream development.
Throughout the length of the valley, rock outcrops are found near the
surface, showing that the changes produced by the glacier were due to
scouring rather than to the accumulation of glacial material. Except
where stratified drift is collected locally in considerable quantity,
the glacial mantle is thin. On the other hand, it has been shown that
glacial gouging was not sufficient in amount to affect the course of
the stream. The glacier simply cleaned off the soil and rotten rock
from the surface, slackening the stream here and hastening it there,
and by blocking the course with drift it forced the river at several
places to depart slightly from its preglacial course.
The evidence shows, therefore, that if Still River has suffered
reversal, glaciation is not responsible for the change, and thus the
first two hypotheses for explaining the history of the valley are
eliminated. There remain for discussion the third and fourth
hypotheses; the former being that reversal was effected in a very
early stage in the development of the drainage, the latter that no
reversal has occurred. The choice between these two hypotheses rests
on evidence obtained in the Umpog, Croton, and other valleys of the
Danbury region. This evidence is presented in the three following
sections, after which the former courses of Still River will be
discussed.