As possible former outlets for the Housatonic, Hobbs has suggested the
Still-Umpog-Saugatuck valley or the Still-Croton valley (by way of the
East Branch Reservoir)[12], whereas Crosby has suggested the Ten
Mile-Swamp River-Muddy Brook-Croton River valley (by way of Webatuck,
Wing's Station, and Pawling), or the Fall's Village-Limerock-Sharon-
Webatuck Creek-Ten Mile valley.[13] The sketch map, fig. 10, indicates
the courses just outlined and one other by way of the Norwalk. The
latter is the route followed by the Danbury and Norwalk Division of the
Housatonic Railroad. It is natural to assume that the Housatonic might
have occupied anyone of these lines of valleys, particularly where they
are developed on limestone and seem too broad for the streams now
occupying them. Nevertheless, although each of these routes is on soft
rock and some give shorter distances to the sea than the present course,
it is highly improbable that the Housatonic ever occupied any of these
valleys. For had the river once become located in a path of least
resistance, such as is furnished by any of these suggested routes, it
could not have been dislodged and forced to cut its way for 25 miles
through a massive granitic formation, as it does between Still River
and Derby, without great difficulty (Pl. IV, A).
[Illustration: ~Fig. 10.~ Five suggested outlets of Housatonic River.]
An inspection of the larger river systems of Connecticut shows that
the streams composing them exhibit two main trends. Likewise, the
courses, of the larger rivers themselves, whether trunk streams or
tributaries, combine these two trends, one of which is
northwest-southeast and the other nearly north-south.
The north-south drainage lines are the result of geologic structure,
and many broad, flat-floored valleys, often apparently out of
proportion to the streams occupying them, have this direction. On the
other hand, the northwest-southeast drainage lines across the strike
of formations, coincide with the slope toward the sea of the uplifted
peneplain whose dissected surface is represented by the crests of the
uplands. The valleys of streams with this trend are generally narrow,
and some are gorges where resistant rock masses are crossed. The
northwest-southeast trends of master streams thus were determined
initially by the slope of the peneplain, whereas the north-south
trends represent later adjustments to structure.
It is concluded, therefore, that the Housatonic between Bulls' Bridge
and Derby (fig. 10), had its course determined by the slope of the
uplifted peneplain and is antecedent in origin. The old headwaters
extended northwest from the turn in the river near Bull's Bridge,
whereas that part of the river above Bull's Bridge was initially a
minor tributary. This tributary, because of its favorable situation,
in time captured all the drainage of the extensive limestone belt to
the north and then became part of the main stream. The lower
Housatonic, therefore, has always maintained its ancient course
diagonal to the strike of formations, and differential erosion, which
reaches its maximum expression in limestone areas, is responsible for
the impression that the Still River lowland and other valleys west of
the Housatonic may once have been occupied by the latter stream.
[Illustration: ~State Geol. Nat. Hist. Survey Bull. 30. Plate IV.~
A. View down the Housatonic Valley from a point one-half mile
below Still River station. Pumpkin Hill, a ridge of resistant
schist and quartzite, stands on right. A small island lies in
the river.
B. Part of the morainal ridge north of Danbury. Till capped by
stratified drift one mile north of Shelter Rock.]
[Footnote 12: Hobbs, W. H., Still rivers of western Connecticut: Bull.
Geol. Soc. Am., vol. 13, p. 25, 1901.]
[Footnote 13: Crosby, W. O., Notes on the geology of the sites of the
proposed dams in the valleys of the Housatonic and Ten Mile rivers:
Tech. Quart., vol. 13, p. 120, 1900.]