It is evident that before the advent of the glacier a stream must have
flowed through the Still-Croton valley past the present divide in
order to have excavated the rock valley there found. The Housatonic
could not have flowed west through this valley if it was as narrow and
shallow as is indicated by known rock outcrops; the river could have
flowed through it only in a deep narrow gorge which was later buried
under drift, but the evidence at hand does not support this view.
[Illustration: ~State Geol. Nat. Hist. Survey Bull. 30. Plate III.~
Limestone Plain southwest of Danbury, in which are situated the
Danbury Fair Grounds and Lake Kanosha.]
It is most probable that this valley was made by the preglacial
Croton River. This explanation demands no change in the direction of
Still and Croton Rivers but calls for a divide at some point east of
the present one. From a divide between the Fair Grounds and Danbury, a
small stream may be supposed to have flowed toward the east, joining
the larger northern branch of the Still at a point near the middle of
the city of Danbury. The stream flowing westward from this divide
formed the headwaters of one branch of the Croton system.
The presence of till in a ravine can be used as a criterion for
locating the site of a former divide, for where till is present in
the bed of a stream the channel is of preglacial date. Where the river
crosses a divide it should be cutting through rock, though till may be
present on the valley slopes. Judged by this test, the old divide was
situated either just east of the Fair Grounds plain or at the east end
of the ravine described in the preceding topic. Of these two positions
the one near the Fair Grounds seems the more likely (fig. 1), for at
this place the river has excavated a recent channel with steep sides
in gneissoid rock. The absence of the limestone at this point may be
sufficient in itself to explain the location of the divide.
Exact measurements of the drift in the upper Still valley are needed
in order to establish this hypothesis completely and to plot the old
channel, but the position of the rock floor of the former channel
extending westward from the Fair Grounds may be fixed approximately.
The rock at the assumed divide now stands at 420 feet above sea-level
and it is reasonable to assume that ten feet has been removed by
glacial scouring and postglacial erosion, making the original
elevation 430 feet. The present divide between Andrew Pond and Haines'
Pond has an elevation of 460, but the bedrock at this place is buried
under 60 feet of drift, so that the valley floor lies at 400 feet.
According to these estimates the stream which headed east of the Fair
Grounds had a fall of 30 feet before reaching the site of the present
Haines' Pond (fig. 8, B).