Drainage Modifications and Glaciation in the Danbury Region Connecticut Part 4

Between Andrew Pond and Haines' Pond is the divide which separates the waters of the Still system from those of the Croton. It consists of a jumbled ma.s.s of morainal hills, seemingly of boulder clay, that rise from 50 to 60 feet above the level of the ponds. The divide is thus merely a local obstruction in what was formerly a through drainage channel.

THE STILL-CROTON VALLEY

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.

[Ill.u.s.tration: ~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 a.s.sumed divide now stands at 420 feet above sea-level and it is reasonable to a.s.sume 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).

GLACIAL LAKE KANOSHA

When the Croton Branch was beheaded by drift choking up its valley west of Andrew Pond, the ponded waters rose to a height of from 20 to 30 feet and then overflowed the basin on the side toward Danbury. The outlet was established across the old divide, and as the gorge by which the water escaped was cut down, the level of the ponded waters was lowered. At the same time, also, the lake was filled by debris washed into it from the surrounding slopes. Thus the present flat plain was formed and the old valley floor, a local peneplain developed on the limestone, was hidden.

DIVIDES IN THE HIGHLANDS SOUTH OF DANBURY

The mountain ma.s.s to the south and southwest of Danbury, including Town Hill and Spruce, Moses, and Thomas mountains, is traversed by a series of parallel gorges trending nearly north and south (fig. 2).

About midway in each valley is a col, separating north and south-flowing streams. Two of the valleys, those between Spruce and Moses mountains, and Thomas Mountain and Town Hill, form fairly low and broad pa.s.ses. They were examined to see whether either could have afforded a southerly outlet for Still River.

The rock composing the mountains is granite-gneiss and schist with an average strike of N 30 W, or very nearly in line with the trend of the valleys. The gneiss was found to be characteristic of the high ridges and schist to be more common in the valleys. No outcrops of limestone were found on the ridges, but at two or three localities limestone in place was found on low ground. From the facts observed it is evident that the stronger features of the relief are due to the presence of bodies of resistant rock, whereas the valleys are due to the presence of softer rock. The series of deep parallel valleys is attributed to the presence of limestone rather than schist.

The gorge between Spruce and Moses mountains, locally called "Sugar Hollow," narrows southward as it rises to the col, and the rock floor is buried under till and stratified drift to depths of 25 to 50 feet.

Nevertheless it is probable that the valley was no deeper in preglacial time than it is now. The plan of the valley with its broad mouth to the north favored glacial scour so that the ice widened and deepened the valley and gave it a U form. Scouring and filling are believed to have been about equal in amount, and the present height of the divide, about 470 feet, may be taken as the preglacial elevation. This is 70 feet higher than the rock floor of the divide at West Redding. The pa.s.s could not, therefore, have served as an outlet for Still River.

The valley west of Town Hill is similar in form and origin to Sugar Hollow. The water parting occurs in a swamp, from each end of which a small brook flows. The height of the pa.s.s in this valley--590 feet-- precludes its use as an ancient outlet for Still River. Likewise the valley east of Town Hill affords no evidence of occupation by a southward through-flowing stream.

THE ANCIENT STILL RIVER

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 relations.h.i.+p 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.

DEPARTURES OF STILL RIVER FROM ITS PREGLACIAL CHANNEL

Between Danbury and Beaver Brook Mountain the Still departs widely from its former channel, as shown in fig. 6. At the foot of Liberty Street in Danbury the river makes a sharp turn to the southeast, flows through a flat plain, and for some distance follows the limestone valley of the Umpog, meeting the latter stream in a swampy meadow. It then cuts across the western end of Shelter Rock in a gorge-like valley not over 200 feet wide. Outcrops of a gneissoid schist on the valley sides and rapids in the stream bear witness to the youthfulness of this portion of the river channel.

An open valley which extends from the foot of Liberty Street in a northeasterly direction (the railroad follows it) marks the former course of Still River, but after the stream was forced out of this course and superimposed across the end of Shelter Rock by the acc.u.mulation of drift in the central and northern parts of the valley, it was unable to regain its old channel until near Beaver Brook Mountain. The deposits of drift not only have kept the Still confined to the eastern side of its valley but have forced a tributary from the west to flow along the edge of the valley for a mile before it joins its master stream.

About a mile north of Brookfield Junction, Still River valley begins to narrow, and at Brookfield the river, here crowded to the extreme eastern side, is cutting a gorge through limestone. The preglacial course of the Still in the Brookfield region seems to have been near the center of the valley where it was joined by Long Brook and other short, direct streams draining the hillsides. The glacier, however, left a thick blanket of drift in the middle of the valley which turned the Still to the east over rock and forced Long Brook to flow for more than a mile along the extreme western side of the valley.

[Ill.u.s.tration: ~Fig. 9.~ Early stage of the Rocky-Still River, antedating preglacial course shown in figure 4.]

The broad valley through which the Still flows in the lower part of its course extends northward beyond it for over two miles, bordering the Housatonic River. At Lanesville near the mouth of the Still, the river has cut a gorge 30 feet deep and one-quarter mile long in the limestone. Upstream from this gorge the river meanders widely in a flat valley, whereas on the downstream side it has cut a deep channel in the drift in order to reach the level of the Housatonic. There is room in the drift-covered plain to the west for a buried channel of Still River which could join the Housatonic at any point between New Milford and Stillriver station. If the depth of the drift be taken at 25 feet, there would seem to be no objection to the supposition that the Still initially joined its master stream opposite New Milford, as shown in fig 6. After the limestone had been worn down to approximate baselevel, the tendency of the Still would have been to seek an outlet farther south in order to shorten its course and reach a lower level on the Housatonic. This stage in the evolution of the river may not have been reached before the ice age, and it is thus possible that glacial deposits may have pushed the river to the extreme southern side of its valley, superimposed it over rock, and forced it to cut its way down to grade.

SUGGESTED COURSES OF HOUSATONIC RIVER

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 a.s.sume 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 ma.s.sive granitic formation, as it does between Still River and Derby, without great difficulty (Pl. IV, A).

[Ill.u.s.tration: ~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 ma.s.ses 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.

[Ill.u.s.tration: ~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.]

GLACIAL DEPOSITS