Drainage Modifications and Glaciation in the Danbury Region Connecticut Part 3

3. REGIONAL SLOPE NOT IN ACCORD WITH COURSE OF THE STILL

Although the regional slope of western Connecticut as a whole is contrary to that of Still River, there is no marked lowering of the hill summits between the source of the river and its mouth. As branches on the south side of the Housatonic are naturally to be expected, there is nothing unusual in the Still flowing in opposition to the regional slope, except that it flows toward the north instead of the northeast.

4. EVIDENCE OF GLACIAL FILLING AND DEGRADING OF THE RIVER BED

Hobbs has suggested that the waters of the Housatonic may have been ponded at a point near West Redding until they rose high enough to overflow into the "fault gorge" below Still River Station, thus giving the streams of the Danbury region an outlet to the Sound by this route. This hypothesis calls for a glacial dam which has not been found. It is true there are glacial deposits in the Umpog valley south of Bethel. The Umpog flows as it does, however, not because of a glacial "dam" but in spite of it. The river heads on rock beyond and above the glacial deposits and picks its way through them (fig. 7).

Drift forms the divide at the western end of Still River valley beyond Mill Plain, but the ponded water which it caused did not extend as far as Danbury (see discussion of Still-Croton valley). The Sugar Hollow pa.s.s is also filled with a heavy mantle of drift, but the valley is both too high and too narrow at the col to have been the outlet of the Housatonic.

It might be a.s.sumed that just previous to the advent of the ice sheet Still River headed south of its present mouth and flowed southward. In this case the Still, when reversed, should have overflowed at the lowest point on the divide between it and the Housatonic. It should have deepened its channel over the former divide, and the result would have been a gorge if the divide were high, or at least some evidence of river cutting even if the divide were low. On the contrary, Still River joins the Housatonic in a low, broad, and poorly drained plain.

The existing relief is due to the uneven distribution of drift. The river is now cutting a gorge at Lanesville, but the appearance of the valley to the west indicates that glacial deposits forced the river out of its former bed (fig. 6) and that no barrier lay between the preglacial Still River valley and the Housatonic Valley.

5. GLACIAL SCOURING

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 acc.u.mulation of glacial material. Except where stratified drift is collected locally in considerable quant.i.ty, 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.

THE STILL-SAUGATUCK DIVIDE

FEATURES OF THE UMPOG VALLEY

The valley of the Umpog, which extends from Still River to the source of the Saugatuck near West Redding (fig. 7), is a critical area in the study of the Still River system. It is possible that this valley once afforded an outlet for Still River, and it has been suggested that the Housatonic formerly followed this route to Long Island Sound. The relation of this valley to the former drainage system of the Danbury region demands, therefore, a careful examination of the features of the valleys occupied by Umpog Creek and the upper waters of the Saugatuck, and of the divide between those streams.

[Ill.u.s.tration: ~Fig. 7.~ Map of Umpog Swamp and vicinity.]

North of Bethel the Umpog occupies an open valley developed in limestone. Knolls of limestone rise to heights of about 40 feet above the floor of the valley and their upper surfaces are cut across the highly, tilted beds. This truncation, together with a general correspondence in height, suggests that these knolls, as well as the rock terraces found between Bethel and West Redding, and the limestone ridge which forms the divide itself, are portions of what was once a more continuous terrace produced by stream erosion and that they determine a former river level. The absence of accurate elevations and the probability of glacial scour make conclusions regarding the direction of slope of this dissected rock terrace somewhat uncertain.

As will be indicated later, however, it seems likely that these terrace remnants mark the course of a southward flowing river that existed in a very early stage in the development of the drainage.

South of Bethel the old Umpog valley, has lost from one-third to one-half its width through deposits of stratified drift (Pl. II, A and B). On the west, gravel beds lie against rock and till; on the east, deposits of sand and coa.r.s.e gravel form a bench or terrace from 500 to 700 feet broad, which after following the side of the valley for one-half mile, crosses it diagonally and joins the western slope as a row of rounded hills. Through this drift the present stream has cut a narrow channel.

The narrowest part of the Umpog valley is about one mile south of Bethel. Farther upstream the valley expands into the flat occupied by Umpog Swamp, which presents several interesting features. The eastern, southern, and western sides of the swamp are formed of irregular ma.s.ses of limestone and granite-gneiss 20 to 60 feet high. Near the northwestern edge of the swamp is a terrace-like surface cut on limestone. Its elevation is about the same as that of the beveled rock remnants lying in Umpog valley north of Bethel.

[Ill.u.s.tration: ~State Geol. Nat. Hist. Survey. Bull. 30. Plate II.~ A. View up the valley of Umpog Creek. The valley dwindles in the distance to the "railroad divide." In the middle distance is Umpog Swamp; in the foreground the edge of the southern end of row of Kames which points down the valley.

B. View down the valley of Umpog Creek. To the left is the edge of limestone terrace; in the middle distance is the Catholic cemetery situated on a terrace of stratified drift; on the right are mounds of stratified drift; in the distance is the granite ridge bounding the valley on the east.]

[Ill.u.s.tration: ~Fig. 8.~ Profiles of rivers.

A. Profile of present Still River and buried channel of Umpog-Still River.

B. Profile of preglacial Croton-Still River.

C. Profile of preglacial Umpog-Still River.

Solid lines show the present levels.

Dotted lines show preglacial levels.]

Umpog Swamp was formerly a lake but is now nearly filled with organic matter so that only a small remnant of the old water body remains.

Soundings have revealed no bottom at 43 feet[10] and the depth to rock bottom is not less than 45 feet. The swamp situated one-half mile southwest of Bethel has a depth to rock of 35 feet. In their relation to the Still River system these two swamps may be regarded simply as extensions of the Umpog Creek channel, but when the elevations of their bottoms are compared with that of points to the north and south, where the river flows on rock, it will be seen that a profile results which is entirely out of harmony with the present profile of the river. Thus Umpog Creek falls 40 feet at the point where it spills over the rock ledge into the swamp, and if the 45 feet which measures the depth of Umpog Swamp be added, the difference in level is seen to be at least 85 feet. A similar calculation locates the bottom of the smaller swamp near Bethel at an elevation of 340 feet above sea-level or on the same level as the bottom of Umpog Swamp. In a straight line 2-1/4 miles north of Bethel, Still River crosses rock at a level of 350 feet, or 10 feet higher than the bottom of Umpog Swamp. At Brookfield, 6-1/2 miles north of the mouth of the Umpog, the Still crosses rock at 260 feet, and 4-1/2 miles farther north, it joins the Housatonic on a rock floor 200 feet above sea-level (fig. 8, A). Such a profile can be explained in either of two ways: glaciers gouged out rock basins in the weak limestone, or the river in its lower part has been forced out of its graded bed onto rock at a higher level.

Probably both causes have operated, but the latter has produced more marked effects.

Umpog Creek has its source in a small forked stream which rises in the granite hills east of the south end of Umpog Swamp. After pa.s.sing westward through a flat swampy area, where it is joined by the waters from Todd Pond, the stream turns north and follows a shallow rock gorge until Umpog Swamp is reached. The divide which separates the present headwaters of the Umpog from those of the Saugatuck is a till-covered swampy flat about one-quarter mile east of Todd Pond.

This arrangement of tributary streams is correctly shown in fig. 7 and differs essentially from that shown on the Danbury atlas sheet. This divide owes its position to the effects of glaciation. Deposits of till and the scouring of the bed rock so modified the preglacial surface that the upper part of the Saugatuck was cut off and made tributary to the Umpog.

[Footnote 10: Report by T. T. Giffen, 1907.]

THE PREGLACIAL DIVIDE

In order to determine whether Still River flowed southward through the Saugatuck Valley just before the advent of the ice sheet, the borders of Umpog Swamp and the region to the south and east were examined. It was found that Umpog Swamp is walled in on the south by ledges of firm crystalline limestone and that the rock-floored ravine leading southward from the swamp, and occupied by the railroad, lies at too high an elevation to have been the channel of a through-flowing stream. A south-flowing Still River, and much less an ancient Housatonic, could not have had its course through this ravine just previous to glaciation. A course for these rivers through the short valley which extends southeastward from Umpog Swamp is also ruled out, because the bedrock floor of this hypothetical pa.s.sageway is 20 feet higher than the floor of the ravine through which the railroad pa.s.ses.

The eastern border of Umpog Swamp is determined by a ridge of limestone which separates the swamp from lowlying land beyond. This ridge is continuous, except for the postglacial gorge cut by the tributary entering from the east, and must have been in existence in preglacial times. The entire lowland east of this limestone ridge possesses a unity that is not in harmony with the present division of the drainage. The streams from this hillside and those from the west may have joined in the flat-floored valley at the head of the Saugatuck and from there flowed into the Saugatuck system. The former divide then lay in a line connecting the limestone rim of the swamp with the tongue of highland which the highway crosses south of Todd Pond (fig. 7).

THE STILL-CROTON DIVIDE

INTRODUCTION

The deep valley extending from the Danbury Fair Grounds to the East Branch Reservoir in the Croton River system, has given rise to the suggestion that the course of the Housatonic formerly may have been along the line of Still and Croton rivers and thence to the Hudson.[11] From the evidence of the topographic map alone, this hypothesis appears improbable. The trend of the larger streams in western Connecticut is to the south and southeast; a southwesterly course, therefore, would be out of harmony with the prevailing direction of drainage. Also, the distance from the present mouth of Still River to tidewater by the Still-Croton route is longer than the present route by way of the Housatonic.

[Footnote 11: Hobbs, W. H., Still rivers of western Connecticut: Bull.

Geol. Soc. Am., vol. 13, p. 25, 1901.]

FEATURES OF STILL RIVER VALLEY WEST OF DANBURY

From Danbury to its source Still River occupies a valley whose features are significant in the history of the drainage. Between Danbury and the Fair Grounds (fig. 1) the valley is a V-shaped ravine 1-1/2 miles long, well proportioned to the small stream now occupying it but entirely too narrow for the channel of a large river. Along the valley are outcrops of schist, and granite rock is present on both sides of the valley for a distance of about one-quarter mile. Part of the valley is a mere cleft cut in the rock and is unglaciated. At the Danbury Fair Grounds the valley opens out into a marshy plain, through which the river meanders and receives two tributaries from the south.

The plain, which extends beyond Lake Kanosha on the west, has a generally level surface but is diversified in places by mounds of stratified drift.

Near the railroad a rock outcrop was found which gives a clue to the nature of the broad lowland. The rock consists mainly of schist, but on the side next the valley there is a facing of rotten limestone.

This plain, like all the others in this region, is a local peneplain developed on soluble limestone. A better example could not be found to prove the fallacy of the saying that "a broad valley proves the existence of a large river." The plain is simply a local expansion of a valley which on each side is much narrower. No other river than the one flowing through it can have been responsible for the erosion, for the plain is enclosed by hills of gneiss and schist (Pl. III).

At Mill Plain the valley is crowded by ragged rock outcrops which jut into the lowland. Here the river occupies a ravine cut in till near the north side of the valley. West of Mill Plain station the valley is enc.u.mbered with ridges of stratified drift, interspersed with heavy acc.u.mulations of till. Near Andrew Pond the true width of the valley--one-eighth mile--is shown by rock outcrops on both the north and south slopes. The valley at this point gives no indication of narrowing toward the headwaters; in fact, it becomes broader toward the west.