Drainage Modifications and Glaciation in the Danbury Region Connecticut / State of Connecticut State Geological and Natural History Survey Bulletin No. 30

by Ruth Sawyer Harvey

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.

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 coarse 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 masses 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.

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.

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 2¼ 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, 6½ miles north of the mouth of the Umpog, the Still crosses rock at 260 feet, and 4½ 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.

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 Creek has its source in a small forked stream which rises in the granite hills east of the south end of Umpog Swamp. After passing 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.

[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 passageway is 20 feet higher than the floor of the ravine through which the railroad passes.

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]).