Drainage Modifications and Glaciation in the Danbury Region Connecticut.
by Ruth Sawyer-Harvey.
INTRODUCTION
The Danbury region of Connecticut presents many features of geographic and geologic interest. It may be regarded as a type area, for the history of its streams and the effects of glaciation are representative of those of the entire State. With this idea in mind, the field work on which this study is based included a traverse of each stream valley and an examination of minor features, as well as a consideration of the broader regional problems. Much detailed and local description, therefore, is included in the text.
The matter in the present bulletin formed the main theme of a thesis on "Drainage and Glaciation in the Central Housatonic Basin" which was submitted in partial fulfillment of the requirements for the degree of doctor of philosophy at Yale University.
The field work was done in 1907 and 1908 under the direction of Professor Herbert E. Gregory. I am also indebted to the late Professor Joseph Barrell and to Dr. Isaiah Bowman for helpful cooperation in the preparation of the original thesis, and to Dr. H. H. Robinson for a.s.sistance in preparing this paper for publication.
DRAINAGE MODIFICATIONS AND GLACIATION IN THE DANBURY REGION, CONNECTICUT -------- By Ruth S. Harvey
REGIONAL RELATIONS
The region discussed in this bulletin is situated in western Connecticut and is approximately 8 miles wide and 18 miles long in a north-south direction, as shown on fig. 1.[1] Throughout, the rocks are crystalline and include gneiss, schist, and marble--the metamorphosed equivalents of a large variety of ancient sedimentary and igneous rocks.
For the purposes of this report, the geologic history may be said to begin with the regional uplift which marked the close of the Mesozoic.
By that time the mountains formed by Tria.s.sic and Jura.s.sic folding and faulting had been worn down to a peneplain, now much dissected but still recognizable in the accordant level of the mountain tops.
Erosion during Cretaceous time resulted in the construction of a piedmont plain extending from an undetermined line 30 to 55 miles north of the present Connecticut sh.o.r.e to a point south of Long Island.[2] This plain is thought to have been built up of unconsolidated sands, clays, and gravels, the debris of the Jura.s.sic mountains. Inland the material consisted of river-made or land deposits; outwardly it merged into coastal plain deposits. When the plain was uplifted, these loose gravels were swept away. In New York, Pennsylvania, and New Jersey, however, portions of the Cretaceous deposits are still to be found. Such deposits are present, also, on the north sh.o.r.e of Long Island, and a well drilled at Barren Island on the south sh.o.r.e revealed not less than 500 feet of Cretaceous strata.[3] The existence of such thick deposits within 30 miles of the Connecticut sh.o.r.e and certain peculiarities in the drainage have led to the inference that the Cretaceous cover extended over the southern part of Connecticut.
[Footnote 1: The streams and other topographic features of the Danbury region are shown in detail on the Danbury and the New Milford sheets of the United States Topographic Atlas. These sheets may be obtained from the Director of the United States Geological Survey, Washington, D. C.]
[Footnote 2: It was probably not less than 30 miles, for that is the distance from the mouth of Still River, where the Housatonic enters a gorge in the crystallines, to the sea. Fifty-five miles is the distance to the sea from the probable old head of Housatonic River on Wa.s.saic Creek, near Amenia, New York.]
[Footnote 3: Veatch, A. C., Slichter, C. S., Bowman, Isaiah, Crosby, W. O., and Horton. R. E., Underground water resources of Long Island: U. S. G. S., PP. 44, p. 188 and fig. 24, 1906.]
A general uplift of the region brought this period of deposition to a close. As the peneplain, probably with a mantle of Cretaceous deposits, was raised to its present elevation, the larger streams kept pace with the uplift by incising their valleys. The position of the smaller streams, however, was greatly modified in the development of the new drainage system stimulated by the uplift. The modern drainage system may be a.s.sumed to have been at first consequent, that is, dependent for its direction on the slope of the uplifted plain, but it was not long before the effect of geologic structure began to make itself felt. In the time when all the region was near baselevel, the harder rocks had no advantage over the softer ones, and streams wandered where they pleased. But after uplift, the streams began to cut into the plain, and those flowing over limestone or schist deepened, then widened their valleys much faster than could the streams which flowed over the resistant granite and gneiss. By a system of stream piracy and shifting, similar to that which has taken place throughout the Newer Appalachians, the smaller streams in time became well adjusted to the structure. They are of the cla.s.s called subsequents; on the other hand, the Housatonic, which dates at least from the beginning of the uplift if not from the earlier period of peneplanation, is an antecedent stream.
The complex rock surface of western Connecticut had reached a stage of mature dissection when the region was invaded by glaciers.[4] The ice sheet sc.r.a.ped off and redistributed the mantle of decayed rock which covered the surface and in places gouged out the bedrock. The resulting changes were of a minor order, for the main features of the landscape and the princ.i.p.al drainage lines were the same in preglacial time as they are today. It is thus seen that the history of the smaller streams like those considered in this report involves three factors: (1) the normal tendencies of stream development, (2) the influence of geologic structure, and (3) the effect of glaciation.
The cover of glacial deposits is generally thin, but marked variations exist. The fields are overspread with coa.r.s.e till containing pebbles 6 inches in diameter to huge boulders of 12 feet or more. The abundance, size, and composition of the boulders in the till of a given locality is well represented by the stone fences which border fields.
[Footnote 4: This stage of glaciation is presumably Wisconsin. No definite indication of any older glacial deposits was found.]
[Ill.u.s.tration: ~Fig. 1.~ Present drainage of the Danbury region.]
The regional depression which marked the close of the glacial period slackened the speed of many rivers and caused them to deposit great quant.i.ties of modified or a.s.sorted drift. Since glacial time, these deposits have been dissected and formed into the terraces which are characteristic of the rivers of the region. A form of terrace even more common than the river-made terrace is the kame terrace found along borders of the lowlands. Eskers in the Danbury region have not the elongated snake-like form by which they are distinguished in some parts of the country, notably Maine; on the contrary, they are characteristically short and broad, many having numerous branches at the southern end like the distributaries of an aggrading river. The material of the eskers ranges from coa.r.s.e sand to pebbles four inches in diameter, the average size being from one to two inches. No exposures were observed which showed a regular diminution in the coa.r.s.eness of the material toward their southern end. The clean-washed esker gravels afford little encouragement to plant growth, and the rain water drains away rapidly through the porous gravel.
Consequently, acc.u.mulations of stratified drift are commonly barren places. A desert vegetation of coa.r.s.e gra.s.ses, a kind of wiry moss, and "everlastings" (_Gnaphalius decurrens_) are the princ.i.p.al growth.
Rattlebox (_Crotolaria sagittalis_), steeplebush (_Spiraea tomentosa_), sweet fern (_Comptonia asplenifolia_), and on the more fertile eskers--especially on the lower, wetter part of the slope--golden rod, ox-eyed daisy, birch, and poplar are also present. All the eskers observed were found to be similar: they ranged in breadth across the top from 100 to 150 feet and the side slopes were about 20 degrees.
Only a single heavily wooded esker was found, and this ran through a forest region.
The acc.u.mulations of stratified drift are distinguished from other features in the landscape by their smoother and rounder outlines, by their habit of lying unconformably on the bedrock without reference to old erosion lines, and by a slightly different tone in the color of the vegetation covering the water-laid material. The difference in color, which is due to the unique elements in the flora of these areas, may cause a hill of stratified drift in summer to present a lighter green color than that of surrounding hills of boulder clay or of the original rock slopes; in winter the piles of stratified drift stand out because of the uniform light tawny red of the dried gra.s.s.
[Ill.u.s.tration: ~State Geol. Nat. Hist. Survey Bull. 30. Plate I.~ View south on the highland northeast of Neversink Pond. The base of a ridge in which rock is exposed is seen at the left; a crescent-shaped lateral moraine bordering the valley lies at the right.]
ROCKY RIVER
DESCRIPTION OF THE RIVER AND ITS VALLEY
Rocky River begins its course as a rapid mountain brook in a rough highland, where the mantle of till in many places is insufficient to conceal the rock ledges (fig. 1). Near Sherman, about four miles from its source, it enters a broad flood plain and meanders over a flat, swampy floor which is somewhat enc.u.mbered with deposits of stratified drift and till. Rocky hills border the valley and rise abruptly from the lowland. The few tributaries of the river in this part of its course are normal in direction.
About six miles below Sherman, Rocky River enters Wood Creek Swamp, which is 5-1/2 miles long by about one mile wide and completely covers the valley floor, extending even into tributary valleys. Within the swamp the river is joined by Squantz Pond Brook and Wood Creek.
Tributaries to Wood Creek include Mountain Brook and the stream pa.s.sing through Ba.r.s.es Pond and Neversink Pond. The head of Ba.r.s.es Pond is separated from the swamp only by a low ridge of till.
Neversink Pond with its inlet gorge and its long southern tributary record significant drainage modifications, as described in the section ent.i.tled "The Neversink-Danbury Valley."
Within and along the margin of Wood Creek Swamp, also east of Wood Creek and at Ba.r.s.es Pond, are rounded, elongated ridges of till, some of which might be called drumlins. East of Neversink Pond is the lateral moraine shown in Pl. I. From the mouth of Wood Creek to Jerusalem, Rocky River is a quiet stream wandering between low banks through flat meadows, which are generally swampy almost to the foot of the bordering hills.
Near Jerusalem bridge two small branches enter Rocky River.
Immediately north of the bridge is a level swampy area about one-half mile in length. Where the valley closes in again, bedrock is exposed near the stream, and beginning at a point one-half mile below (north of) Jerusalem, Rocky River--a swift torrent choked by boulders of great size--deserves its name.
In spite of its rapid current, however, the river is unable to move these boulders, and for nearly three miles one can walk dry-shod on those that lie in midstream.
At two or three places below Jerusalem, in quiet reaches above rapids, the river has taken its first step toward making a flood plain by building tiny beaches. One-half mile above the mouth of the river the valley widens and on the gently rising south bank there are several well-marked terraces about three feet in height and shaped out of glacial material. A delta and group of small islands at the mouth of Rocky River indicate the transporting power of the stream and the relative weakness of the slow-moving Housatonic.
RELATIONS OF THE VALLEY TO GEOLOGIC STRUCTURE
Rocky River is cla.s.sed with streams which are comformable to the rock structure. This conclusion rests largely on the a.n.a.logy between Rocky River and other rivers of this region. The latter very commonly are located on belts of limestone, or limestone and schist, and their extension is along the strike. The interfluvial ridges are generally composed of the harder rocks. The valleys of the East Aspetuck and Womenshenuck Brook on the north side of the Housatonic, and of the Still, the Umpog, Beaver Brook, the upper Saugatuck, and part of Rocky River are on limestone beds (fig. 2). In the valleys between Town Hill and Spruce Mountain (south of Danbury), two ravines northwest of Gra.s.sy Plain (near Bethel), and the Saugatuck valley north of Umpawaug Pond, the limestone bed is largely buried under drift, talus, and organic deposits, but remnants which reveal the character of the valley floors have been found. The parallelism between the courses of these streams and that of Rocky River and the general resemblance in the form of their valleys, flat-floored with steep-sided walls, as well as the scattered outcrops of limestone in the valley, have led to the inference that Rocky River, like the others, is a subsequent stream developed on beds of weaker rock along lines of foliation.
[Ill.u.s.tration: ~Fig. 2.~ Geological map of Still River Valley.]
The Geological Map of Connecticut[5] shows that the valleys of Still River, Womenshenuck Brook, Aspetuck River, and upper Rocky River are developed on Stockbridge limestone. The lower valley of Rocky River is, however, mapped as Becket gneiss and Thomaston granite gneiss.
Although the only outcrops along lower Rocky River are of granite, it is believed that a belt of limestone or schist, now entirely removed, initially determined the course of the river. The a.s.sumption of an irregular belt of limestone in this position would account for the series of gorges and flood plains in the vicinity of Jerusalem bridge and for the broad drift-filled valley at the mouth of Rocky River.