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 Triassic and Jurassic 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 shore 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 Jurassic
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 shore of Long Island, and a well drilled at Barren Island on
the south shore revealed not less than 500 feet of Cretaceous
strata.[3] The existence of such thick deposits within 30 miles of the
Connecticut shore 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 Wassaic 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 assumed 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 class 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 scraped 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 principal 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 coarse 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.]
[Illustration: ~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
quantities of modified or assorted 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 coarse 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
coarseness 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, accumulations of stratified drift are commonly barren
places. A desert vegetation of coarse grasses, a kind of wiry moss,
and "everlastings" (_Gnaphalius decurrens_) are the principal 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 accumulations 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 grass.
[Illustration: ~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.]