the separate deposition of iron and manganese; and also that it
explains the occurrence of large deposits of manganese ore in
regions where the iron ore contains least of that ingredient.
(See Elements of Chemistry and Phys. Geol., Vol. III., pp.
531–532.)
[30] In some cases these iron and manganese deposits are undoubtedly
formed by the replacement of limestone or other rocks, as is
further discussed on pages -- to --.
[31] Kentucky Geol. Survey, Report of Progress, Vol. III., New
Series, 1877, p. 164.
[32] U. S. Geol. Survey, Tenth Annual Report, 1888–1889, Vol. I.,
pp. 409–422.
SOME RIVERS OF CONNECTICUT.[33]
_Outline._--Introduction.--Topography of Connecticut: The upland
plateau, its origin, date, elevation, valleys sunk beneath its
surface.--Lowland on the Triassic area.--Later oscillations.--
Résumé of the topography.--Early drainage.--Re-adjusted streams.
--Revived streams.--Unconformable rivers, consequent or
superimposed.--Pleistocene changes; the Farmington, Quinnipiac,
Scantic.--Abandoned gaps.
_Introduction._ In order to study intelligently the history of a river,
one must first become acquainted with the present physical geography
of the region in which the river lies, and know the stages of its
development. Therefore, before classifying the rivers of Connecticut,
I shall consider the topography of the state, and in a few paragraphs
outline the successive cycles in the history of its growth. The scope
of this article will not permit a discussion or even a full statement
of the evidence on which these conclusions are based. They have been
stated at considerable length by Professor W. M. Davis,[34] and the
reader is referred to his papers for the complete discussion. His
conclusions in respect to the physical geography are accepted here
without question, and form the basis for the discussion on the rivers
of the state.
[Illustration: FIG 2]
_Topography of Connecticut._ Connecticut can be said to consist of two
great areas quite distinct in topography and geologic structure.[35] On
the east and on the west are the crystalline uplands which rise from
sea level along the Sound to 1,700 and 1,800 feet in the northwestern
part of the state, and to 600 and 700 feet in the northeastern.
These uplands consist chiefly of gneiss and granite, probably of
pre-Paleozoic age, which are now much folded, faulted and crumpled.
Between these two areas of crystallines is a lowland belt of Triassic
sandstone and shale, twenty to twenty-five miles wide, extending from
New Haven north through the center of the state and including in its
borders New Haven, Meriden, Hartford, New Britain and many towns of
lesser note. These sandstones form a monocline with an eastward dip
of 10° to 30°, and in addition to being tilted they have been faulted
since their deposition in a shallow, slowly-subsiding trough of
crystallines. Their thickness is variously estimated--3,000 to 5,000
feet, Dana; 10,000 or more, Davis. This lowland is interrupted by a
series of trap ridges, which in general present steep faces toward the
west, whereas their eastward slope is gradual, less than the dip of the
sandstones.
_The upland plateau._ Suppose we ascend the highest point of these trap
ridges, the old tower on Talcott Mt., nine miles west of Hartford; we
are 900 feet above the sea level and more than 600 above the plain at
our feet. A few miles to the west across the sandstone valley, rise the
crystalline uplands, which extend far to the north and to the south.
On the east across the Connecticut we see the eastern uplands. The
first impression, which comes to one as he gazes upon these uplands
and which is strengthened with each view, is that few hills rise above
the general level of the plateau; the crest line is nearly horizontal,
declining gently to Long Island Sound. Above this general level are
a few rounded domes, but no sharp, towering peaks. Below it valleys
have been cut, but they do not destroy the plateau-like appearance. A
view from the western plateau across the sandstone valley shows the
remarkably even crest line of the trap ridges, a crest line which
approximates in height the uplands on the east and west. A nearer view
of the upland corroborates our first impressions of the gently rolling
character of the inter-stream surfaces, but we have a better view of
the valleys which have been sunk beneath the general level and of the
low rounded hills which rise above it. In popular parlance the country
is “hilly.” It is uneven, not because there are high hills, but rather
because there are deep valleys. If in imagination we fill up these
valleys and the wide Triassic lowland to the general level of the broad
inter-stream surfaces, we shall have constructed a gently undulating
plateau, dipping to the south and east--a peneplain.[36]
_Origin of the peneplain._ This is not a constructional surface,
for the rocks are greatly tilted, folded and faulted, so that the
surface consequent upon such disturbance must have been complex and
mountainous. Long subaërial denudation upon a folded and faulted
mass when the land stood much lower than at present produced this
plateau. Evidently it could be produced by denudation only at or near
baselevel, for the effect of erosion upon a mass high above baselevel
is to accentuate its topographic relief, not to reduce it. We naturally
ask ourselves, “At what stage in geologic history did this denudation
occur?”
_Date of the peneplain._ The erosion which accomplished this great
work must have commenced after the formation and dislocation of the
Triassic beds, for the even crest line of the trap ridges, a part of
which--perhaps all--were contemporaneous with the sandstones, is a part
of the dissected peneplain; but to fix the date of the completion of
the peneplain, we must turn to evidence presented in New Jersey.[37]
There we learn that by the close of Cretaceous times, the country was
eroded nearly to baselevel, and we may therefore speak of the relative
position of the land and sea, to which the land was at this time
reduced, as the Cretaceous baselevel, and this land surface as the
Cretaceous peneplain.
_Elevation of the peneplain._ In post-Cretaceous, presumably early
Tertiary[38] times, the land was elevated to nearly its present height
and remained at that altitude, so far as topographic evidence shows,
during Tertiary times. The proofs of this elevation are the valleys
which the streams have sunk below the general level. That this was
not a simple uplift, but was accompanied with tilting and warping, is
clear from the following considerations. The depth to which a stream
can cut its valley depends directly upon its height above baselevel.
If the present surface were a peneplain uniformly elevated, the head
waters and middle courses of a river would not be cut so deep in the
surrounding plain as its lower course. But the reverse is true of the
rivers of Connecticut. The depth of the valley increases inland, being
greater in those regions where the peneplain was raised the highest. A
comparison of the upper and lower valleys of the Housatonic, Naugatuck,
Quinnebaug, and of the Connecticut at Middletown, where it enters the
plateau, and at its mouth, will give some idea of the amount of the
warping. It will not give an _exact measure_ of it for several reasons:
first, the upper courses of the rivers have not yet reached the present
baselevel; second, the present altitude of the uplands is the result
of the post-Cretaceous uplift and warping, plus a probable later
post-Tertiary uplift (to be mentioned later), besides several minor
oscillations, the last of which was downward, and is recorded near the
coast in the drowned condition of the rivers. As has been already said,
the peneplain is highest in the northwest, and gradually declines to
sea level toward the south and east.
_Consequences of the uplift._ The consequences of this uplift are
seen in the valleys, which are cut into the peneplain, and which have
destroyed the level character of the country. In the hard crystalline
rocks the valleys are generally narrow and deep, with bold slopes;[39]
where they are cut in the crystalline limestone, they are wider and
more open. In marked contrast, however, is the lowland on the Triassic
area in which only the trap ridges remain to tell of the former
altitude of the general surface, and the immense amount of erosion
which has taken place on the soft sandstones and shales. Indeed erosion
has progressed so rapidly on these soft rocks, that they have been worn
down almost to a new baselevel in the same length of time in which the
hard crystallines have been only trenched. This fact cannot be too
strongly emphasized. The broad sandstone lowland from New Haven north
into Massachusetts has been carved out of the uplifted peneplain in
soft rocks, during the same time in which the Connecticut has excavated
its gorge in the crystallines below Middletown, and the Housatonic has
opened its upland valley on the limestones. The difference in results
is due not to a difference of time, but to the difference in the
relative hardness of the rocks.
On the basis of this principle the age of certain river gorges to which
reference will be made later can be fixed. The _narrow_ passage of
the Quinnipiac through a sandstone ridge southwest of Meriden cannot
belong to the same cycle of erosion as the _broad sandstone lowland_
on either side of it, but manifestly must be much younger. So, also,
the narrow passage of the Farmington at Tariffville, where it crosses
the trap ridge through a gorge free from drift, is of much later date
than the _broader_ valley more or less encumbered with drift which the
upper part of the same river has cut in the hard crystalline schists.
Cook’s Gap in the trap sheet west of New Britain is much broader than
either of the above, and belongs to the Tertiary cycle of erosion,
although as I shall endeavor to show later, it was probably not
occupied by a stream during the whole cycle. In marked contrast, also,
with the Tariffville gorge is the gap by which the Westfield river in
Massachusetts cuts the trap ridge. This gap was formerly broad and
open--the result of Tertiary erosion--but is now filled with drift,
in which the river is at present working. Since these two rivers are
essentially the same in size, are now at the same level, and the rock
is the same in both cases, the only explanation for the difference in
the two passages is that they belong to different cycles.
To recapitulate, the results of the post-Cretaceous uplift are seen in
the valleys which have been cut in the peneplain. The narrow valleys
in the gneisses and schists, the upland valleys in the limestones, the
wide open, drift encumbered gaps in the trap ridge,--Cook’s and the
Westfield river gaps,--the broad open lowland on the sandstones, are
all the result of erosion in this cycle. The Quinnipiac gorge in the
sandstone, and the Tariffville gorge in the trap are just as surely of
a later date. They do not at all accord with the work of the earlier
cycle either in size, angle of slope, or depth.
This conclusion is somewhat at variance with an opinion expressed by
Professor J. D. Dana,[40] but it seems justifiable in view of the
successive cycles in the physical development of the region. In another
part of this article I shall consider these gaps again in connection
with their river histories, and shall give additional reasons why I
venture to differ from so eminent an authority.
_Length of this cycle._ This cycle of erosion beginning with the
post-Cretaceous uplift was not so long as the preceding cycle. In
the earlier one the whole state was reduced to a peneplain; in the
later cycle only the soft Triassic sandstones were brought near to
baselevel. It probably lasted through Tertiary times, and was brought
to a close by a slight uplift. The result of this uplift is well
shown in Pennsylvania[41] and New Jersey.[42] It is not well shown in
Connecticut, but there seem to be some traces of it in the trenches the
rivers have cut below the level of the sandstone peneplain. However,
these trenches are so much obscured by drift that a positive statement
is not warranted. It may, however, be spoken of provisionally as the
post-Tertiary uplift. There may have been later oscillations of small
amount, probably were; here and there are shreds of evidence which
point to such oscillations, but only one movement has had an effect
upon the topography, which can be recognized. The fjorded condition
of all the rivers along the Sound--the Norwalk, Saugatuck, New Haven
bay, Niantic and Thames are the best examples--shows that within
comparatively recent time there has been a slight subsidence of the
land. But this movement is not to be compared in amount with those of
the earlier cycles.
_The drift._ Over all the state in varying thickness lies the glacial
drift, either in its typical unmodified development as till, or in its
modified form, as river terraces, kames, eskers and sand-plains. It is
of importance in this connection only as it has affected the topography
of the country and so modified the drainage. Examples of these
modifications will be mentioned later.
_Résumé._ There was first a long cycle of denudation in pre-Triassic
times, during which the contorted crystallines were worn down to a
comparative level; second, a cycle of subsidence, deposition and
volcanic outburst, during which the sea entered the crystalline
trough, and the Triassic conglomerates, sandstones and shales were
deposited with the intercalated layers of lava; third, a long cycle of
elevation, folding, faulting and erosion, during which the sedimentary
beds were elevated--tilted into the present faulted monocline, and
this constructional surface worn down to a baselevel of erosion in
late Cretaceous times. Each of these cycles probably represents
the sum total of several subordinate cycles. There was, fourth, a
post-Cretaceous uplift inaugurating a period of erosion lasting
through Tertiary times and resulting in the formation of valleys in
the hardest rocks, and a lowland approaching baselevel on the Triassic
sandstones and shales; fifth, a probable late or post-Tertiary uplift,
when the valleys were deepened and the lowlands trenched--obscure in
Connecticut, but well shown farther south; sixth, the land, near the
coast at least, is now slightly lower than it has been in the not
remote past, as is shown by the fjords.
With the changes of the physical geography clearly in mind, the rivers
of Connecticut may now be examined in respect to their conditions of
origin, the number of cycles through which they have lived, and the
approach they have made to mature old age. But at the very outset a
serious difficulty is encountered, for the geological structure of
the state is nowhere well described, nor have topographic maps of
all the districts yet been issued. Since the structural details are
to some extent unknown it is unwise in many cases to attempt more
than tentative conclusions. Several of the problems to be presented
cannot be considered as settled. Considerable progress toward a
final settlement will have been made, however, if the conditions of
the problems are made clear, various hypotheses suggested, and the
attention of workers in this field called to these questions.
_Early drainage._ Of the drainage of Connecticut during Jurassic and
Cretaceous times very little can be said. It is not even known whether
it was consequent upon the Jurassic tilting and faulting, or whether
these deformations were so slow in their movement that the rivers
persisted in spite of them. It may have been that the larger rivers
were victorious, while the smaller were conquered and compelled to
assume new consequent courses. Whatever was their origin there must
have been abundant opportunities during the long erosion which resulted
in the Cretaceous baselevel, and again in the period of revived and
quickened degradation succeeding the post-Cretaceous uplift, for the
streams to adjust themselves in a large degree to the geological
structure. The contrast of hard and soft beds and the great elevation
must have been potent factors in bringing to pass such a result. We
expect to find the streams so far re-adjusted as to render improbable
the discovery of their manner of origin.
_The Housatonic, a re-adjusted stream._ The best example of
re-adjustment is found in the northwestern part of the state where the
Housatonic and some of its branches follow well adjusted courses. From
its headwaters, near Pittsfield, Mass., to New Milford, Conn., it has
nearly all the way chosen its course along the Cambrian crystalline
limestones in preference to the harder granites and gneisses on either
side. The stratigraphical relationships of the limestone are not fully
understood, but they seem to be deeply eroded anticlines and synclines,
whose axes plunge north or south at various angles. The course of
the river, if the drainage was consequent, was at first along the
synclinal valleys, passing from one to another across the lowest points
in the anticlinal ridge between them. But by a series of changes[43],
resulting from the differential rates of erosion as hard or soft beds
became exposed, the river previously to the Cretaceous baseleveling,
seems to have re-adjusted its course to the softer limestones. However,
there are several places where this conformity to structure does not
seem to be the law; where the river departs from a limestone valley to
flow for a time in the crystallines, only to return to the limestone
again. The most marked instance of this is in the towns of Sharon and
Cornwall, where the river leaves the limestone valley, which continues
to the southwest, and flows for ten miles in a narrow gorge in the
gneiss, only to again enter at its northern end a long narrow bed of
limestone. The following seems to be the probable explanation. When the
land stood at the elevation represented by the Cretaceous peneplain,
these hard beds were below or but very slightly above baselevel, and
were therefore undiscovered by the stream or had just begun to make
themselves known late in the cycle. Had they been reached early in the
cycle, when the stream was far above baselevel and presumably before
many of its tributaries had been developed, and when it was therefore a
smaller river, it is quite probable that further re-adjustments would
have occurred, and the stream been led away from the hard rocks onto
the softer beds to the west; but when they were reached the stream
had cut so deeply and so nearly to baselevel that it was safe from
capture. After the elevation of the peneplain the stream was revived
and disclosed more and more of these hard beds, but was then, owing to
the development and head-water growth of its tributaries, too important
a river to be diverted by any rival. A river of this kind may be
said to be “conformably superimposed” in distinction to one which is
superimposed from an unconformable cover.
_Revived streams._ It is important to recognize the effect of the
post-Cretaceous uplift upon the rivers at that time established. As
the land was baseleveled and the velocity of the streams decreased,
they lost in large degree their cutting power and sluggishly meandered
more or less in broad flood-plains. During and for a period after the
uplift, their cutting power was restored to them by virtue of their
increased velocity and they excavated the deep narrow valleys which we
find in the crystalline highlands. The upper course of the Housatonic
is a good example of a river re-adjusted to the structure during one
cycle, revived by uplift to a second cycle of erosion, and in places
“conformably superimposed” upon structures from which it would have
been led away in the ordinary course of re-adjustment. Its tributaries,
the East Aspetuck, Still, Shepaug, and Pomeraug follow courses
re-adjusted in one cycle and revived in a later uplift.
We can assert with the more confidence that such was the history of
the upper Housatonic, because we find in other states, in regions
whose history has been the same, similar examples of “conformably
superimposed” and “revived” streams. The Musconetcong and Pequest,
highland rivers of New Jersey, are streams “revived” from mature old
age to vigorous youth and “conformably superimposed” upon saddles of
gneiss between two limestone valleys.[44]
_Unconformable rivers._ In considering the course of the lower
Housatonic we meet with some difficulty at the outset. In the southern
part of the town of New Milford the river leaves the limestone belt
which continues with some slight interruptions to the Hudson, and
swings sharply into the crystalline plateau in a southeasterly course
until it is joined by the Naugatuck, when their united waters flow
south for a few miles to the sound. The course of the lower Connecticut
is even more surprising. At Middletown it leaves the broad open
Triassic sandstone lowland, and through a gorge enters the plateau,
which has an average elevation of 600 to 700 feet. In this plateau of
crystallines the river has sunk its valley nearly to sea-level. The
slopes are steep compared to the lines in the sandstone lowland, and
the contrast between the two parts of the river is one of the striking
features of Connecticut scenery. Several theories may be framed to
account for the curious behavior of these two rivers, but none of them
are free from all difficulty.
_As a consequent river._ The lower Connecticut has been thought[45] to
be a revived river, whose course was consequent upon the post-Triassic
tilting and faulting. The faulted monocline seems to have had the shape
of a half-boat, ends to the north and south, and one gunwale rising
toward the west, the combined effect of the tilting and faulting being
to swing the river to the southeast, where the keel of the boat was
lowest. The probable existence of faults, with upthrow on the east,
along the eastern margin of the Triassic rocks, is a difficulty in the
way of the complete acceptance of this theory. Unfortunately too little
is known about the structure of the western plateau to say whether
the course of the lower Housatonic could be accounted for on such an
hypothesis. On this theory the Connecticut would be consequent upon the
Jurassic deformation, and revived by the post-Cretaceous uplift.
It may be suggested that the southeast courses are due to the tilting
of the peneplain at the time of elevation, the plateau now being, as
we have seen, much higher in the northwestern part of the state than
elsewhere. But the acceptance of this theory necessitates a degree of
smoothness and absence of even mild relief in the peneplain, which is
hardly possible. The present average slope of the plateau is but a
few feet per mile, and it seems incredible that so gentle a tilting
could force rivers as large as these to take new courses. Besides,
if the Housatonic and Connecticut were deflected, why were not the
smaller streams--the Naugatuck and Quinnebaug--also given southeastern
deflections? Clearly, this explanation is not the correct one.