described in which the Juniata left its original syncline for others to
the south. The general case may be stated in a few words. A stream
flowing along a syncline of hard beds (Carboniferous sandstones)
develops side streams which breach the adjacent anticlines and open
lowlands in the underlying softer beds (Devonian and Silurian). On
these lowlands, the headwaters of side streams from other synclines are
encountered and a contest ensues as to possession of the drainage
territory. The divides are pushed away from those headwaters whose
lower course leads them over the fewest hard barriers; this conquest
goes on until the upper course of the initial main stream is diverted
to a new and easier path than the one it chose in its youth in
obedience to the first deformation of the region. Thus the Juniata now
avoids the center and once deepest part of the old Broad Top lake,
because in the general progress of erosion, lowlands on soft Devonian
beds were opened all around the edge of the great mass of sandstones
that held the lake; the original drainage across the lake, from its
western slopes to its outlet just south of the Jack's mountain
anticline, has now taken an easier path along the Devonian beds to the
west of the old lake basin, and is seen in the Little Juniata, flowing
along the outer side of Terrace mountain and rounding the northern
synclinal point where Terrace mountain joins Sideling hill. It then
crosses Jack's mountain at a point where the hard Medina sandstones of
the mountain were still buried at the time of the choice of this
channel. In the same way, the drainage of the subordinate basin,
through which the main lake discharged eastward, is now not along the
axis of the Juniata-Catawissa syncline, but on the softer beds along
one side of it; and along the southern side because the easier escape
that was provided for it lay on that side, namely, via the Tuscarora
and Wiconisco synclines, as already described. The much broader change
from the Anthracite to the Susquehanna was only another form of the
same process. Taking a transverse view of the whole system of central
folds, it is perceived that their axes descend into the Anthracite
district from the east and rise westward therefrom; it is as if the
whole region had received a slight transverse folding, and the
transverse axis of depression thus formed defined the initial course of
the first master stream. But this master stream deserted its original
course on the transverse axis of depression because a lateral course
across lowlands on softer beds was opened by its side streams; and in
the contest on these lowlands with an external stream, the Susquehanna,
the upper portion of the Anthracite was diverted from the hard rocks
that had appeared on the transverse axis. The distance of diversion
from the axial to the lateral course in this case was great because of
the gentle quality of the transverse folding; or, better said, because
of the gentle dips of the axes of the longitudinal folds. This
appearance of systematic re-arrangement in the several river courses
where none was expected is to my mind a strong argument in favor of the
originally consequent location of the rivers and their later mutual
adjustment. It may perhaps be conceived that antecedent streams might
imitate one another roughly in the attitude that they prophetically
chose with regard to folds subsequently formed, but no reason has been
suggested for the imitation being carried to so remarkable and definite
a degree as that here outlined.
35. _Superimposition of the Susquehanna on two synclinal
ridges_.--There is however one apparently venturesome postulate that
may have been already noted as such by the reader; unless it can be
reasonably accounted for and shown to be a natural result of the long
sequence of changes here considered, it will seriously militate against
the validity of the whole argument. The present course of the middle
Susquehanna leads it through the apical curves of two Pocono synclinal
ridges, which were disregarded in the statement given above. It was
then assumed that the embryonic Susquehanna gained possession of the
Siluro-Devonian lowland drainage by gnawing out a course to the west of
these synclinal points; for it is not to be thought of that any
conquest of the headwaters of the Anthracite river could have been made
by the Susquehanna if it had had to gnaw out the existing four
traverses of the Pocono sandstones before securing the drainage of the
lowlands above them. The backward progress of the Susquehanna could not
in that case have been nearly fast enough to reach the Anthracite
before the latter had sunk its channel to a safe depth. It is therefore
important to justify the assumption as to the more westerly location of
the embryonic Susquehanna; and afterwards, to explain how it should
have since then been transferred to its present course. A short cut
through all this round-about method is open to those who adopt in the
beginning the theory that the Susquehanna was an antecedent river; but
as I have said at the outset of this inquiry, it seems to me that such
a method is not freer from assumption, even though shorter than the one
here adopted; and it has the demerit of not considering all the curious
details that follow the examination of consequent and adjusted courses.
The sufficient reason for the assumption that the embryonic Susquehanna
lay farther west than the present one in the neighborhood of the Pocono
synclinals is simply that--in the absence of any antecedent stream--it
must have lain there. The whole explanation of the development of the
Siluro-Devonian lowlands between the Pocono and Medina ridges depends
simply on their being weathered out where the rocks are weak enough to
waste faster than the enclosing harder ridges through which the streams
escape. In this process, the streams exercise no control whatever over
the direction in which their headwaters shall grow; they leave this
entirely to the structure of the district that they drain. It thus
appears that, under the postulate as to the initial location of the
Susquehanna as one of the many streams descending the great slope of
the Kittatinny (Cumberland) highland into the Swatara syncline, its
course being reversed from northward to southward by the Newark
depression, we are required to suppose that its headwater (northward)
growth at the time of the Jurassic elevation must have been on the
Siluro-Devonian beds, so as to avoid the harder rocks on either side.
Many streams competed for the distinction of becoming the master, and
that one gained its ambition whose initial location gave it the best
subsequent opportunity. It remains then to consider the means by which
the course of the conquering Susquehanna may have been subsequently
changed from the lowlands on to the two Pocono synclines that it now
traverses. Some departure from its early location may have been due to
eastward planation in its advanced age, when it had large volume and
gentle slope and was therefore swinging and cutting laterally in its
lower course. This may have had a share in the result, but there is
another process that seems to me more effective.
In the latter part of the Jura-Cretaceous cycle, the whole country
hereabout suffered a moderate depression, by which the Atlantic
transgressed many miles inland from its former shoreline, across the
lowlands of erosion that had been developed on the litoral belt. Such a
depression must have had a distinct effect on the lower courses of the
larger rivers, which having already cut their channels down close to
baselevel and opened their valleys wide on the softer rocks, were then
"estuaried," or at least so far checked as to build wide flood-plains
over their lower stretches. Indeed, the flood-plains may have been
begun at an earlier date, and have been confirmed and extended in the
later time of depression. Is it possible that in the latest stage of
this process, the almost baselevelled remnants of Blue mountain and the
Pocono ridges could have been buried under the flood-plain in the
neighborhood of the river?
If this be admitted, it is then natural for the river to depart from
the line of its buried channel and cross the buried ridges on which it
might settle down as a superimposed river in the next cycle of
elevation. It is difficult to decide such general questions as these;
and it may be difficult for the reader to gain much confidence in the
efficacy of the processes suggested; but there are certain features in
the side streams of the Susquehanna that lend some color of probability
to the explanation as offered.
Admit, for the moment, that the aged Susquehanna, in the later part of
the Jura-Cretaceous cycle, did change its channel somewhat by cutting
to one side, or by planation, as it is called. Admit, also, that in the
natural progress of its growth it had built a broad flood plain over
the Siluro-Devonian lowlands, and that the depth of this deposit was
increased by the formation of an estuarine delta upon it when the
country sank at the time of the mid-Cretaceous transgression of the
sea. It is manifest that one of the consequences of all this might be
the peculiar course of the river that is to be explained, namely, its
superimposition on the two Pocono synclinal ridges in the next cycle of
its history, after the Tertiary elevation had given it opportunity to
re-discover them. It remains to inquire what other consequences should
follow from the same conditions, and from these to devise tests of the
hypothesis.
36. _Evidence of superimposition in the Susquehanna tributaries_.--One
of the peculiarities of flood-plained rivers is that the lateral
streams shift their points of union with the main stream farther and
farther down the valley, as Lombardini has shown in the case of the Po.
If the Susquehanna were heavily flood-plained at the close of the
Jura-Cretaceous cycle, some of its tributaries should manifest signs of
this kind of deflection from their structural courses along the strike
of the rocks. Side streams that once joined the main stream on the line
of some of the softer northeast-southwest beds, leaving the stronger
beds as faint hills on either side, must have forgotten such control
after it was baselevelled and buried; as the flood plain grew, they
properly took more and more distinctly downward deflected courses, and
these deflections should be maintained in subsequent cycles as
superimposed courses independent of structural guidance. Such I believe
to be the fact. The downstream deflection is so distinctly a
peculiarity of a number of tributaries that join the Susquehanna on the
west side (see figure 1) that it cannot be ascribed to accident, but
must be referred to some systematic cause. Examples of deflection are
found in Penn's creek, Middle creek and North Mahantango creek in
Snyder county; West Mahantango between the latter and Juniata county;
and in the Juniata and Little Juniata rivers of Perry county. On the
other side of the Susquehanna, the examples are not so distinct, but
the following may be mentioned: Delaware and Warrior runs,
Chillisquaque creek and Little Shamokin creek, all in Northumberland
county. It may be remarked that it does not seem impossible that the
reason for the more distinct deflection of the western streams may be
that the Susquehanna is at present east of its old course, and hence
towards the eastern margin of its flood plain, as, indeed its position
on the Pocono synclinals implies. A reason for the final location of
the superimposed river on the eastern side of the old flood plain may
perhaps be found in the eastward tilting that is known to have
accompanied the elevation of the Cretaceous lowland.
It follows from the foregoing that the present lower course of the
Susquehanna must also be of superimposed origin; for the flood plain of
the middle course must have extended down stream to its delta, and
there have become confluent with the sheet of Cretaceous sediments that
covered all the southeastern lowland, over which the sea had
transgressed. McGee has already pointed out indications of superimposed
stream courses in the southeastern part of the State;[22] but I am not
sure that he would regard them as of the date here referred to.
[Footnote 22: Amer. Journ. Science, xxxv, 1888, 121, 134.]
The theory of the location of the Susquehanna on the Pocono synclinal
ridges therefore stands as follows. The general position of the river
indicates that it has been located by some process of slow
self-adjusting development and that it is not a persistent antecedent
river; and yet there is no reason to think that it could have been
brought into its present special position by any process of shifting
divides. The processes that have been suggested to account for its
special location, as departing slightly from a location due to slow
adjustments following an ancient consequent origin, call for the
occurrence of certain additional peculiarities in the courses of its
tributary streams, entirely unforeseen and unnoticed until this point
in the inquiry is reached; and on looking at the map to see if they
occur, they are found with perfect distinctness. The hypothesis of
superimposition may therefore be regarded as having advanced beyond the
stage of mere suggestion and as having gained some degree of
confirmation from the correlations that it detects and explains. It
only remains to ask if these correlations might have originated in any
other way, and if the answer to this is in the negative, the case may
be looked upon as having a fair measure of evidence in its favor. The
remaining consideration may be taken up at once as the first point to
be examined in the Tertiary cycle of development.
37. _Events of the Tertiary cycle_.--The elevation given to the region
by which Cretaceous baselevelling was terminated, and which I have
called the early Tertiary elevation, offered opportunity for the
streams to deepen their channels once more. In doing so, certain
adjustments of moderate amount occurred, which will be soon examined.
As time went on, much denudation was effected, but no wide-spread
baselevelling was reached, for the Cretaceous crest lines of the hard
sandstone ridges still exist. The Tertiary cycle was an incomplete one.
At its close, lowlands had been opened only on the weaker rocks between
the hard beds. Is it not possible that the flood-plaining of the
Susquehanna and the down-stream deflection of its branches took place
in the closing stages of this cycle, instead of at the end of the
previous cycle? If so, the deflection might appear on the branches, but
the main river would not be transferred to the Pocono ridges. This
question may be safely answered in the negative; for the Tertiary
lowland is by no means well enough baselevelled to permit such an
event. The beds of intermediate resistance, the Oriskany and certain
Chemung sandstones, had not been worn down to baselevel at the close of
the Tertiary cycle; they had indeed lost much of the height that they
possessed at the close of the previous cycle, but they had not been
reduced as low as the softer beds on either side. They were only
reduced to ridges of moderate and unequal height over the general plain
of the Siluro-Devonian low country, without great strength of relief
but quite strong enough to call for obedience from the streams along
side of them. And yet near Selin's Grove, for example, in Snyder
county, Penn's and Middle creeks depart most distinctly from the strike
of the local rocks as they near the Susquehanna, and traverse certain
well-marked ridges on their way to the main river. Such aberrant
streams cannot be regarded as superimposed at the close of the
incomplete Tertiary cycle; they cannot be explained by any process of
spontaneous adjustment yet described, nor can they be regarded as
vastly ancient streams of antecedent courses; I am therefore much
tempted to consider them as of superimposed origin, inheriting their
present courses from the flood-plain cover of the Susquehanna in the
latest stage of the Jura-Cretaceous cycle. With this tentative
conclusion in mind as to the final events of Jura-Cretaceous time, we
may take up the more deliberate consideration of the work of the
Tertiary cycle.
The chief work of the Tertiary cycle was merely the opening of the
valley lowlands; little opportunity for river adjustment occurred
except on a small scale. The most evident cases of adjustment have
resulted in the change of water-gaps into wind-gaps, of which several
examples can be given, the one best known being the Delaware wind-gap
between the Lehigh and Delaware water-gaps in Blue mountain. The
wind-gap marks the unfinished notch of some stream that once crossed
the ridge here and whose headwaters have since then been diverted,
probably to the Lehigh. The difficulty in the case is not at all how
the stream that once flowed here was diverted, but how a stream that
could be diverted in the Tertiary cycle could have escaped diversion at
some earlier date. The relative rarity of wind-gaps indicates that
nearly all of the initial lateral streams, which may have crossed the
ridges at an early epoch in the history of the rivers, have been
beheaded in some cycle earlier than the Tertiary and their gaps
thereafter obliterated. Why the Delaware wind-gap stream should have
endured into a later cycle does not at present appear. Other wind-gaps
of apparently similar origin may be found in Blue mountain west of the
Schuylkill and east of the Susquehanna. It is noteworthy that if any
small streams still persevere in their gaps across a hard ridge, they
are not very close to any large river-gap; hence it is only at the very
headwaters of Conedogwinet creek, in the northern part of Franklin
county, that any water is still drawn from the back of Blue mountain.
Again, these small stream gaps do not lie between large river-gaps and
wind-gaps, but wind-gaps lie between the gaps of large rivers and those
of small streams that are not yet diverted. Excellent illustration of
this is found on the "Piedmont sheet" of the contoured maps issued by
the United States Geological Survey. The sheet covers part of Maryland
and West Virginia, near where the North Branch of the Potomac comes out
of the plateau and crosses New Creek mountain. Eleven miles south of
the Potomac gap there is a deep wind-gap; but further on, at twenty,
twenty-five and twenty-nine miles from the river-gap are three fine
water-gaps occupied by small streams. This example merely shows how
many important points in the history of our rivers will be made clear
when the country is properly portrayed on contoured maps.
A few lines may be given to the general absence of gaps in Blue
Mountain in Pennsylvania. When the initial consequent drainage was
established, many streams must have been located on the northward slope
of the great Cumberland highland, C, C, fig. 21; they must have gullied
the slope to great depths and carried away great volumes of the weak
Cambrian beds that lay deep within the hard outer casings of the mass.
Minor adjustments served to diminish the number of these streams, but
the more effective cause of their present rarity lay in the natural
selection of certain of them to become large streams; the smaller ones
were generally beheaded by these. The only examples of streams that
still cross this ridge with their initial Permian direction of flow to
the northwest are found in two southern branches of Tuscarora creek at
the southern point of Juniata county; and these survive because of
their obscure location among the many Medina ridges of that district,
where they were not easily accessible to capture by other streams.
38. _Tertiary adjustment of the Juniata on the Medina anticlines_.--The
lower course of the Juniata presents several examples of adjustment
referable to the last part of the Jura-Cretaceous cycle and to the
Tertiary cycle. The explanation offered for the escape of this river
from its initial syncline did not show any reason for its peculiar
position with respect to the several Medina anticlines that it now
borders, because at the time when it was led across country to the
Wiconisco syncline, the hard Medina beds of these anticlines were not
discovered. It is therefore hardly to be thought that the location of
the Juniata in the Narrows below Lewistown between Blue Ridge and Shade
mountain and its avoidance of Tuscarora mountain could have been
defined at that early date. But all these Medina anticlines rise more
or less above the Cretaceous baselevel, and must have had some effect
on the position taken by the river about the middle of that cycle when
its channel sank upon them. Blue Ridge and Black Log anticlines rise
highest. The first location of the cross-country stream that led the
early Juniata away from its initial syncline probably traversed the
Blue Ridge and Black Log anticlines while they were yet buried; but its
channel-cutting was much retarded on encountering them, and some branch
stream working around from the lower side of the obstructions may have
diverted the river to an easier path. The only path of the kind is the
narrow one between the overlapping anticlines of Blue Ridge and Shade
mountains, and there the Juniata now flows. If another elevation should
occur in the future, it might happen that the slow deepening of the
channel in the hard Medina beds which now floor the Narrows would allow
Middle creek of Snyder county to tap the Juniata at Lewistown and lead
it by direct course past Middleburgh to the Susquehanna; thus it would
return to the path of its youth.
The location of the Juniata at the end of Tuscarora mountain is again
so definite that it can hardly be referred to a time when the mountain
had not been revealed. The most likely position of the original
cross-country stream which brought the Juniata into the Wiconisco
syncline was somewhere on the line of the existing mountain, and
assuming it to have been there, we must question how it has been
displaced. The process seems to have been of the same kind as that just
given; the retardation of channel-cutting in the late Cretaceous cycle,
when the Medina beds of Tuscarora anticline were discovered, allowed a
branch from the lower part of the river to work around the end of the
mountain and lead the river out that way. The occurrence of a shallow
depression across the summit of the otherwise remarkably even crest of
Tuscarora mountain suggests that this diversion was not finally
accomplished until shortly after the Tertiary elevation of the country;
but at whatever date the adjustment occurred, it is natural that it
should pass around the eastern end of the mountain and not around the
western end, where the course would have been much longer, and
therefore not successfully to be taken by a diverting stream.
While the quality of these processes appears satisfactory, I am not
satisfied as to the sufficiency of their quantity. If diversion was
successfully practiced at the crossing of the Tuscarora anticline, why
not also at the crossing of Jack's mountain anticline, on which the
river still perseveres. It is difficult here to decide how much
confidence may be placed in the explanation, because of its giving
reason for the location of certain streams, and how much doubt must be
cast upon it, because it seems impossible and is not of universal
application.