may be next noted that the drainage of the Broad Top region does not
follow a single syncline to the Anthracite region, as it should have in
the initial stage of the consequent Permian drainage, but soon turns
aside from the syncline in which it starts and runs across country to
the Susquehanna. It is true that in its upper course the Juniata
departs from the Broad Top region by one of the two synclines that were
indicated as the probable line of discharge of the ancient Broad Top
lake in our restoration of the constructional topography of the State;
there does not appear to be any significant difference between the
summit altitudes of the Tuscarora-Mahanoy and the Juniata-Catawissa
synclinal axes and hence the choice must have been made for reasons
that cannot be detected; or it may be that the syncline lying more to
the northwest was raised last, and for this reason was taken as the
line of overflow. The beginning of the river is therefore not
discordant with the hypothesis of consequent drainage, but the
southward departure from the Catawissa syncline at Lewistown remains to
be explained. It seems to me that some reason for the departure may be
found by likening it to the case already given in figs. 16-18. The
several synclines with which the Juniata is concerned have precisely
the relative attitudes that are there discussed. The Juniata-Catawissa
syncline has parallel sides for many miles about its middle, and hence
must have long maintained the initial Juniata well above baselevel over
all this distance; the progress of cutting down a channel through all
the hard Carboniferous sandstones for so great a distance along the
axis must have been exceedingly slow. But the synclines next south, the
Tuscarora-Mahanoy and the Wiconisco, plunge to the northeast more
rapidly, as the rapid divergence of their margins demonstrates, and
must for this reason have carried the hard sandstones below baselevel
in a shorter distance and on a steeper slope than in the Catawissa
syncline. The further southwestward extension of the Pocono sandstone
ridges in the southern than in the northern syncline gives further
illustration of this peculiarity of form. Lateral capture of the
Juniata by a branch of the initial Tuscarora, and of the latter by a
branch of the Wiconisco therefore seems possible, and the accordance of
the facts with so highly specialized an arrangement is certainly again
indicative of the correctness of the hypothesis of consequent drainage,
and this time in a larger stream than before. At first sight, it
appears that an easier lateral capture might have been made by some of
the streams flowing from the outer slope of the Nittany highland; but
this becomes improbable when it is perceived that the heavy Medina
sandstone would here have to be worn through as well as the repeated
arches of the Carboniferous beds in the many high folds of the Seven
Mountains. Again, as far as present appearances go, we can give no
sufficient reason to explain why possession of the headwaters of the
Juniata was not gained by some subsequent stream of its own, such as G,
fig. 18, instead of by a side-stream of the river in the neighboring
syncline; but it may be admitted, on the other hand, that as far as we
can estimate the chances for conquest, there was nothing distinctly in
favor of one or the other of the side-streams concerned; and as long as
the problem is solved indifferently in favor of one or the other, we
may accept the lead of the facts and say that some control not now
apparent determined that the diversion should be, as drawn, through D
and not through G. The detailed location of the Juniata in its middle
course below Lewistown will be considered in a later section.
[Illustration: FIG. 22.]
[Illustration: FIG. 23.]
[Illustration: FIG. 24.]
30. _Avoidance of the Broad Top basin by the Juniata
headwaters_.--Another highly characteristic change that the Juniata has
suffered is revealed by examining the adjustments that would have taken
place in the general topography of the Broad Top district during the
Perm-Triassic cycle of erosion. When the basin, BT, fig. 22, was first
outlined, centripetal streams descended its slopes from all sides and
their waters accumulated as a lake in the center, overflowing to the
east into the subordinate basin, A, in the Juniata syncline along side
of the larger basin, and thence escaping northeast. In due time, the
breaching of the slopes opened the softer Devonian rocks beneath and
peripheral lowlands were opened on them. The process by which the
Juniata departed from its original axial location, J, fig. 22, to a
parallel course on the southeastern side of the syncline, J, fig. 23,
has been described (fig. 18). The subsequent changes are manifest. Some
lateral branch of the Juniata, like N, fig. 23, would work its way
around the northern end of the Broad Top canoe on the soft underlying
rocks and capture the axial stream, C, that came from the depression
between Nittany and Kishicoquillas highlands; thus reënforced, capture
would be made of a radial stream from the west, Tn, the existing Tyrone
branch of the Juniata; in a later stage the other streams of the
western side of the basin would be acquired, their divertor
constituting the Little Juniata of to-day; and the end would be when
the original Juniata, A, fig. 22, that once issued from the subordinate
synclinal as a large stream, had lost all its western tributaries, and
was but a shrunken beheaded remnant of a river, now seen in Aughwick
creek, A, fig. 24. In the meantime, the former lake basin was fast
becoming a synclinal mountain of diminishing perimeter. The only really
mysterious courses of the present streams are where the Little Juniata
runs in and out of the western border of the Broad Top synclinal, and
where the Frankstown (FT) branch of the Juniata maintains its
independent gap across Tussey's mountain (Medina), although diverted to
the Tyrone or main Juniata (Tn) by Warrior's ridge (Oriskany) just
below. At the time of the early predatory growth of the initial
divertor, N, its course lay by the very conditions of its growth on
only the weakest rocks; but after this little stream had grown to a
good-sized river, further rising of the land, probably in the time of
the Jurassic elevation, allowed the river to sink its channel to a
greater depth, and in doing so, it encountered the hard Medina
anticline of Jack's mountain; here it has since persisted, because, as
we may suppose, there has been no stream able to divert the course of
so large a river from its crossing of a single hard anticlinal.
The doubt that one must feel as to the possibility of the processes
just outlined arises, if I may gauge it by my own feeling, rather from
incredulity than from direct objections. It seems incredible that the
waste of the valley slopes should allow the backward growth of N at
such a rate as to enable it to capture the heads of C, Tn, F, and so
on, before they had cut their beds down close enough to the baselevel
of the time to be safe from capture. But it is difficult to urge
explicit objections against the process or to show its quantitative
insufficiency. It must be remembered that when these adjustments were
going on, the region was one of great altitude, its rocks then had the
same strong contrasts of strength and weakness that are so apparent in
the present relief of the surface and the streams concerned were of
moderate size; less than now, for at the time, the Tyrone, Frankstown
and Bedford head branches of the Juniata had not acquired drainage west
of the great Nittany-Bedford anticlinal axis, but were supplied only by
the rainfall on its eastern slope (see section 39)--and all these
conditions conspired to favor the adjustment. Finally, while apparently
extraordinary and difficult of demonstration, the explanation if
applicable at all certainly gives rational correlation to a number of
peculiar and special stream courses in the upper Juniata district that
are meaningless under any other theory that has come to my notice. It
is chiefly for this reason that I am inclined to accept the
explanation.
31. _Reversal of larger rivers to southeast courses_.--Our large rivers
at present flow to the southeast, not to the northwest. It is difficult
to find any precise date for this reversal of flow from the initial
hypothetical direction, but it may be suggested that it occurred about
the time of the Triassic depression of the Newark belt. We have been
persuaded that much time elapsed between the Permian folding and the
Newark deposition, even under the most liberal allowance for
pre-Permian erosion in the Newark belt; hence when the depression
began, the rivers must have had but moderate northwestward declivity.
The depression and submergence of the broad Newark belt may at this
time have broken the continuity of the streams that once flowed across
it. The headwater streams from the ancient Archean country maintained
their courses to the depression; the lower portions of the rivers may
also have gone on as before; but the middle courses were perhaps turned
from the central part of the state back of the Newark belt. No change
of attitude gives so fitting a cause of the southeastward flow of our
rivers as this. The only test that I have been able to devise for the
suggestion is one that is derived from the relation that exists between
the location of the Newark belt along the Atlantic slope and the course
of the neighboring transverse rivers. In Pennsylvania, where the belt
reaches somewhat beyond the northwestern margin of the crystalline
rocks in South mountain, the streams are reversed, as above stated; but
in the Carolinas where the Newark belt lies far to the east of the
boundary between the Cambrian and crystalline rocks, the Tennessee
streams persevere in what we suppose to have been their original
direction of flow. This may be interpreted as meaning that in the
latter region, the Newark depression was not felt distinctly enough, if
at all, within the Alleghany belt to reverse the flow of the streams;
while in the former region, it was nearer to these streams and
determined a change in their courses. The original Anthracite river ran
to the northwest, but its middle course was afterwards turned to the
southeast.
I am free to allow that this has the appearance of heaping hypothesis
on hypothesis; but in no other way does the analysis of the history of
our streams seem possible, and the success of the experiment can be
judged only after making it. At the same time, I am constrained to
admit that this is to my own view the least satisfactory of the
suggestions here presented. It may be correct, but there seems to be no
sufficient exclusion of other possibilities. For example, it must not
be overlooked that, if the Anthracite river ran southeast during Newark
deposition, the formation of the Newark northwestward monocline by the
Jurassic tilting would have had a tendency to turn the river back again
to its northwest flow. But as the drainage of the region is still
southeastward, I am tempted to think that the Jurassic tilting was not
here strong enough to reverse the flow of so strong and mature a river
as the Anthracite had by that time come to be; and that the elevation
that accompanied the tilting was not so powerful in reversing the river
to a northwest course as the previous depression of the Newark basin
had been in turning it to the southeast. If the Anthracite did continue
to flow to the southeast, it may be added that the down-cutting of its
upper branches was greatly retarded by the decrease of slope in its
lower course when the monocline was formed.
The only other method of reversing the original northwestward flow of
the streams that I have imagined is by capture of their headwaters by
Atlantic rivers. This seems to me less effective than the method just
considered; but they are not mutually exclusive and the actual result
may be the sum of the two processes. The outline of the idea is as
follows. The long continued supply of sedimentary material from the
Archean land on the southeast implies that it was as continually
elevated. But there came a time when there is no record of further
supply of material, and when we may therefore suppose the elevation was
no longer maintained. From that time onward, the Archean range must
have dwindled away, what with the encroachment of the Atlantic on its
eastern shore and the general action of denuding forces on its surface.
The Newark depression was an effective aid to the same end, as has been
stated above, and for a moderate distance westward of the depressed
belt, the former direction of the streams must certainly have been
reversed; but the question remains whether this reversal extended as
far as the Wyoming basin, and whether the subsequent formation of the
Newark monocline did not undo the effect of the Newark depression. It
is manifest that as far as our limited knowledge goes, it is impossible
to estimate these matters quantitatively, and hence the importance of
looking for additional processes that may supplement the effect of the
Newark depression and counteract the effect of the Newark uplift in
changing the course of the rivers. Let it be supposed for the moment
that at the end of the Jurassic uplift by which the Newark monocline
was formed, the divide between the Ohio and the Atlantic drainage lay
about the middle of the Newark belt. There was a long gentle descent
westward from this watershed and a shorter and hence steeper descent
eastward. Under such conditions, the divide must have been pushed
westward, and as long as the rocks were so exposed as to open areas of
weak sediments on which capture by the Atlantic streams could go on
with relative rapidity, the westward migration of the divide would be
important. For this reason, it might be carried from the Newark belt as
far as the present Alleghany front, beyond which further pushing would
be slow, on account of the broad stretch of country there covered by
hard horizontal beds.
The end of this is that, under any of the circumstances here detailed,
there would be early in the Jurassic-Cretaceous cycle a distinct
tendency to a westward migration of the Atlantic-Ohio divide; it is the
consequences of this that have now to be examined.
32. _Capture of the Anthracite headwaters by the growing
Susquehanna_.--Throughout the Perm-Triassic period of denudation, a
great work was done in wearing down the original Alleghanies.
Anticlines of hard sandstone were breached, and broad lowlands were
opened on the softer rocks beneath. Little semblance of the early
constructional topography remained when the period of Newark depression
was brought to a close; and all the while the headwater streams of the
region were gnawing at the divides, seeking to develop the most perfect
arrangement of waterways. Several adjustments have taken place, and the
larger streams have been reversed in the direction of their flow; but a
more serious problem is found in the disappearance of the original
master stream, the great Anthracite river, which must have at first led
away the water from all the lateral synclinal streams. Being a large
river, it could not have been easily diverted from its course, unless
it was greatly retarded in cutting down its channel by the presence of
many beds of hard rocks on its way. The following considerations may
perhaps throw some light on this obscure point.
[Illustration: FIG. 25. General distribution of high and low land and
drainage in early Jurassic time.]
It may be assumed that the whole group of mountains formed by the
Permian deformation had been reduced to a moderate relief when the
Newark deposition was stopped by the Jurassic elevation. The harder
ribs of rock doubtless remained as ridges projecting above the
intervening lowlands, but the strength of relief that had been given by
the constructional forces had been lost. The general distribution of
residual elevations then remaining unsubdued is indicated in fig. 25,
in which the Crystalline, the Medina, and the two Carboniferous
sandstone ridges are denoted by appropriate symbols. In restoring this
phase of the surface form, when the country stood lower than now, I
have reduced the anticlines from their present outlines and increased
the synclines, the change of area being made greatest where the dips
are least, and hence most apparent at the ends of the plunging
anticlines and synclines. Some of the Medina anticlines of Perry and
Juniata counties are not indicated because they were not then
uncovered. The country between the residual ridges of Jurassic time was
chiefly Cambrian limestone and Siluro-Devonian shales and soft
sandstones. The moderate ridges developed on the Oriskany and Chemung
sandstones are not represented. The drainage of this stage retained the
original courses of the streams, except for the adjustments that have
been described, but the great Anthracite river is drawn as if it had
been controlled by the Newark depression and reversed in the direction
of its flow, so that its former upper course on the Cambrian rocks was
replaced by a superimposed Newark lower course. Fig. 25 therefore
represents the streams for the most part still following near their
synclinal axes, although departing from them where they have to enter a
synclinal cove-mountain ridge; the headwaters of the Juniata avoid the
mass of hard sandstones discovered in the bottom of old Broad Top lake,
and flow around them to the north, and then by a cross-country course
to the Wiconisco synclinal, as already described in detail. Several
streams come from the northeast, entering the Anthracite district after
the fashion generalized in fig. 13. Three of the many streams that were
developed on the great Kittatinny slope are located, with their
direction of flow reversed; these are marked Sq, L and D, and are
intended to represent the ancestors of the existing Susquehanna, Lehigh
and Delaware. We have now to examine the opportunities offered to these
small streams to increase their drainage areas.
The Jurassic elevation, by which the Newark deposition was stopped,
restored to activity all the streams that had in the previous cycle
sought and found a course close to baselevel. They now all set to work
again deepening their channels. But in this restoration of lost
activity with reference to a new baselevel, there came the best
possible chance for numerous re-arrangements of drainage areas by
mutual adjustment into which we must inquire.
I have already illustrated what seems to me to be the type of the
conditions involved at this time in figs. 19 and 20. The master stream,
A, traversing the synclines, corresponds to the reversed Anthracite
river; the lowlands at the top are those that have been opened out on
the Siluro-Devonian beds of the present Susquehanna middle course
between the Pocono and the Medina ridges. The small stream, B, that is
gaining drainage area in these lowlands, corresponds to the embryo of
the present Susquehanna, Sq, fig. 25, this having been itself once a
branch on the south side of the Swatara synclinal stream, fig. 21, from
which it was first turned by the change of slope accompanying the
Newark depression; but it is located a little farther west than the
actual Susquehanna, so as to avoid the two synclinal cove mountains of
Pocono sandstone that the Susquehanna now traverses, for reasons to be
stated below (section 35). This stream had to cross only one bed of
hard rock, the outer wall of Medina sandstone, between the broad inner
lowlands of the relatively weak Siluro-Devonian rocks and the great
valley lowlands on the still weaker Cambrian limestones. Step by step
it must have pushed its headwater divide northward, and from time to
time it would have thus captured a subsequent stream, that crossed the
lowlands eastward, and entered a Carboniferous syncline by one of the
lateral gaps already described. With every such capture, the power of
the growing stream to capture others was increased. Fig. 19 represents
a stage after the streams in the Swatara and Wiconisco synclines (the
latter then having gained the Juniata) had been turned aside on their
way to the Carboniferous basins. On the other hand, the Anthracite
river, rising somewhere on the plains north of the Wyoming syncline and
pursuing an irregular course from one coal basin to another, found an
extremely difficult task in cutting down its channel across the
numerous hard beds of the Carboniferous sandstones, so often repeated
in the rolling folds of the coal fields. It is also important to
remember that an aid to other conditions concerned in the diversion of
the upper Anthracite is found in the decrease of slope that its lower
course suffered in crossing the coal fields, if that area took any part
in the deformation that produced the Newark monocline--whichever theory
prove true in regard to the origin of the southeastward flow of the
rivers--for loss of slope in the middle course, where the river had to
cross many reefs of hard sandstone, would have been very effective in
lengthening the time allowed for the diversion of the headwaters.
The question is, therefore, whether the retardation of down-cutting
here experienced by the Anthracite was sufficient to allow the capture
of its headwaters by the Susquehanna. There can be little doubt as to
the correct quality of the process, but whether it was quantitatively
sufficient is another matter. In the absence of any means of testing
its sufficiency, may the result not be taken as the test? Is not the
correspondence between deduction and fact close enough to prove the
correctness of the deduction?
33. _Present outward drainage of the Anthracite basins_.--The Lehigh,
like the Susquehanna, made an attempt to capture the headwaters of
adjacent streams, but failed to acquire much territory from the
Anthracite because the Carboniferous sandstones spread out between the
two in a broad plateau of hard rocks, across which the divide made
little movement. The plateau area that its upper branches drain is, I
think, the conquest of a later cycle of growth. The Delaware had little
success, except as against certain eastern synclinal branches of the
Anthracite, for the same reason. The ancestor of the Swatara of to-day
made little progress in extending its headwaters because its point of
attack was against the repeated Carboniferous sandstones in the Swatara
synclinal. One early stream alone found a favorable opportunity for
conquest, and thus grew to be the master river--the Susquehanna of
to-day. The head of the Anthracite was carried away by this captor, and
its beheaded lower portion remains in our Schuylkill. The Anthracite
coal basins, formerly drained by the single master stream, have since
been apportioned to the surrounding rivers. As the Siluro-Devonian
lowlands were opened around the coal-basins, especially on the north
and west, the streams that formerly flowed into the basins were
gradually inverted and flowed out of them, as they still do. The extent
of the inversion seems to be in a general way proportionate to its
opportunity. The most considerable conquests were made in the upper
basins, where the Catawissa and Nescopec streams of to-day drain many
square miles of wide valleys opened on the Mauch Chunk red shale
between the Pocono and Pottsville sandstone ridges; the ancient middle
waters of the Anthracite here being inverted to the Susquehanna
tributaries, because the northern coal basins were degraded very slowly
after the upper Anthracite had been diverted. The Schuylkill as the
modern representative of the Anthracite retains only certain streams
south of a medial divide between Nescopec and Blue mountains. The only
considerable part of the old Anthracite river that still retains a
course along the axis of a synclinal trough seems to be that part which
follows the Wyoming basin; none of the many other coal basins are now
occupied by the large stream that originally followed them. The reason
for this is manifestly to be found in the great depth of the Wyoming
basin, whereby the axial portion of its hard sandstones are even now
below baselevel, and hence have never yet acted to throw the river from
its axial course. Indeed, during the early cycles of denudation, this
basin must have been changed from a deep lake to a lacustrine plain by
the accumulation in it of waste from the surrounding highlands, and for
a time the streams that entered it may have flowed in meandering
courses across the ancient alluvial surface; the lacustrine and
alluvial condition may have been temporarily revived at the time of the
Jurassic elevation. It is perhaps as an inheritance from a course thus
locally superimposed that we may come to regard the deflection of the
river at Nanticoke from the axis of the syncline to a narrow shale
valley on its northern side, before turning south again and leaving the
basin altogether. But like certain other suggestions, this can only be
regarded as an open hypothesis, to be tested by some better method of
river analysis than we now possess; like several of the other
explanations here offered, it is presented more as a possibility to be
discussed than as a conclusion to be accepted.
I believe that it was during the earlier part of the great
Jura-Cretaceous cycle of denudation that the Susquehanna thus became
the master stream of the central district of the state. For the rest of
the cycle, it was occupied in carrying off the waste and reducing the
surface to a well finished baselevel lowland that characterized the end
of Cretaceous time. From an active youth of conquest, the Susquehanna
advanced into an old age of established boundaries; and in later times,
its area of drainage does not seem to have been greatly altered from
that so long ago defined; except perhaps in the districts drained by
the West and North Branch headwaters.
34. _Homologies of the Susquehanna and Juniata_.--Looking at the change
from the Anthracite to the Susquehanna in a broad way, one may perceive
that it is an effect of the same order as the peripheral diversion of
the Broad Top drainage, illustrated in figures 22, 23 and 24; another
example of a similar change is seen in the lateral diversion of the
Juniata above Lewistown and its rectilinear continuation in Aughwick
creek, from their original axial location when they formed the initial
Broad Top outlet. They have departed from the axis of their syncline to
the softer beds on its southern side; FE of fig. 17 has been diverted
to FD of fig. 18.