Medlicott and others showed the validity of the explanation in other
regions than the one for which it was first proposed, it has found much
acceptance. Löwl's objection to it does not seem to me to be nearly so
well founded as his suggestion of an additional method of river
development by means of backward headwater erosion and subsequent
capture of other streams, as already described. And yet I cannot help
thinking that the explanation of transverse valleys as antecedent
courses savors of the Gordian method of explaining a difficult matter.
The case of the Green river, to which Powell first gave this
explanation, seems well supported; the examples given by Medlicott in
the Himalayas are as good: but still it does not seem advisable to
explain all transverse streams in this way, merely because they are
transverse. Perhaps one reason why the explanation has become so
popular is that it furnishes an escape from the old catastrophic idea
that fractures control the location of valleys, and is at the same time
fully accordant with the ideas of the uniformitarian school that have
become current in this half of our century. But when it is remembered
that most of the streams of a region are extinguished at the time of
mountain growth, that only a few of the larger ones can survive, and
that there are other ways in which transverse streams may
originate,[20] it is evident that the possibility of any given
transverse stream being antecedent must be regarded only as a
suggestion, until some independent evidence is introduced in its favor.
This may be difficult to find, but it certainly must be searched for;
if not then forthcoming, the best conclusion may be to leave the case
open until the evidence appears. Certainly, if we find a river course
that is accordant in its location with the complicated results of other
methods of origin, then the burden of proof may be said to lie with
those who would maintain that an antecedent origin would locate the
river in so specialized a manner. Even if a river persist for a time in
an antecedent course, this may not prevent its being afterwards
affected by the various adjustments and revivals that have been
explained above: rivers so distinctly antecedent as the Green and the
Sutlej may hereafter be more or less affected by processes of
adjustment, which they are not yet old enough to experience. Hence in
mountains as old as the Appalachians the courses of the present rivers
need not coincide with the location of the pre-Permian rivers, even if
the latter persisted in their courses through the growth of the Permian
folding; subsequent elevations and adjustments to hard beds, at first
buried and unseen, may have greatly displaced them, in accordance with
Löwl's principle.
[Footnote 20: Hilber, Pet. Mitth., xxxv, 1889, 13.]
When the deeper channelling of a stream discovers an unconformable
subjacent terrane, the streams persist at least for a time in the
courses that were determined in the overlying mass; they are then
called superimposed (Powell), inherited (Shaler), or epigenetic
(Richthofen). Such streams are particularly liable to readjustment by
transfer of channels from courses that lead them over hard beds to
others on which the hard beds are avoided; for the first choice of
channels, when the unconformable cover was still present, was made
without any knowledge of the buried rock structure or of the
difficulties in which the streams would be involved when they
encountered it. The examples of falls produced when streams terrace
their flood-plains and run on buried spurs has already been referred to
as superimposed; and the rivers of Minnesota now disclosing half-buried
ledges here and there may be instanced as illustrating the transition
stage between simple consequent courses, determined by the form of the
drift sheet on which their flow began, and the fully inconsequent
courses that will be developed there in the future.
22. _Simple, compound, composite and complex rivers_.--We have thus far
considered an ideal river. It now seems advisable to introduce a few
terms with which to indicate concisely certain well marked
peculiarities in the history of actual rivers.
An original river has already been defined as one which first takes
possession of a land area, or which replaces a completely extinguished
river on a surface of rapid deformation.
A river may be simple, if its drainage area is of practically one kind
of structure and of one age; like the rivers of southern New Jersey.
Such rivers are generally small. It may be composite, when drainage
areas of different structure are included in the basin of a single
stream. This is the usual case.
A compound river is one which is of different ages in its different
parts; as certain rivers of North Carolina, which have old headwaters
rising in the mountains, and young lower courses traversing the coastal
plain.
A river is complex when it has entered a second or later cycle of
development; the headwaters of a compound river are therefore complex,
while the lower course may be simple, in its first cycle. The degree of
complexity measures the number of cycles that the river has entered.
When the study of rivers is thus attempted, its necessary complications
may at first seem so great as to render it of no value; but in answer
to this I believe that it may be fairly urged that, although
complicated, the results are true to nature, and if so, we can have no
ground of complaint against them. Moreover, while it is desirable to
reduce the study of the development of rivers to its simplest form, in
order to make it available for instruction and investigation, it must
be remembered that this cannot be done by neglecting to investigate the
whole truth in the hope of avoiding too great complexity, but that
simplicity can be reached safely only through fullness of knowledge, if
at all.
It is with these points in mind that I have attempted to decipher the
history of the rivers of Pennsylvania. We find in the Susquehanna,
which drains a great area in the central part of the state, an example
of a river which is at once composite, compound and highly complex. It
drains districts of divers structure; it traverses districts of
different ages; and it is at present in its fourth or fifth degree of
complexity, its fourth or fifth cycle of development at least. In
unravelling its history and searching out the earlier courses of
streams which may have long since been abandoned in the processes of
mature adjustment, it will be seen that the size of the present streams
is not always a measure of their previous importance, and to this we
may ascribe the difficulty that attends the attempt to decipher a
river's history from general maps of its stream lines. Nothing but a
detailed examination of geological structure and history suffices to
detect facts and conditions that are essential to the understanding of
the result.
If the postulates that I shall use seem unsound and the arguments seem
overdrawn, error may at least be avoided by not holding fast to the
conclusions that are presented, for they are presented only
tentatively. I do not feel by any means absolutely persuaded of the
correctness of the results, but at the same time deem them worth giving
out for discussion. The whole investigation was undertaken as an
experiment to see where it might lead, and with the hope that it might
lead at least to a serious study of our river problems.
PART FOURTH. _The development of the rivers of Pennsylvania_.
23. _Means of distinguishing between antecedent and adjusted consequent
rivers_.--The outline of the geological history of Pennsylvania given
above affords means of dividing the long progress of the development of
our rivers into the several cycles which make up their complete life.
We must go far back into the past and imagine ancient streams flowing
down from the Archean land towards the paleozoic sea; gaining length by
addition to their lower portions as the land grew with the building on
of successive mountain ranges; for example, if there were a
Cambro-Silurian deformation, a continuation of the Green Mountains into
Pennsylvania, we suppose that the pre-existent streams must in some
manner have found their way westward to the new coastline; and from the
date of this mountain growth, it is apparent that any streams then born
must have advanced far in their history before the greater Appalachian
disturbance began. At the beginning of the latter, as of the former,
there must have been streams running from the land into the sea, and at
times of temporary elevation of the broad sand-flats of the coal
measures, such streams must have had considerable additions to their
lower length; rising in long-growing Archean highlands or mountains,
snow-capped and drained by glaciers for all we can say to the contrary,
descending across the Green Mountain belt, by that time worn to
moderate relief in the far advanced stage of its topographic
development, and finally flowing across the coal-measure lowlands of
recent appearance. It was across the lower courses of such rivers that
the Appalachian folds were formed, and the first step in our problem
consists in deciding if possible whether the streams held their courses
after the antecedent fashion, or whether they were thrown into new
courses by the growing folds, so that a new drainage system would be
formed. Possibly both conditions prevailed; the larger streams holding
their courses little disturbed, and the smaller ones disappearing, to
be replaced by others as the slopes of the growing surface should
demand. It is not easy to make choice in this matter. To decide that
the larger streams persisted and are still to be seen in the greater
rivers of to-day, only reversed in direction of flow, is certainly a
simple method of treating the problem, but unless some independent
reasons are found for this choice, it savors of assumption. Moreover,
it is difficult to believe that any streams, even if antecedent and
more or less persistent for a time during the mountain growth, could
preserve till now their pre-Appalachian courses through all the varying
conditions presented by the alternations of hard and soft rocks through
which they have had to cut, and at all the different altitudes above
baselevel in which they have stood. A better means of deciding the
question will be to admit provisionally the occurrence of a completely
original system of consequent drainage, located in perfect accord with
the slopes of the growing mountains; to study out the changes of
stream-courses that would result from later disturbances and from the
mutual adjustments of the several members of such a system in the
different cycles of its history; and finally to compare the courses
thus deduced with those now seen. If there be no accord, either the
method is wrong or the streams are not consequent but of some other
origin, such as antecedent; if the accord between deduction and fact be
well marked, varying only where no definite location can be given to
the deduced streams, but agreeing where they can be located more
precisely, then it seems to me that the best conclusion is distinctly
in favor of the correctness of the deductions. For it is not likely,
even if it be possible, that antecedent streams should have
accidentally taken, before the mountains were formed, just such
locations as would have resulted from the subsequent growth of the
mountains and from the complex changes in the initial river courses due
to later adjustments. I shall therefore follow the deductive method
thus indicated and attempt to trace out the history of a completely
original, consequent system of drainage accordant with the growth of
the central mountain district.
In doing this, it is first necessary to restore the constructional
topography of the region; that is, the form that the surface would have
had if no erosion had accompanied its deformation. This involves
certain postulates which must be clearly conceived if any measure of
confidence is to be gained in the results based upon them.
24. _Postulates of the argument_.--In the first place, I assume an
essential constancy in the thickness of the paleozoic sediments over
the entire area in question. This is warranted here because the known
variations of thickness are relatively of a second order, and will not
affect the distribution of high and low ground as produced by the
intense Permian folding. The reasons for maintaining that the whole
series had a considerable extension southeast of the present margin of
the Medina sandstone have already been presented.
In the second place, I shall assume that the dips and folds of the beds
now exposed at the surface of the ground may be projected upwards into
the air in order to restore the form of the eroded beds. This is
certainly inadmissible in detail, for it cannot be assumed that the
folded slates and limestones of the Nittany valley, for instance, give
any close indication of the form that the coal measures would have
taken, had they extended over this district, unworn. But in a general
way, the Nittany massif was a complex arch in the coal measures as well
as in the Cambrian beds; for our purpose and in view of the moderate
relief of the existing topography, it suffices to say that wherever the
lower rocks are now revealed in anticlinal structure, there was a great
upfolding and elevation of the original surface; and wherever the
higher rocks are still preserved, there was a relatively small
elevation.
In the third place, I assume that by reconstructing from the completed
folds the form which the country would have had if unworn, we gain a
sufficiently definite picture of the form through which it actually
passed at the time of initial and progressive folding. The difference
between the form of the folds completely restored and the form that the
surface actually reached is rather one of degree than of kind; the two
must correspond in the general distribution of high and low ground and
this is the chief consideration in our problem. When we remember how
accurately water finds its level, it will be clearer that what is
needed in the discussion is the location of the regions that were
relatively raised and lowered, as we shall then have marked out the
general course of the consequent water ways and the trend of the
intervening constructional ridges.
Accepting these postulates, it may be said in brief that the outlines
of the formations as at present exposed are in effect so many contour
lines of the old constructional surface, on which the Permian rivers
took their consequent courses. Where the Trenton limestone is now seen,
the greatest amount of overlying strata must have been removed; hence
the outline of the Trenton formation is our highest contour line. Where
the Helderberg limestone appears, there has been a less amount of
material removed; hence the Helderberg outcrop is a contour of less
elevation. Where the coal beds still are preserved, there has been
least wasting, and these beds therefore mark the lowest contour of the
early surface. It is manifest that this method assumes that the present
outcrops are on a level surface; this is not true, for the ridges
through the State rise a thousand feet more or less over the
intervening valley lowlands, and yet the existing relief does not count
for much in discussing the enormous relief of the Permian surface that
must have been measured in tens of thousands of feet at the time of its
greatest strength.
[Illustration: FIG. 21. Constructional Permian topography of
Pennsylvania.]
25. _Constructional Permian topography and consequent drainage_.--A
rough restoration of the early constructional topography is given in
fig. 21 for the central part of the State, the closest shading being
the area of the Trenton limestone, indicating the highest ground, or
better, the places of greatest elevation, while the Carboniferous area
is unshaded, indicating the early lowlands. The prevalence of northeast
and southwest trends was then even more pronounced than now. Several of
the stronger elements of form deserve names, for convenient reference.
Thus we have the great Kittatinny or Cumberland highland, C, C, on the
southeast, backed by the older mountains of Cambrian and Archean rocks,
falling by the Kittatinny slope to the synclinal lowland troughs of the
central district. In this lower ground lay the synclinal troughs of the
eastern coal regions, and the more local Broad Top basin, BT, on the
southwest, then better than now deserving the name of basins. Beyond
the corrugated area that connected the coal basins rose the great
Nittany highland, N, and its southwest extension in the Bedford range,
with the less conspicuous Kishicoquilas highland, K, in the foreground.
Beyond all stretched the great Alleghany lowland plains. The names thus
suggested are compounded of the local names of to-day and the
morphological names of Permian time.
What would be the drainage of such a country? Deductively we are led to
believe that it consisted of numerous streams as marked in full lines
on the figure, following synclinal axes until some master streams led
them across the intervening anticlinal ridges at the lowest points of
their crests and away into the open country to the northwest. All the
enclosed basins would hold lakes, overflowing at the lowest part of the
rim. The general discharge of the whole system would be to the
northwest. Here again we must resort to special names for the easy
indication of these well-marked features of the ancient and now
apparently lost drainage system. The master stream of the region is the
great Anthracite river, carrying the overflow of the Anthracite lakes
off to the northwest and there perhaps turning along one of the faintly
marked synclines of the plateau and joining the original Ohio, which
was thus confirmed in its previous location across the Carboniferous
marshes. The synclinal streams that entered the Anthracite lakes from
the southwest may be named, beginning on the south, the Swatara, S,
fig. 21, the Wiconisco, Wo, the Tuscarora-Mahanoy, M, the
Juniata-Catawissa, C, and the Wyoming, Wy. One of these, probably the
fourth, led the overflow from the Broad Top lake into the Catawissa
lake on the middle Anthracite river. The Nittany highland formed a
strong divide between the central and northwestern rivers, and on its
outer slope there must have been streams descending to the Alleghany
lowlands; and some of these may be regarded as the lower courses of
Carboniferous rivers, that once rose in the Archean mountains, now
beheaded by the growth of mountain ranges across their middle.
26. _The Jura mountains homologous with the Permian
Alleghanies_.--However willing one may be to grant the former existence
of such a drainage system as the above, an example of a similar one
still in existence would be acceptable as a witness to the
possibilities of the past. Therefore we turn for a moment to the Jura
mountains, always compared to the Appalachians on account of the
regular series of folds by which the two are characterized. But while
the initial topography is long lost in our old mountains, it is still
clearly perceptible in the young Jura, where the anticlines are still
ridges and the longitudinal streams still follow the synclinal troughs;
while the transverse streams cross from one synclinal valley to another
at points where the intervening anticlinal arches are lowest.[21] We
could hardly ask for better illustration of the deductive drainage
system of our early Appalachians than is here presented.
[Footnote 21: This is beautifully illustrated in the recent monograph
by La Noë and Margerie on "Les Formes du Terrain."]
27. _Development and adjustment of the Permian drainage_.--The problem
is now before us. Can the normal sequence of changes in the regular
course of river development, aided by the post-Permian deformations and
elevations, evolve the existing rivers out of the ancient ones?
In order to note the degree of comparison that exists between the two,
several of the larger rivers of to-day are dotted on the figure. The
points of agreement are indeed few and small. Perhaps the most
important ones are that the Broad Top region is drained by a stream,
the Juniata, which for a short distance follows near the course
predicted for it; and that the Nittany district, then a highland, is
still a well-marked divide although now a lowland. But there is no
Anthracite river, and the region of the ancient coal-basin lakes is now
avoided by large streams; conversely, a great river--the
Susquehanna--appears where no consequent river ran in Permian time, and
the early synclinal streams frequently turn from the structural troughs
to valleys located on the structural arches.
28. _Lateral water-gaps near the apex of synclinal ridges_.--One of the
most frequent discrepancies between the hypothetical and actual streams
is that the latter never follow the axis of a descending syncline along
its whole length, as the original streams must have done, but depart
for a time from the axis and then return to it, notching the ridge
formed on any hard bed at the side instead of at the apex of its curve
across the axis of the syncline. There is not a single case in the
state of a stream cutting a gap at the apex of such a synclinal curve,
but there are perhaps hundreds of cases where the streams notch the
curve to one side of the apex. This, however, is precisely the
arrangement attained by spontaneous adjustment from an initial axial
course, as indicated in figure 13. The gaps may be located on small
transverse faults, but as a rule they seem to have no such guidance. It
is true that most of our streams now run out of and not into the
synclinal basins, but a reason for this will be found later; for the
present we look only at the location of the streams, not at their
direction of flow. As far as this illustration goes, it gives evidence
that the smaller streams at least possess certain peculiarities that
could not be derived from persistence in a previous accidental
location, but which would be necessarily derived from a process of
adjustment following the original establishment of strictly consequent
streams. Hence the hypothesis that these smaller streams were long ago
consequent on the Permian folding receives confirmation; but this says
nothing as to the origin of the larger rivers, which might at the same
time be antecedent.