Vol. I. No. 3.
THE NATIONAL GEOGRAPHIC MAGAZINE.
PUBLISHED BY THE
NATIONAL GEOGRAPHIC SOCIETY.
WASHINGTON, D. C.
Price 50 Cents.
CONTENTS.
The Rivers and Valleys of Pennsylvania: William Morris Davis
(Illustrated by one map and twenty-five cuts.)
Topographic Models: Cosmos Mindeleff
(Illustrated by two plates.)
National Geographic Society--Abstract of Minutes
International Literary Contest to be held at Madrid, Spain
July, 1889.
PRESS OF TUTTLE, MOREHOUSE & TAYLOR, NEW HAVEN, CONN.
THE NATIONAL GEOGRAPHIC MAGAZINE.
Vol. I. 1889. No. 3.
THE RIVERS AND VALLEYS OF PENNSYLVANIA.[1]
BY WILLIAM MORRIS DAVIS.
"In Faltensystemen von sehr hohem Alter wurde die ursprüngliche
Anordnung der Langenthäler durch das Ueberhandnehmen der transversalen
Erosionsfurchen oft ganz und gar verwischt."
LÖWL. Petermann's Mittheilungen, xxviii, 1882, 411.
[Footnote 1: The substance of this essay was presented to the Society
in a lecture on February 8th, 1889, but since then it has been much
expanded.]
CONTENTS.
PART FIRST. _Introductory_.
1. Plan of work here proposed.
2. General description of the topography of Pennsylvania.
3. The drainage of Pennsylvania.
4. Previous studies of Appalachian drainage.
PART SECOND. _Outline of the geological history of the region_.
5. Conditions of formation.
6. Former extension of strata to the southeast.
7. Cambro-Silurian and Permian deformations.
8. Perm-Triassic denudation.
9. Newark deposition.
10. Jurassic tilting.
11. Jura-Cretaceous denudation.
12. Tertiary elevation and denudation.
13. Later changes of level.
14. Illustrations of Pennsylvanian topography.
PART THIRD. _General conception of the history of a river_.
15. The complete cycle of river life: youth, adolescence,
maturity and old age.
16. Mutual adjustment of river courses.
17. Terminology of rivers changed by adjustment.
18. Examples of adjustments.
19. Revival of rivers by elevation and drowning by depression.
20. Opportunity for new adjustments with revival.
21. Antecedent and superimposed rivers.
22. Simple, compound, composite and complex rivers.
PART FOURTH. _The development of the rivers of Pennsylvania_.
23. Means of distinguishing between antecedent and adjusted
consequent rivers.
24. Postulates of the argument.
25. Constructional Permian topography and consequent drainage.
26. The Jura mountains homologous with the Permian Alleghanies.
27. Development and adjustment of the Permian drainage.
28. Lateral water-gaps near the apex of synclinal ridges.
29. Departure of the Juniata from the Juniata-Catawissa syncline.
30. Avoidance of the Broad Top basin by the Juniata headwaters.
31. Reversal of larger rivers to southeast courses.
32. Capture of the Anthracite headwaters by the growing Susquehanna.
33. Present outward drainage of the Anthracite basins.
34. Homologies of the Susquehanna and Juniata.
35. Superimposition of the Susquehanna on two synclinal ridges.
36. Evidence of superimposition in the Susquehanna tributaries.
37. Events of the Tertiary cycle.
38. Tertiary adjustment of the Juniata on the Medina anticlines.
39. Migration of the Atlantic-Ohio divide.
40. Other examples of adjustments.
41. Events of the Quaternary cycle.
42. Doubtful cases.
43. Complicated history of our actual rivers.
44. Provisional conclusions.
PART FIRST. _Introductory_.
1. _Plan of work here proposed_.--No one now regards a river and its
valley as ready-made features of the earth's surface. All are convinced
that rivers have come to be what they are by slow processes of natural
development, in which every peculiarity of river-course and valley-form
has its appropriate cause. Being fully persuaded of the gradual and
systematic evolution of topographic forms, it is now desired, in
studying the rivers and valleys of Pennsylvania, to seek the causes of
the location of the streams in their present courses; to go back if
possible to the early date when central Pennsylvania was first raised
above the sea and trace the development of the several river systems
then implanted upon it from their ancient beginning to the present
time.
The existing topography and drainage system of the State will first be
briefly described. We must next inquire into the geological structure
of the region, follow at least in a general way the deformations and
changes of attitude and altitude that it has suffered, and consider the
amount of denudation that has been accomplished on its surface. We must
at the same time bear in mind the natural history of rivers, their
morphology and development; we must recognize the varying activities of
a river in its youth and old age, the adjustments of its adolescence
and maturity, and the revival of its decrepit powers when the land that
it drains is elevated and it enters a new cycle of life. Finally we
shall attempt to follow out the development of the rivers of
Pennsylvania by applying the general principles of river history to the
special case of Pennsylvania structure.
2. _General description of the topography of Pennsylvania_.--The
strongly marked topographic districts of Pennsylvania can hardly be
better described than by quoting the account given over a century ago
by Lewis Evans, of Philadelphia, in his "Analysis of a map of the
middle British colonies in America" (1755), which is as valuable from
its appreciative perception as it is interesting from its early date.
The following paragraphs are selected from his early pages:
"The land southwestward of Hudson's River is more regularly divided and
into a greater number of stages than the other. The first object worthy
of regard in this part is a rief or vein of rocks of the talky or
isinglassy kind, some two or three or half a dozen miles broad; rising
generally some small matter higher than the adjoining land; and
extending from New York city southwesterly by the lower falls of
Delaware, Schuylkill, Susquehanna, Gun-Powder, Patapsco, Potomack,
Rapahannock, James river and Ronoak. This was the antient maritime
boundary of America and forms a very regular curve. The land between
this rief and the sea and from the Navesink hills southwest ... may be
denominated the Lower Plains, and consists of soil washt down from
above and sand accumulated from the ocean. Where these plains are not
penetrated by rivers, they are a white sea-sand, about twenty feet deep
and perfectly barren, as no mixture of soil helps to enrich them. But
the borders of the rivers, which descend from the uplands, are rendered
fertile by the soil washt down with the floods and mixt with the sands
gathered from the sea. The substratum of sea-mud, shells and other
foreign subjects is a perfect confirmation of this supposition. And
hence it is that for 40 or 50 miles inland and all the way from the
Navesinks to Cape Florida, all is a perfect barren where the wash from
the uplands has not enriched the borders of the rivers; or some ponds
and defiles have not furnished proper support for the growth of white
cedars....
"From this rief of rocks, over which all the rivers fall, to that chain
of broken hills, called the South mountain, there is the distance of
50, 60 or 70 miles of very uneven ground, rising sensibly as you
advance further inland, and may be denominated the Upland. This
consists of veins of different kinds of soil and substrata some scores
of miles in length; and in some places overlaid with little ridges and
chains of hills. The declivity of the whole gives great rapidity to the
streams; and our violent gusts of rain have washt it all into gullies,
and carried down the soil to enrich the borders of the rivers in the
Lower Plains. These inequalities render half the country not easily
capable of culture, and impoverishes it, where torn up by the plow, by
daily washing away the richer mould that covers the surface.
"The South mountain is not in ridges like the Endless mountains, but in
small, broken, steep, stoney hills; nor does it run with so much
regularity. In some places it gradually degenerates to nothing, not to
appear again for some miles, and in others it spreads several miles in
breadth. Between South mountain and the hither chain of the Endless
mountains (often for distinction called the North mountain, and in some
places the Kittatinni and Pequélin), there is a valley of pretty even
good land, some 8, 10 or 20 miles wide, and is the most considerable
quantity of valuable land that the English are possest of; and runs
through New Jersey, Pensilvania, Mariland and Virginia. It has yet
obtained no general name, but may properly enough be called Piemont,
from its situation. Besides conveniences always attending good land,
this valley is everywhere enriched with Limestone.
"The Endless mountains, so called from a translation of the Indian name
bearing that signification, come next in order. They are not confusedly
scattered and in lofty peaks overtopping one another, but stretch in
long uniform ridges scarce half a mile perpendicular in any place above
the intermediate vallies. Their name is expressive of their extent,
though no doubt not in a literal sense.... The mountains are almost all
so many ridges with even tops and nearly of a height. To look from
these hills into the lower lands is but, as it were, into an ocean of
woods, swelled and deprest here and there by little inequalities, not
to be distinguished one part from another any more than the waves of
the real ocean. The uniformity of these mountains, though debarring us
of an advantage in this respect, makes some amends in another. They are
very regular in their courses, and confine the creeks and rivers that
run between; and if we know where the gaps are that let through these
streams, we are not at a loss to lay down their most considerable
inflections....
"To the northwestward of the Endless mountains is a country of vast
extent, and in a manner as high as the mountains themselves. To look at
the abrupt termination of it, near the sea level, as is the case on the
west side of Hudson's river below Albany, it looks as a vast high
mountain; for the Kaats Kills, though of more lofty stature than any
other mountains in these parts of America, are but the continuation of
the Plains on the top, and the cliffs of them in the front they present
towards Kinderhook. These Upper Plains are of extraordinary rich level
land, and extend from the Mohocks river through the country of the
Confederates.[2] Their termination northward is at a little distance
from Lake Ontario; but what it is westward is not known, for those most
extensive plains of Ohio are part of them."
[Footnote 2: Referring to the league of Indian tribes, so-called.]
These several districts recognized by Evans may be summarized as the
coastal plain, of nearly horizontal Cretaceous and later beds, just
entering the southeastern corner of Pennsylvania; the marginal upland
of contorted schists of disputed age; the South Mountain belt of
ancient and much disturbed crystalline rocks, commonly called Archean;
a space between these two traversed by the sandstone lowland of the
Newark formation;[3] the great Appalachian valley of crowded Cambrian
limestones and slates; the region of the even-crested, linear Paleozoic
ridges, bounded by Kittatinny or Blue mountain on the southeast and by
Alleghany mountain on the northwest, this being the area with which we
are here most concerned; and finally the Alleghany plateau, consisting
of nearly horizontal Devonian and Carboniferous beds and embracing all
the western part of the state. The whole region presents the most
emphatic expression not only of its structure but also of the more
recent cycles of development through which it has passed. Fig. 1
represents the stronger ridges and larger streams of the greater part
of the central district: it is reproduced from the expressive
Topographic Map of Pennsylvania (1871) by Lesley. The Susquehanna flows
down the middle, receiving the West Branch from Lock Haven and
Williamsport, the East Branch from Wilkes-Barre in the Wyoming basin,
and the Juniata from the Broad Top region, south of Huntingdon. The
Anthracite basins lie on the right, enclosed by zigzag ridges of Pocono
and Pottsville sandstone; the Plateau, trenched by the West Branch of
the Susquehanna is in the northwest. Medina sandstone forms most of the
central ridges.
[Footnote 3: Russell has lately recommended the revival of this term,
proposed many years ago by Redfield, as a non-committal name for the
"New red sandstones" of our Atlantic slope, commonly called Triassic.]
[Illustration: FIG. 1. Part of Topographic Map of Pennsylvania, by J.
P. Lesley (1871).]
3. _The drainage of Pennsylvania_.--The greater part of the Alleghany
plateau is drained westward into the Ohio, and with this we shall have
little to do. The remainder of the plateau drainage reaches the
Atlantic by two rivers, the Delaware and the Susquehanna, of which the
latter is the more special object of our study. The North and West
Branches of the Susquehanna rise in the plateau, which they traverse in
deep valleys; thence they enter the district of the central ranges,
where they unite and flow in broad lowlands among the even-crested
ridges. The Juniata brings the drainage of the Broad Top region to the
main stream just before their confluent current cuts across the
marginal Blue Mountain. The rock-rimmed basins of the anthracite region
are drained by small branches of the Susquehanna northward and
westward, and by the Schuylkill and Lehigh to the south and east. The
Delaware, which traverses the plateau between the Anthracite region and
the Catskill Mountain front, together with the Lehigh, the Schuylkill,
the little Swatara and the Susquehanna, cut the Blue Mountain by fine
water-gaps, and cross the great limestone valley. The Lehigh then turns
eastward and joins the Delaware, and the Swatara turns westward to the
Susquehanna; but the Delaware, Schuylkill and Susquehanna all continue
across South Mountain and the Newark belt, and into the low plateau of
schists beyond. The Schuylkill unites with the Delaware near
Philadelphia, just below the inner margin of the coastal plain; the
Delaware and the Susquehanna continue in their deflected estuaries to
the sea. All of these rivers and many of their side streams are at
present sunk in small valleys of moderate depth and width, below the
general surface of the lowlands, and are more or less complicated with
terrace gravels.
4. _Previous studies of Appalachian drainage_.--There have been no
special studies of the history of the rivers of Pennsylvania in the
light of what is now known of river development. A few recent essays of
rather general character as far as our rivers are concerned, may be
mentioned.
Peschel examined our rivers chiefly by means of general maps with
little regard to the structure and complicated history of the region.
He concluded that the several transverse rivers which break through the
mountains, namely, the Delaware, Susquehanna and Potomac, are guided by
fractures, anterior to the origin of the rivers.[4] There does not seem
to be sufficient evidence to support this obsolescent view, for most of
the water-gaps are located independently of fractures; nor can
Peschel's method of river study be trusted as leading to safe
conclusions.
[Footnote 4: Physische Erdkunde, 1880, ii, 442.]
Tietze regards our transverse valleys as antecedent;[5] but this was
made only as a general suggestion, for his examination of the structure
and development of the region is too brief to establish this and
exclude other views.
[Footnote 5: Jahrbuch Geol. Reichsanstalt, xxviii, 1878, 600.]
Löwl questions the conclusion reached by Tietze and ascribes the
transverse gaps to the backward or headwater erosion of external
streams, a process which he has done much to bring into its present
important position, and which for him replaces the persistence of
antecedent streams of other authors.[6]
[Footnote 6: Pet. Mitth., 1882, 405; Ueber Thalbildung, Prag, 1884.]
A brief article[7] that I wrote in comment on Löwl's first essay
several years ago now seems to me insufficient in its method. It
exaggerated the importance of antecedent streams; it took no sufficient
account of the several cycles of erosion through which the region has
certainly passed; and it neglected due consideration of the
readjustment of initial immature stream courses during more advanced
river-life. Since then, a few words in Löwl's essay have come to have
more and more significance to me; he says that in mountain systems of
very great age, the original arrangement of the longitudinal valleys
often becomes entirely confused by means of their conquest by
transverse erosion gaps. This suggestion has been so profitable to me
that I have placed the original sentence at the beginning of this
paper. Its thesis is the essential element of my present study.
[Footnote 7: Origin of Cross-valleys. Science, i, 1883, 325.]
Phillipson refers to the above-mentioned authors and gives a brief
account of the arrangement of drainage areas within our Appalachians,
but briefly dismisses the subject.[8] His essay contains a serviceable
bibliography.
[Footnote 8: Studien über Wasserscheiden. Leipsig, 1886, 149.]
If these several earlier essays have not reached any precise
conclusion, it may perhaps be because the details of the geological
structure and development of Pennsylvania have not been sufficiently
examined. Indeed, unless the reader has already become familiar with
the geological maps and reports of the Pennsylvania surveys and is
somewhat acquainted with its geography, I shall hardly hope to make my
case clear to him. The volumes that should be most carefully studied
are, first, the always inspiring classic, "Coal and its Topography"
(1856), by Lesley, in which the immediate relation of our topography to
the underlying structure is so finely described; the Geological Map of
Pennsylvania (1856), the result of the labors of the first survey of
the state; and the Geological Atlas of Counties, Volume X of the second
survey (1885). Besides these, the ponderous volumes of the final report
of the first survey and numerous reports on separate counties by the
second survey should be examined, as they contain many accounts of the
topography although saying very little about its development. If, in
addition to all this, the reader has seen the central district of the
state and marvelled at its even-crested, straight and zigzag ridges,
and walked through its narrow water-gaps into the enclosed coves that
they drain, he may then still better follow the considerations here
presented.
PART SECOND. _Outline of the geological history of the region_.
5. _Conditions of formation_.--The region in which the Susquehanna and
the neighboring rivers are now located is built in chief part of marine
sediments derived in paleozoic time from a large land area to the
southeast, whose northwest coast-line probably crossed Pennsylvania
somewhere in the southeastern part of the state; doubtless varying its
position, however, by many miles as the sea advanced and receded in
accordance with the changes in the relative altitudes of the land and
water surfaces, such as have been discussed by Newberry and Claypole.
The sediments thus accumulated are of enormous thickness, measuring
twenty or thirty thousand feet from their crystalline foundation to the
uppermost layer now remaining. The whole mass is essentially
conformable in the central part of the state. Some of the formations
are resistent, and these have determined the position of our ridges;
others are weaker and are chosen as the sites of valleys and lowlands.
The first are the Oneida and Medina sandstones, which will be here
generally referred to under the latter name alone, the Pocono sandstone
and the Pottsville conglomerate; to these may be added the fundamental
crystalline mass on which the whole series of bedded formations was
deposited, and the basal sandstone that is generally associated with
it. Wherever we now see these harder rocks, they rise above the
surrounding lowland surface. On the other hand, the weaker beds are the
Cambrian limestones (Trenton) and slates (Hudson River), all the
Silurian except the Medina above named, the whole of the Devonian--in
which however there are two hard beds of subordinate value, the
Oriskany sandstone and a Chemung sandstone and conglomerate, that form
low and broken ridges over the softer ground on either side of
them--and the Carboniferous (Mauch Chunk) red shales and some of the
weaker sandstones (Coal measures).
6. _Former extension of strata to the southeast_.--We are not much
concerned with the conditions under which this great series of beds was
formed; but, as will appear later, it is important for us to recognize
that the present southeastern margin of the beds is not by any means
their original margin in that direction. It is probable that the whole
mass of deposits, with greater or less variations of thickness,
extended at least twenty miles southeast of Blue Mountain, and that
many of the beds extended much farther. The reason for this conclusion
is a simple one. The several resistant beds above-mentioned consist of
quartz sand and pebbles that cannot be derived from the underlying beds
of limestones and shales; their only known source lay in the
crystalline rocks of the paleozoic land to the southeast. South
Mountain may possibly have made part of this paleozoic land; but it
seems more probable that it was land only during the earlier Archean
age, and that it was submerged and buried in Cambrian time and not
again brought to the light of day until it had been crushed into many
local anticlines[9] whose crests were uncovered by Permian and later
erosion. The occurrence of Cambrian limestone on either side of South
Mountain, taken with its compound anticlinal structure, makes it likely
that Medina time found this crystalline area entirely covered by the
Cambrian beds; Medina sands must therefore have come from farther still
to the southeast. A similar argument applies to the source of the
Pocono and Pottsville beds. The measure of twenty miles as the former
southeastern extension of the paleozoic formations therefore seems to
be a moderate one for the average of the whole series; perhaps forty
would be nearer the truth.
[Footnote 9: Lesley, as below.]
7. _Cambro-Silurian and Permian deformations_.--This great series of
once horizontal beds is now wonderfully distorted; but the distortions
follow a general rule of trending northeast and southwest, and of
diminishing in intensity from southeast to northwest. In the Hudson
Valley, it is well known that a considerable disturbance occurred
between Cambrian and Silurian time, for there the Medina lies
unconformably on the Hudson River shales. It seems likely, for reasons
that will be briefly given later on, that the same disturbance extended
into Pennsylvania and farther southwest, but that it affected only the
southeastern corner of the State; and that the unconformities in
evidence of it, which are preserved in the Hudson Valley, are here lost
by subsequent erosion. Waste of the ancient land and its
Cambro-Silurian annex still continued and furnished vast beds of
sandstone and sandy shales to the remaining marine area, until at last
the subsiding Paleozoic basin was filled up and the coal marshes
extended broadly across it. At this time we may picture the drainage of
the southeastern land area wandering rather slowly across the great
Carboniferous plains to the still submerged basin far to the west; a
condition of things that is not imperfectly represented, although in a
somewhat more advanced stage, by the existing drainage of the mountains
of the Carolinas across the more modern coastal plain to the Atlantic.
This condition was interrupted by the great Permian deformation that
gave rise to the main ranges of the Appalachians in Pennsylvania,
Virginia and Tennessee. The Permian name seems appropriate here, for
while the deformation may have begun at an earlier date, and may have
continued into Triassic time, its culmination seems to have been within
Permian limits. It was characterized by a resistless force of
compression, exerted in a southeast-northwest line, in obedience to
which the whole series of Paleozoic beds, even twenty or more thousand
feet in thickness, was crowded gradually into great and small folds,
trending northeast and southwest. The subjacent Archean terrane
doubtless shared more or less in the disturbance: for example, South
Mountain is described by Lesley as "not one mountain, but a system of
mountains separated by valleys. It is, geologically considered, a
system of anticlinals with troughs between.... It appears that the
South Mountain range ends eastward [in Cumberland and York Counties] in
a hand with five [anticlinal] fingers."[10]
[Footnote 10: Proc. Amer. Phil. Soc., xiii, 1873, 6.]
It may be concluded with fair probability that the folds began to rise
in the southeast, where they are crowded closest together, some of them
having begun here while coal marshes were still forming farther west;
and that the last folds to be begun were the fainter ones on the
plateau, now seen in Negro mountain and Chestnut and Laurel ridges. In
consequence of the inequalities in the force of compression or in the
resistance of the yielding mass, the folds do not continue indefinitely
with horizontal axes, but vary in height, rising or falling away in
great variety. Several adjacent folds often follow some general control
in this respect, their axes rising and falling together. It is to an
unequal yielding of this kind that we owe the location of the
Anthracite synclinal basins in eastern Pennsylvania, the Coal Measures
being now worn away from the prolongation of the synclines, which rise
in either direction.
8. _Perm-Triassic denudation_.--During and for a long time after this
period of mountain growth, the destructive processes of erosion wasted
the land and lowered its surface. An enormous amount of material was
thus swept away and laid down in some unknown ocean bed. We shall speak
of this as the Perm-Triassic period of erosion. A measure of its vast
accomplishment is seen when we find that the Newark formation, which is
generally correlated with Triassic or Jurassic time, lies unconformably
on the eroded surface of Cambrian and Archean rocks in the southeastern
part of the State, where we have concluded that the Paleozoic series
once existed; where the strata must have risen in a great mountain mass
as a result of the Appalachian deformations; and whence they must
therefore have been denuded before the deposition of the Newark beds.
Not only so; the moderate sinuosity of the southeastern or under
boundary of the Newark formation indicates clearly enough that the
surface on which that portion of the formation lies is one of no great
relief or inequality; and such a surface can be carved out of an
elevated land only after long continued denudation, by which
topographic development is carried beyond the time of its greatest
strength or maturity into the fainter expression of old age. This is a
matter of some importance in our study of the development of the rivers
of Pennsylvania; and it also constitutes a good part of the evidence
already referred to as indicating that there must have been some
earlier deformations of importance in the southeastern part of the
State; for it is hardly conceivable that the great Paleozoic mass could
have been so deeply worn off of the Newark belt between the making of
the last of the coal beds and the first of the Newark. It seems more in
accordance with the facts here recounted and with the teachings of
geological history in general to suppose, as we have here, that
something of the present deformation of the ancient rocks underlying
the Newark beds was given at an early date, such as that of the Green
Mountain growth; and that a certain amount of the erosion of the folded
beds was thus made possible in middle Paleozoic time; then again at
some later date, as Permian, a second period of mountain growth
arrived, and further folding was effected, and after this came deeper
erosion; thus dividing the destructive work that was done into several
parts, instead of crowding it all into the post-Carboniferous time
ordinarily assigned to it. It is indeed not impossible that an
important share of what we have called the Permian deformation was, as
above suggested, accomplished in the southeastern part of the State
while the coal beds were yet forming in the west; many grains of sand
in the sandstones of the Coal Measures may have had several temporary
halts in other sandstone beds between the time of their first erosion
from the Archean rocks and the much later time when they found the
resting place that they now occupy.[11]
[Footnote 11: These considerations may have value in showing that the
time in which the lateral crushing of the Appalachians was accomplished
was not so brief as is stated by Reade in a recent article in the
American Geologist, iii, 1889, 106.]
9. _Newark deposition_.--After the great Paleozoic and Perm-Triassic
erosions thus indicated, when the southeastern area of ancient
mountains had been well worn down and the Permian folds of the central
district had acquired a well developed drainage, there appeared an
opportunity for local deposition in the slow depression of a
northeast-southwest belt of the deeply wasted land, across the
southeastern part of the State; and into this trough-like depression,
the waste from the adjacent areas on either side was carried, building
the Newark formation. This may be referred to as the Newark or
Trias-Jurassic period of deposition. The volume of this formation is
unknown, as its thickness and original area are still undetermined; but
it is pretty surely of many thousand feet in vertical measure, and its
original area may have been easily a fifth or a quarter in excess of
its present area, if not larger yet. So great a local accumulation
seems to indicate that while the belt of deposition was sinking, the
adjacent areas were rising, in order to furnish a continual supply of
material; the occurrence of heavy conglomerates along the margins of
the Newark formation confirms this supposition, and the heavy breccias
near Reading indicate the occurrence of a strong topography and a
strong transporting agent to the northwest of this part of the Newark
belt. It will be necessary, when the development of the ancestors of
our present rivers is taken up, to consider the effects of the
depression that determined the locus of Newark deposition and of the
adjacent elevation that maintained a supply of material.
10. _Jurassic tilting_.--Newark deposition was stopped by a gradual
reversal of the conditions that introduced it. The depression of the
Newark belt was after a time reversed into elevation, accompanied by a
peculiar tilting, and again the waste of the region was carried away to
some unknown resting place. This disturbance, which may be regarded as
a revival of the Permian activity, culminated in Jurassic, or at least
in post-Newark time, and resulted in the production of the singular
monoclinal attitude of the formation; and as far as I can correlate it
with the accompanying change in the underlying structures, it involved
there an over-pushing of the closed folds of the Archean and Paleozoic
rocks. This is illustrated in figs. 2 and 3, in which the original and
disturbed attitudes of the Newark and the underlying formations are
roughly shown, the over-pushing of the fundamental folds causing the
monoclinal and probably faulted structure in the overlying beds.[12] If
this be true, we might suspect that the unsymmetrical attitude of the
Appalachian folds, noted by Rogers as a characteristic of the range, is
a feature that was intensified if not originated in Jurassic and not in
Permian time.
[Footnote 12: Amer. Journ. Science, xxxii, 1886, 342; and Seventh Ann.
Rept. U. S. Geol. Survey, 1888, 486.]
[Illustration: FIG. 2.]
[Illustration: FIG. 3.]
It is not to be supposed that the Jurassic deformation was limited to
the area of the Newark beds; it may have extended some way on either
side; but it presumably faded out at no great distance, for it has not
been detected in the history of the Atlantic and Mississippi regions
remote from the Newark belt. In the district of the central folds of
Pennsylvania, with which we are particularly concerned, this
deformation was probably expressed in a further folding and
over-pushing of the already partly folded beds, with rapidly decreasing
effect to the northwest; and perhaps also by slip-faults, which at the
surface of the ground nearly followed the bedding planes: but this is
evidently hypothetical to a high degree. The essential point for our
subsequent consideration is that the Jurassic deformation was probably
accompanied by a moderate elevation, for it allowed the erosion of the
Newark beds and of laterally adjacent areas as well.
11. _Jura-Cretaceous denudation_.--In consequence of this elevation, a
new cycle of erosion was entered upon, which I shall call the
Jura-Cretaceous cycle. It allowed the accomplishment of a vast work,
which ended in the production of a general lowland of denudation, a
wide area of faint relief, whose elevated remnants are now to be seen
in the even ridge-crests that so strongly characterize the central
district, as well as in certain other even uplands, now etched by the
erosion of a later cycle of destructive work. I shall not here take
space for the deliberate statement of the argument leading to this end,
but its elements are as follows: the extraordinarily persistent
accordance among the crest-line altitudes of many Medina and
Carboniferous ridges in the central district; the generally
corresponding elevation of the western plateau surface, itself a
surface of erosion, but now trenched by relatively deep and narrow
valleys; the generally uniform and consistent altitude of the uplands
in the crystalline highlands of northern New Jersey and in the South
Mountains of Pennsylvania; and the extension of the same general
surface, descending slowly eastward, over the even crest-lines of the
Newark trap ridges. Besides the evidence of less continental elevation
thus deduced from the topography, it may be noted that a lower stand of
the land in Cretaceous time than now is indicated by the erosion that
the Cretaceous beds have suffered in consequence of the elevation that
followed their deposition. The Cretaceous transgression in the western
states doubtless bears on the problem also. Finally it may be fairly
urged that it is more accordant with what is known about old mountains
in general to suppose that their mass has stood at different attitudes
with respect to base level during their long period of denudation than
to suppose that they have held one attitude through all the time since
their deformation.
It is natural enough that the former maintenance of some lower altitude
than the present should have expression in the form of the country, if
not now extinguished by subsequent erosion. It is simply the reverse of
this statement that leads us to the above-stated conclusion. We may be
sure that the long maintained period of relative quiet was of great
importance in allowing time for the mature adjustment of the rivers of
the region, and hence due account must be taken of it in a later
section. I say relative quiet, for there were certainly subordinate
oscillations of greater or less value; McGee has detected records of
one of these about the beginning of Cretaceous time, but its effects
are not now known to be of geographic value; that is, they do not now
manifest themselves in the form of the present surface of the land, but
only in the manner of deposition and ancient erosion of certain
deposits.[13] Another subordinate oscillation in the sense of a
moderate depression seems to have extended through middle and later
Cretaceous time, resulting in an inland transgression of the sea and
the deposit of the Cretaceous formation unconformably on the previous
land surface for a considerable distance beyond the present margin of
the formation.[14] This is important as affecting our rivers. Although
these oscillations were of considerable geological value, I do not
think that for the present purposes they call for any primary division
of the Jura-Cretaceous cycle; for as the result of this long period of
denudation we find but a single record in the great lowland of erosion
above described, a record of prime importance in the geographic
development of our region, that will often be referred to. The surface
of faint relief then completed may be called the Cretaceous baselevel
lowland. It may be pictured as a low, undulating plain of wide extent,
with a portion of its Atlantic margin submerged and covered over with a
relatively thin marine deposit of sands, marls and clays.
[Footnote 13: Amer. Jour. Science, xxxv, 1888, 367, 448.]
[Footnote 14: This statement is based on a study of the geographic
evolution of northern New Jersey, in preparation for publication.]
12. _Tertiary elevation and denudation_.--This broad lowland is a
lowland no longer. It has been raised over the greater part of its area
into a highland, with an elevation of from one to three thousand feet,
sloping gently eastward and descending under the Atlantic level near
the present margin of the Cretaceous formation. The elevation seems to
have taken place early in Tertiary time, and will be referred to as of
that date. Opportunity was then given for the revival of the previously
exhausted forces of denudation, and as a consequence we now see the
formerly even surface of the plain greatly roughened by the incision of
deep valleys and the opening of broad lowlands on its softer rocks.
Only the harder rocks retain indications of the even surface which once
stretched continuously across the whole area. The best indication of
the average altitude at which the mass stood through the greater part
of post-Cretaceous time is to be found on the weak shales of the Newark
formation in New Jersey and Pennsylvania, and on the weak Cambrian
limestones of the great Kittatinny valley; for both of these areas have
been actually almost baselevelled again in the Tertiary cycle. They
will be referred to as the Tertiary baselevel lowlands; and the valleys
corresponding to them, cut in the harder rocks, as well as the rolling
lowlands between the ridges of the central district of Pennsylvania
will be regarded as of the same date. Whatever variations of level
occurred in this cycle of development do not seem to have left marks of
importance on the inland surface, though they may have had greater
significance near the coast.
13. _Later changes of level_.--Again at the close of Tertiary time,
there was an elevation of moderate amount, and to this may be referred
the trenches that are so distinctly cut across the Tertiary baselevel
lowland by the larger rivers, as well as the lateral shallower channels
of the smaller streams. This will be called the Quaternary cycle; and
for the present no further mention of the oscillations known to have
occurred in this division of time need be considered; the reader may
find careful discussion of them in the paper by McGee, above referred
to. It is proper that I should add that the suggestion of baselevelling
both of the crest-lines and of the lowlands, that I have found so
profitable in this and other work, is due largely to personal
conference with Messrs. Gilbert and McGee of the Geological Survey; but
it is not desired to make them in any way responsible for the
statements here given.
[Illustration: FIG. 4.]
[Illustration: FIG. 5.]
14. _Illustrations of Pennsylvanian topography_.--A few sketches made
during a recent recess-trip with several students through Pennsylvania
may be introduced in this connection. The first, fig. 4, is a view from
Jenny Jump mountain, on the northwestern side of the New Jersey
highlands, looking northwest across the Kittatinny valley-lowland to
Blue or Kittatinny mountain, where it is cut at the Delaware Water-gap.
The extraordinarily level crest of the mountain preserves record of the
Cretaceous baselevel lowland; since the elevation of this ancient
lowland, its softer rocks have, as it were, been etched out, leaving
the harder ones in relief; thus the present valley-lowland is to be
explained. In consequence of the still later elevation of less amount,
the Delaware has cut a trench in the present lowland, which is partly
seen to the left in the sketch. Fig. 5 is a general view of the Lehigh
plateau and cañon, looking south from Bald Mountain just above Penn
Haven Junction. Blue mountain is the most distant crest, seen for a
little space. The ridges near and above Mauch Chunk form the other
outlines; all rising to an astonishingly even altitude, in spite of
their great diversity of structure. Before the existing valleys were
excavated, the upland surface must have been an even plain--the
Cretaceous baselevel lowland elevated into a plateau. The valleys cut
into the plateau during the Tertiary cycle are narrow here, because the
rocks are mostly hard. The steep slopes of the cañon-like valley of the
Lehigh and the even crests of the ridges manifestly belong to different
cycles of development. Figs. 6 and 7 are gaps cut in Black Log and
Shade mountain, by a small upper branch stream of the Juniata in
southeastern Huntingdon county. The stream traverses a breached
anticlinal of Medina sandstone, of which these mountains are the
lateral members. A long narrow valley is opened on the axial Trenton
limestone between the two. The gaps are not opposite to each other, and
therefore in looking through either gap from the outer country the even
crest of the further ridge is seen beyond the axial valley. The gap in
Black Log mountain, fig. 6, is located on a small fracture, but in this
respect it is unlike most of its fellows.[15] The striking similarity
of the two views illustrates the uniformity that so strongly
characterizes the Medina ridges of the central district. Fig. 8 is in
good part an ideal view, based on sketches on the upper Susquehanna,
and designed to present a typical illustration of the more significant
features of the region. It shows the even crest-lines of a high Medina
or Pocono ridge in the background, retaining the form given to it in
the Cretaceous cycle; the even lowlands in the foreground, opened on
the weaker Siluro-Devonian rocks in the Tertiary cycle; and the uneven
ridges in the middle distance marking the Oriskany and Chemung beds of
intermediate hardness that have lost the Cretaceous level and yet have
not been reduced to the Tertiary lowland. The Susquehanna flows
distinctly below the lowland plain, and the small side streams run in
narrow trenches of late Tertiary and Quaternary date.
[Footnote 15: Second Geol. Surv. Pa., Report T_{3}, 19.]
[Illustration: FIG. 6.]
[Illustration: FIG. 7.]
[Illustration: FIG. 8.]
If this interpretation is accepted, and the Permian mountains are seen
to have been once greatly reduced and at a later time worn out, while
the ridges of to-day are merely the relief left by the etching of
Tertiary valleys in a Cretaceous baselevelled lowland, then we may well
conclude with Powell that "mountains cannot remain long as mountains;
they are ephemeral topographic forms."[16]
[Footnote 16: Geol. Uinta Mountains, 1876, 196.]
PART THIRD. _General conception of the history of a river_.
15. _The complete cycle of river life: youth, adolescence, maturity and
old age_.--The general outline of an ideal river's history may be now
considered, preparatory to examining the special history of the rivers
of Pennsylvania, as controlled by the geological events just narrated.
Rivers are so long lived and survive with more or less modification so
many changes in the attitude and even in the structure of the land,
that the best way of entering on their discussion seems to be to
examine the development of an ideal river of simple history, and from
the general features thus discovered, it may then be possible to
unravel the complex sequence of events that leads to the present
condition of actual rivers of complicated history.
A river that is established on a new land may be called an original
river. It must at first be of the kind known as a consequent river, for
it has no ancestor from which to be derived. Examples of simple
original rivers may be seen in young plains, of which southern New
Jersey furnishes a fair illustration. Examples of essentially original
rivers may be seen also in regions of recent and rapid displacement,
such as the Jura or the broken country of southern Idaho, where the
directly consequent character of the drainage leads us to conclude
that, if any rivers occupied these regions before their recent
deformation, they were so completely extinguished by the newly made
slopes that we see nothing of them now.
Once established, an original river advances through its long life,
manifesting certain peculiarities of youth, maturity and old age, by
which its successive stages of growth may be recognized without much
difficulty. For the sake of simplicity, let us suppose the land mass,
on which an original river has begun its work, stands perfectly still
after its first elevation or deformation, and so remains until the
river has completed its task of carrying away all the mass of rocks
that rise above its baselevel. This lapse of time will be called a
cycle in the life of a river. A complete cycle is a long measure of
time in regions of great elevation or of hard rocks; but whether or not
any river ever passed through a single cycle of life without
interruption we need not now inquire. Our purpose is only to learn what
changes it would experience if it did thus develop steadily from
infancy to old age without disturbance.
In its infancy, the river drains its basin imperfectly; for it is then
embarrassed by the original inequalities of the surface, and lakes
collect in all the depressions. At such time, the ratio of evaporation
to rainfall is relatively large, and the ratio of transported land
waste to rainfall is small. The channels followed by the streams that
compose the river as a whole are narrow and shallow, and their number
is small compared to that which will be developed at a later stage. The
divides by which the side-streams are separated are poorly marked, and
in level countries are surfaces of considerable area and not lines at
all. It is only in the later maturity of a system that the divides are
reduced to lines by the consumption of the softer rocks on either side.
The difference between constructional forms and those forms that are
due to the action of denuding forces is in a general way so easily
recognized, that immaturity and maturity of a drainage area can be
readily discriminated. In the truly infantile drainage system of the
Red River of the North, the inter-stream areas are so absolutely flat
that water collects on them in wet weather, not having either original
structural slope or subsequently developed denuded slope to lead it to
the streams. On the almost equally young lava blocks of southern
Oregon, the well-marked slopes are as yet hardly channeled by the flow
of rain down them, and the depressions among the tilted blocks are
still undrained, unfilled basins.