courses which cannot be definitely referred to any particular cycle,
and which may therefore be mentioned now. Among the greatest are those
by which the divide between the Atlantic and the Ohio streams has been
changed from its initial position on the great constructional Nittany
highland and Bedford range. There was probably no significant change
until after Newark depression, for the branches of the Anthracite river
could not have begun to push the divide westward till after the
eastward flow of the river was determined; until then, there does not
seem to have been any marked advantage possessed by the eastward
streams over the westward. But with the eastward escape of the
Anthracite, it probably found a shorter course to the sea and one that
led it over alternately soft and hard rocks, instead of the longer
course followed by the Ohio streams over continuous sandstones. The
advantage given by the greater extent of soft beds is indicated by the
great breadth of the existing valleys in the central district compared
with the less breadth of those in the plateau to the west. Consider the
effect of this advantage at the time of the Jurassic elevation. As the
streams on the eastern slope of the Nittany divide had the shortest and
steepest courses to the sea, they deepened their valleys faster than
those on the west and acquired drainage area from them; hence we find
reason for the drainage of the entire Nittany and Bedford district by
the Atlantic streams at present. Various branches of what are now the
Alleghany and Monongahela originally rose on the western slope of the
dividing range. These probably reached much farther east in pre-Permian
time, but had their headwaters turned another way by the growth of the
great anticlinal divide; but the smaller anticlines of Laurel ridge and
Negro mountain farther west do not seem to have been strong enough to
form a divide, for the rivers still traverse them. Now as the
headwaters of the Juniata breached the eastern slope of the
Nittany-Bedford range and pushed the divide westward, they at last
gained possession of the Siluro-Devonian monocline on its western
slope; but beyond this it has not been possible for them yet to go. As
the streams cut down deeper and encountered the Medina anticline near
the core of the ridge, they sawed a passage through it; the Cambrian
beds were discovered below and a valley was opened on them as the
Medina cover wore away. The most important point about this is that we
find in it an adequate explanation of the opposite location of
water-gaps in pairs, such as characterize the branches of the Juniata
below Tyrone and again below Bedford. This opposite location has been
held to indicate an antecedent origin of the river that passes through
the gaps, while gaps formed by self-developed streams are not thought
to present such correspondence (Hilber). Yet this special case of
paired gaps in the opposite walls of a breached anticline is manifestly
a direct sequence of the development of the Juniata headwaters. The
settling down of the main Juniata on Jack's mountain anticline below
Huntingdon is another case of the same kind, in which the relatively
low anticlinal crest is as yet not widely breached; the gaps below
Bedford stand apart, as the crest is there higher, and hence wider
opened; and the gaps below Tyrone are separated by some ten or twelve
miles.
When the headwater streams captured the drainage of the Siluro-Devonian
monocline on the western side of the ancient dividing anticline, they
developed subsequent rectangular branches growing like a well-trained
grape vine. Most of this valley has been acquired by the west branch of
the Susquehanna, probably because it traversed the Medina beds less
often than the Juniata. For the same reason, it may be, the West Branch
has captured a considerable area of plateau drainage that must have
once belonged to the Ohio, while the Juniata has none of it; but if so,
the capture must have been before the Tertiary cycle, for since that
time the ability of the West Branch and of the Juniata as regards such
capture appears about alike. On the other hand, Castleman's river, a
branch of the Monongahela, still retains the drainage of a small bit of
the Siluro-Devonian monocline, at the southern border of the State,
where the Juniata headwaters had the least opportunity to capture it;
but the change here is probably only retarded, not prevented entirely;
the Juniata will some day push the divide even here back to the
Alleghany Front, the frontal bluff of the plateau.
[Illustration: FIG. 26.]
40. _Other examples of adjustments_.--Other examples of small
adjustments are found around the Wyoming basin, fig. 26. Originally all
these streams ran centripetally down the enclosing slopes, and in such
locations they must have cut gullies and breaches in the hard
Carboniferous beds and opened low back country on the weaker Devonians.
Some of the existing streams still do so, and these are precisely the
ones that are not easily reached by divertors. The Susquehanna in its
course outside of the basin has sent out branches that have beheaded
all the centripetal streams within reach; where the same river enters
the basin, the centripetal streams have been shortened if not
completely beheaded. A branch of the Delaware has captured the heads of
some of the streams near the eastern end of the basin. Elsewhere, the
centripetal streams still exist of good length. The contrast between
the persistence of some of the centripetal streams here and their
peripheral diversion around Broad Top is a consequence of the
difference of altitude of the old lake bottoms in the two cases. It is
not to be doubted that we shall become acquainted with many examples of
this kind as our intimacy with rivers increases.
41. _Events of the Quaternary cycle_.--The brief quaternary cycle does
not offer many examples of the kind that we have considered, and all
that are found are of small dimensions. The only capturing stream that
need be mentioned has lately been described as a "river pirate;"[23]
but its conquest is only a Schleswig-Holstein affair compared to the
Goth- and Hun-like depredations of the greater streams in earlier
cycles.
[Footnote 23: Science, xiii, 1889, 108.]
The character of the streams and their valleys as they now exist is
strikingly dependent in many ways on the relation of the incipient
quaternary cycle to the longer cycles of the past. No lakes occur,
exception being made only of the relatively small ponds due to drift
obstruction within the glaciated area. Waterfalls are found only at the
headwaters of small streams in the plateau district, exception again
being made only for certain cases of larger streams that have been
thrown from their pre-glacial courses by drift barriers, and which are
now in a very immature state on their new lines of flow. The small
valleys of this cycle are shallow and narrow, always of a size strictly
proportional to the volume of the stream and the hardness of the
enclosing rocks, exception being made only in the case of post-glacial
gorges whose streams have been displaced from their pre-glacial
channels. The terraces that are seen, especially on the streams that
flow in or from the glaciated district, are merely a temporary and
subordinate complication of the general development of the valleys. In
the region that has been here considered, the streams have been seldom
much displaced from their pre-glacial channels; but in the northwestern
part of the State, where the drift in the valleys seems to be heavier,
more serious disturbance of pre-glacial courses is reported. The facts
here referred to in regard to lakes, falls, gorges, terraces and
displaced streams are to be found in the various volumes of the Second
Geological Survey of the State;[24] in regard to the terraces and the
estuarine deflections of the Delaware and Susquehanna, reference should
be made also to McGee's studies.[25]
[Footnote 24: Especially Carll, Reports I_{3}, I_{4}; White, Reports
G_{5}, G_{6}; Lewis, Report Z.]
[Footnote 25: Amer. Journ. Science, xxxv, 1888, 367, 448; Seventh
Annual Rep. U. S. G. S., 1888, 545.]
42. _Doubtful cases_.--It is hardly necessary to state that there are
many facts for which no satisfactory explanation is found under the
theory of adjustments that we have been considering. Some will
certainly include the location of the Susquehanna on the points of the
Pocono synclines under this category; all must feel that such a
location savors of an antecedent origin. The same is true of the
examples of the alignment of water-gaps found on certain streams; for
example, the four gaps cut in the two pairs of Pocono and Pottsville
outcrops at the west end of the Wyoming syncline, and the three gaps
where the Little Schuylkill crosses the coal basin at Tamaqua; the
opposite gaps in pairs at Tyrone and Bedford have already been
sufficiently explained. The location of the upper North Branch of the
Susquehanna is also unrelated to processes of adjustment as far as I
can see them, and the great area of plateau drainage that is now
possessed by the West Branch is certainly difficult to understand as
the result of conquest. The two independent gaps in Tussey's mountain,
maintained by the Juniata and its Frankstown branch below Tyrone are
curious, especially in view of the apparent diversion of the branch to
the main stream on the upper side of Warrior's ridge (Oriskany), just
east of Tussey's mountain.
43. _Complicated history of our actual rivers_.--If this theory of the
history of our rivers is correct, it follows that any one river as it
now exists is of so complicated an origin that its development cannot
become a matter of general study and must unhappily remain only a
subject for special investigation for some time to come. It was my hope
on beginning this essay to find some teachable sequence of facts that
would serve to relieve the usual routine of statistical and descriptive
geography, but this is not the result that has been attained. The
history of the Susquehanna, the Juniata, or the Schuylkill, is too
involved with complex changes, if not enshrouded in mystery, to become
intelligible to any but advanced students; only the simplest cases of
river development can be introduced into the narrow limits of ordinary
instruction. The single course of an ancient stream is now broken into
several independent parts; witness the disjointing and diversion of the
original Juniata, which, as I have supposed, once extended from Broad
Top lake to the Catawissa basin. Now the upper part of the stream,
representing the early Broad Top outlet, is reduced to small volume in
Aughwick creek; the continuation of the stream to Lewistown is first
set to one side of its original axial location and is then diverted to
another syncline; the beheaded portion now represented by Middle creek
is diverted from its course to the Catawissa basin by the Susquehanna;
perhaps the Catawissa of the present day represents the reversed course
of the lower Juniata where it joined the Anthracite. This unserviceably
complicated statement is not much simplified if instead of beginning
with an original stream and searching out its present disjointed parts,
we trace the composition of a single existing stream from its once
independent parts. The Juniata of to-day consists of headwaters
acquired from Ohio streams; the lake in which the river once gathered
its upper branches is now drained and the lake bottom has become a
mountain top; the streams flow around the margin of the lake, not
across its basin; a short course towards Lewistown nearly coincides
with the original location of the stream, but to confound this with a
precise agreement is to lose the true significance of river history;
the lower course is the product of diversion at least at two epochs and
certainly in several places; and where the river now joins the
Susquehanna, it is suspected of having a superimposed course unlike any
of the rest of the stream. This is too complicated, even if it should
ever be demonstrated to be wholly true, to serve as material for
ordinary study; but as long as it has a savor of truth, and as long as
we are ignorant of the whole history of our rivers, through which alone
their present features can be rightfully understood, we must continue
to search after the natural processes of their development as carefully
and thoroughly as the biologist searches for the links missing from his
scheme of classification.
44. _Provisional Conclusions_.--It is in view of these doubts and
complications that I feel that the history of our rivers is not yet
settled; but yet the numerous accordances of actual and deductive
locations appear so definite and in some cases so remarkable that they
cannot be neglected, as they must be if we should adhere to the
antecedent origin of the river courses.
The method adopted on an early page therefore seems to be justified.
The provisional system of ancient consequent drainage, illustrated on
fig. 21, does appear to be sufficiently related to the streams of
to-day to warrant the belief that most of our rivers took their first
courses between the primitive folds of our mountains, and that from
that distant time to the present the changes they have suffered are due
to their own interaction--to their own mutual adjustment more than to
any other cause. The Susquehanna, Schuylkill, Lehigh and Delaware are
compound, composite and highly complex rivers, of repeated mature
adjustment. The middle Susquehanna and its branches and the upper
portions of the Schuylkill and Lehigh are descendants of original
Permian rivers consequent on the constructional topography of that
time; Newark depression reversed the flow of some of the transverse
streams, and the spontaneous changes or adjustments from immature to
mature courses in the several cycles of development are so numerous and
extensive that, as Löwl truly says, the initial drainage has almost
disappeared. The larger westward-flowing streams of the plateau are of
earlier, Carboniferous birth, and have suffered little subsequent
change beyond a loss of headwaters. The lower courses of the Atlantic
rivers are younger, having been much shifted from their Permian or
pre-Permian courses by Newark and Cretaceous superimposition, as well
as by recent downward deformation of the surface in their existing
estuaries. No recognizable remnant of rivers antecedent to the Permian
deformation are found in the central part of the State; and with the
exception of parts of the upper Schuylkill and of the Susquehanna near
Wilkes-Barre, there are no large survivors of Permian consequent
streams in the ordinary meaning of the term "consequent." The shifting
of courses in the progress of mature adjustment has had more to do with
determining the actual location of our rivers and streams than any
other process.
Harvard College, June, 1889.
TOPOGRAPHIC MODELS.
BY COSMOS MINDELEFF.
Of the many methods by which it has been sought to represent the relief
of a country or district, only two have been at all widely used. These
methods are, in the order of their development, by hachured and by
contoured maps. Both have advantages and both have serious
disadvantages. Without entering into the controversy that is even yet
raging over the relative merits of the two systems, some slight notice
of what each claims to accomplish is necessary.
The representation of relief by hachures is a graphic system, and in
the best examples we have is an attempt to show, upon a plane surface,
the actual appearance of a given area under given conditions of
lighting,--as in the Dufour map of the Alps. Of course certain details
that would really disappear if the assumed conditions were actual ones,
must be shown upon the map,--so that it is, after all, but a
conventional representation. The very best examples are, for this and
other reasons, unsatisfactory, and far more so is this the case in the
vastly larger class of medium grade and poor work.
The contour system represents relief by a series of lines, each of
which is, at every point throughout its length, at a certain stated
elevation above sea-level, or some other datum-plane; in other words,
each contour line represents what would be the water's edge, if the sea
were to rise to that elevation. It possesses the advantage of great
clearness, but fails to a large degree in the representation of surface
detail; moreover, one must have considerable knowledge of topography,
in order to read the map correctly.[1]
[Footnote 1: For specimens of representation of the same subject on
different scales, in both the hachure and contour systems, see plate
from "Enthoffer's Topographical Atlas."]
To those who must give first place to the quantity of relief rather
than the quality, as, for example, the geologist or the engineer, a
contoured map is now considered essential. On the other hand, where
quality of relief is the prime consideration and the quantity a
secondary one, as, for example, for the use of the army, a hachured map
is considered the best. The method of hachures may be roughly
characterized as a graphic system with a conventional element, and the
contour method as a conventional system with a graphic element,--for if
the contour interval is small enough a sort of shading is produced
which helps considerably the idea of relief.
In addition to these two great systems, with which everyone is more or
less familiar, there is another method of representing a country or
district,--a method that succeeds where others fail, and which although
by no means new, has not received the attention it deserves: this is
the representation of a country by a model in relief. Certain striking
advantages of models over maps of all kinds are, indeed, so apparent
that one almost loses sight of such slight disadvantages as can, of
course, be urged against them. In the graphic representation of the
surface they are far superior to the hachured map, and they have the
further advantage of expressing the relative relief, which the hachured
map fails to do, except in a very general way. They have also the
advantage of showing actual shadows, exactly as they would be seen in a
bird's-eye view of the district, instead of more or less conventional
ones, and are, consequently, more easily comprehended by the layman,
without becoming any less valuable to the skilled topographer. In
short, they combine all the graphic features of a hachured map with all
the advantages of the best class of contoured maps, and in addition
they show more of the surface detail, upon which so much of the
character of the country depends and which is very inadequately
expressed by hachures and almost completely ignored in a contoured map
of large interval. The contours themselves can be made to appear upon
the model very easily and without interfering with other features.
The uses of models are many and various. Within the past few years
their usefulness has been much extended, and, now that they are
becoming better known, will probably receive a still further extension.
To the geologist they are often of great value in working out the
structure of complicated districts, for the reason that so many
important structural relations can be presented to the eye at a single
glance. Similarly, for the graphic presentation of results there is no
better method, as the topography, the surface geology, and any number
of sections can be shown together and seen in their proper
relationship. To the engineer an accurate model is often of the
greatest assistance in working out his problems, and it is simply
invaluable to explain the details of a plan to anyone who has little or
no technical training; for, as has been stated, a model is easily
comprehended by anyone, while more or less technical knowledge is
required for the proper understanding of even the best maps.
I might go on cataloguing in detail the many uses to which models may
be put, but shall now mention only one more--perhaps the most important
of all--their use in the education of the young. No method has yet been
devised that is capable of giving so clear and accurate a conception of
the principles of physical geography as a series of well selected
models; models have, indeed, already been used for this purpose, but
unfortunately their great cost has prevented their general use in
schools. Since, however, the study of geography has been placed upon a
new basis and a new life has been infused into it, many men have given
their attention to the subject of models, and have experimented with a
view to cheapen the cost of reproduction, which has hitherto prevented
their wide distribution; and probably this objection will soon be
remedied. The ability to read a map correctly,--to obtain from a study
of the map a clear conception of the country represented,--is more
uncommon than is usually supposed. Some of the recent methods of
teaching geography are intended to cultivate this very faculty, but it
is doubtful whether there is any better method than that which consists
in the study of a series of good models in conjunction with a series of
maps, all on the same scale and of the same areas. The value of a
series of good models in teaching geology is so apparent that it need
only be mentioned. It is often, for reasons stated above, far more
valuable even, than field instruction.
For the construction of a good relief map the first requisite is a good
contoured map. To this should be added, when possible, a good hachured
map, upon which the elevations of the principal points are stated,--if
the interval in the contoured map is a large one,--and as much material
in the way of photographs and sketches as it is possible to procure.
The modeler should, moreover, have some personal acquaintance with the
region to be represented, or, failing that, a general knowledge of
topographic forms, and at least a clear conception of the general
character of the country which he seeks to represent. This is very
important, for it is here that many modelers fail: the mechanical
portion of the work any ordinarily intelligent person can do. A model
may be as accurate as the map from which it is made, every contour may
be placed exactly where it belongs, and yet the resulting model may
be,--indeed, often is--"flat," expressionless, and unsatisfactory.
Every topographer in drawing his map is compelled to generalize more or
less, and it is fortunate for the map if this be done in the field
instead of in the draughtsman's office. But topographers differ among
themselves: there may be, and often is, considerable difference in two
maps of the same region made by different men; in other words, the
"personal equation" is a larger element in a map than is usually
supposed. This being the case, there is something more required in a
modeler than the mere transferring of the matter in the map,--giving it
three dimensions instead of two: he must supply through his special
knowledge of the region (or, failing that through his general
knowledge) certain characteristics that do not appear upon the map, and
undo, so far as it is necessary, certain generalizations of the
topographer and draughtsman. This artistic or technical skill required
correctly to represent the _individuality_ of a given district is
especially important in the modeler; it is more important, perhaps, in
small-scale maps of large districts than in large-scale maps of small
ones,--for in the latter the generalizing process has not been carried
so far, and the smaller interval of the contour lines preserves much of
the detail.
The methods by which relief maps are made have always received more
attention than would, at first sight, appear to be their proper
proportion. It may be due, however, to the difficulty of applying any
test to determine the accuracy of the finished model, and perhaps also
to the general impression that any one can make a relief map,--and so
he can, though of course there will be a wide difference in the value
of the results. Some, indeed, have devoted their attention to methods
exclusively, letting the result take care of itself,--and the models
show it. There is no more reason why a modeler should tie himself down
to one method of work, than that a water-colorist, or a chemist, or
anyone engaged in technical work, should do so; though in some cases he
might be required, as the chemist is, to show his methods as well as
his results.