EARTH FEATURES SHAPED BY RUNNING WATER
=The newly incised upland and its sharp salients.=—The successive
stages of incising, sculpturing, and finally of reducing an uplifted
land area, are each of them possessed of distinctive characters
which are all to be read either from the map or in the lines of the
landscape. Upon the newly uplifted plain the incising by the young
rivers is to be found chiefly in the neighborhood of the margins. In
this stage the valleys are described as V-shaped cañons, for the valley
wall meets the upland surface in sharp salients (plate 12 A), and the
lines of the landscape are throughout made up from straight elements.
Though the landscapes of this stage present the grandest scenery that
is known and may be cut out in massive proportions, often with rushing
river or placid lake to enhance the effect of crag and gorge, they lack
the softness and grace of outline which belong only to the maturer
erosion stages. The grand cañon of the Colorado presents the features
characteristic of this stage in the grandest and most sublime of all
examples, and the castled Rhine is a gorge of rugged beauty, carved out
from the newly elevated plateau of western Prussia, through which the
water swirls in eddying rapids (Fig. 175).
[Illustration:
FIG. 175.—Gorge of the River Rhine near St. Goars, incised within an
uplifted plain which forms the hill tops.]
=The stage of adolescence.=—As the upland becomes more largely
invaded as a consequence of the headward advance of the cañons and
their sending out of tributary side cañons, the sharp angles in which
the cañon walls intersect the plain become gradually replaced by
well-rounded shoulders. Thus the lines in the landscape of this stage
are a combination of the straight line with a simple curve convex
toward the sky (Fig. 176). In this stage large sections of the original
plateau remain, though cut into small areas by the extensions of the
tributary valleys.
[Illustration:
FIG. 176.—V-shaped valley with well-rounded shoulders characteristic of
the stage of adolescence. Allegheny plateau in West Virginia.]
=The maturely dissected upland.=—Continued ramifications by the
rivers eventually divide the entire upland area into separated parts,
and the rounding of the shoulders of valleys proceeds simultaneously
until of the original upland no easily recognizable compartments
are to be found. Where before were flat hilltops are now ridges or
watersheds, the well-known _divides_. The upland is now said to be
completely dissected or to have arrived at _maturity_. The streams are
still vigorous, for they make the full descent from the upland level
to base level, and yet a critical turning point of their history has
been reached, and from now on they are to show a steady falling off in
efficiency as sculpturing agents.
[Illustration:
FIG. 177.—View of a maturely dissected upland from one of its
hilltops, Klamath Mountains, California (after a photograph by
Fairbanks).]
Viewed from one of the hilltops, the landscape of this stage bears a
marked resemblance to a sea in which the numberless divides are the
crests of billows, and these, as distance reduces their importance in
the landscape, fade away into the even line of the horizon (Fig. 177).
=The Hogarthian line of beauty.=—Since the youthful stage of the
upland, when the lines of its landscape were straight, its character
rugged, and its rivers wild and turbulent, there has been effected
a complete transformation. The only straight line to be seen is the
distant horizon, for the landscape is now molded in softened outlines,
among which there is a repeated recurrence of the line of beauty made
famous by Hogarth in his “Analysis of Beauty.” As well known to all art
students, this is a sinuous line of reversed or double curvature—a
curve which passes insensibly at a point of inflection from convex
to concave (Fig. 178). The curve of beauty is now found in every
section of the hills, and it imparts to the landscape a gracefulness
and a measure of restfulness as well, which are not to be found in the
landscapes of earlier stages in the erosion cycle. In the bottoms of
the valleys also the initial windings of the rivers within their narrow
flood plains add silver beauty lines which stand out prominently from
the more somber background of the hills.
[Illustration: FIG. 178.—Hogarth’s line of beauty.]
Considered from the commercial viewpoint, the mature upland is one
of the least adaptable as a habitation for highly civilized man.
Direct lines of communication run up hill and down dale in monotonous
alternation, and almost the only way of carrying a railroad through the
region, without an expenditure for trestles which would be prohibitive,
is to follow the tortuous crest of a main divide or the equally winding
bed of one of the larger valleys.
[Illustration:
FIG. 179.—View of the old land of New England, with Mount Monadnock
rising in the distance.]
=The final product of river sculpture—the peneplain.=—When maturity
has been reached in the history of a river, its energies are devoted
to a paring down of the valley slopes and crests so as to reduce the
general level. From this time on hill summits no longer fall into a
common level—that of the original upland—for some mount notably
higher than others, and with increasing age such differences become
accentuated. There is now also a larger aggradation of the valleys to
form the level floors of flood plains, out of which at length the now
slight elevations rise upon such gentle slopes that the process of
land sculpture approaches its end. Gradually the vigor of the stream
has faded away, and can now only be renewed through a fresh uplift of
the land, or, what would amount to the same thing, a depression of
the base level. Upland and river have reached old age together, and
the approximation to a new plain but little elevated above base level
is so marked that the name _peneplain_ is applied to it. Scattered
elevations, which because of some favoring circumstance rise to
greater heights above the general level of the peneplain, are known as
_monadnocks_ after the type example of Mount Monadnock in New Hampshire
(Fig. 179).
[Illustration: FIG. 180.—Comparison of the cross sections of river
valleys for the different stages of the erosion cycle.]
=The river cross sections of successive stages.=—To the successive
stages of a river’s life it has been common to carry over the names
from the well-marked periods of a human life. If neglecting for the
moment the general aspect of the upland, we fix our attention upon the
characteristic cross sections of the river valley, we find that here
also there are clearly marked characters to distinguish each stage
of the river’s life (Fig. 180). In infancy the steep, narrow, and
sharp-angled cañon is a characteristic; with youth the wider V-form
has already developed; in adolescence the angles of the cañon are
transformed into well-rounded shoulders, and the valley broadens so
as in the lower reaches to lay down a flood plain; in maturity the
divides and the double curves of the line of beauty appear; while in
the decline of old age the valleys are extremely broad and flat and are
floored by an extended flood plain.
=The entrenchment of meanders with renewed uplift.=—Upon the reduced
grades which are characteristic of the declining stage of a river’s
life, the current has little power to modify the surface configuration.
On the old land of this stage a renewed uplift starts the streams again
into action. This infusion of driving power into moving water, regarded
as a machine capable of accomplishing certain work, is like winding up
a clock that has run down. Once more the streams acquire a velocity
sufficient to enable them to cut their valleys into the land surface,
and so a new erosional cycle may be inaugurated upon the old land
surface—the peneplain. After such an uplift has been accomplished
and the rivers have sunk their early valleys within the new upland, we
may look out from this now elevated surface and the eye take in but a
single horizontal line, since we view the plain along its edge.
[Illustration: FIG. 181.—The Beavertail Bend of the Yakima Cañon in
central Washington (after George Otis Smith).]
By the uplift the meanders of the earlier rivers may become entrenched
in the new upland, the wide lobes of the individual meanders being now
separated by mountains where before had been plains of silt only. The
New River of the Cumberland plateau and the Yakima River of central
Washington (Fig. 181) furnish excellent American examples of intrenched
meanders, as the Moselle River does in Europe. Upon the course of the
latter river near the town of Zell a tunnel of the railroad a quarter
of a mile in length pierces a mountain in the neck of a meander lobe
in which the river itself travels a distance of more than six miles in
order to make the same advance. The Kaiser Wilhelm tunnel in the same
district penetrates a larger mountain included in a double meander of
the river. Although intrenched, river meanders are still competent to
scour and so undermine the outer bank, and with favoring conditions
they may by this process erode extended “bottoms” out of the plateau.
(See Lockport quadrangle, U. S. G. S.)
=The valley of the rejuvenated river.=—Whenever a new uplift occurs
before an erosional cycle has been completed, the rivers become
intrenched, not in a peneplain, but in the bottoms of broad valleys.
The sweeping curves which characterize mature landscapes may thus be
brought into striking contrast with the straight lines of youthful
cañons which with V-sections descend from their lowest levels (Fig.
182). The full cross section of such a valley shows a central V whose
sharp shoulders are extended outward and upward in the softened curves
of later erosion stages.
[Illustration: FIG. 182.—A rejuvenated river valley (after a
photograph by Fairbanks).]
=The arrest of stream erosion by the more resistant rocks.=—The
capacity of a river to erode and carry away the rock material that
lies along its course is dependent not only upon the velocity of the
current, but also upon the hardness, the firmness of texture, and the
solubility of the material. Particularly in arid and semiarid regions,
where no mantle of vegetation is at hand to mask the surfaces of the
firmer rock masses, differences of this kind are stamped deeply upon
the landscape. The rock terraces in the Grand Cañon of the Colorado
together represent the stronger rock formations of the region, while
sloping talus accumulations bury the weaker beds from sight.
[Illustration: FIG. 183.—Plan of a river narrows.]
Each area of harder rock which rises athwart the course of a stream
causes a temporary arrest in the process of valley erosion and is
responsible for a noteworthy local contraction of the river valley.
The valley is carved less widely as well as less deeply, and since a
river can never corrade below its base, a “temporary base level” is
for a time established above the area of harder rock. Owing to the
contraction of the valley under these conditions, the locality is
described as a river _narrows_ (Fig. 183). The narrows upon the Hudson
River occur in the Highlands where the river leaves a broad expanse
occupied by softer sediments to traverse an island-like area of hard
crystalline rocks. Within the narrows of a river the steep walls,
characteristic of youth and the turbulent current as well, are often
retained long after other portions of the river have acquired the more
restful lines of river maturity. The picturesque crag and the generally
rugged character of river narrows render them points of special
interest upon every navigable river.
[Illustration:
FIG. 184.—Successive diagrams to illustrate repeated river piracy and
the development of “trellis drainage”, (after Russell).]
=The capture of one river’s territory by another.=—The effect of a
hard layer of rock interposed in the course of a stream is thus always
to delay the advance of the erosional process at all levels above the
obstruction. When a stream in incising its valley degrades its channel
through a veneer of softer rocks into harder materials below, it is
technically described as having _discovered_ the harder layer. Where
several neighboring streams flow by similar routes to their common base
level, those which discover a harder rock will advance their headwaters
less rapidly into the upland and so will be at a disadvantage in
extending their drainage territory. A stream which is not thus hindered
will in the course of time rob the others of a portion of their
territory, for it is able to erode its lower reaches nearer to base
level and thus acquire for its upper reaches, where erosion is chiefly
accomplished, an advantage in declivity. The divide which separates its
headwaters from those of its less favored neighbor will in consequence
migrate steadily into the neighbor’s territory. The divide is thus a
sort of boundary wall separating the drainage basins of neighboring
streams, and any migration must extend the territory of the one at
the expense of the other. As more and more territory is brought under
the dominion of the more favored stream, there will come a time when
the divide in its migration will arrive at the channel of the stream
that is being robbed, and so by a sudden act of annexation draw off
all the upper waters into its own basin. By this _capture_ the stream
whose territory has been invaded is said to have been _beheaded_.
By this act of _piracy_ the stronger stream now develops exceptional
activity because of the local steep grades near the point of capture,
and with this newly acquired cutting power the invader is competent to
advance still further and enter the territory of the stream that lies
next beyond. The type of drainage network which results from repeated
captures of this kind is known as “trellis drainage” (Fig. 184), a type
well illustrated by the rivers of the southern Appalachians.
In general it may be said that, other conditions being the same, of
two neighboring streams which have a common base level, that one which
takes the longest route will lose territory to the other, since it must
have the flatter average slope. Stream capture may thus come about
without the discovery of hard rock layers which are more unfavorable to
one stream than another.
[Illustration:
FIG. 185.—Sketch maps to show the earlier and the present drainage
condition about the Blue Ridge near Harper’s Ferry.]
=Water and wind gaps.=—In the Allegheny plateau rivers cross, the
range of harder rocks in deep mountain narrows which upon the horizon
appear as gateways through the barrier of the mountain wall. Such
gateways are sometimes referred to as “water gaps”, of which the
Delaware Water Gap is perhaps the best known example, though the
Potomac crosses the Blue Ridge at the historic Harper’s Ferry through
a similar portal. The valley of the tributary Shenandoah has been the
scene of an interesting episode in the struggle of rival streams which
is typical of others in the same upland region. The records which may
be made out from the landscapes show clearly that in an earlier but
recent period, when the general surface stood at a higher level which
has been called the Kittatinny Plain, the younger Potomac of that time
and a younger but larger ancestor of Beaverdam Creek each crossed
the Blue Ridge of the time through similar water gaps (Fig. 185, map,
and Fig. 186). The Potomac of that time was, however, the more deeply
intrenched, and possessing an advantage in slope it was able to advance
the divide at the head of its tributary, the Shenandoah, into the
territory of Beaverdam Creek. Thus the beheading of the Beaverdam by
the Shenandoah was accomplished (Fig. 185, second map) and its upper
waters annexed to the Potomac system. With the subsequent lowering of
the general level of the country which yielded the present Shenandoah
Plain, the former water gap of Beaverdam Creek was abandoned of its
stream at a high level in the range. Known as Snickers Gap, it may
serve as a type of the “wind gaps” of similar origin which are not
altogether uncommon in the Appalachian Mountain system (Fig. 186).
[Illustration: FIG. 186.—Section to illustrate the history of Snickers
Gap.]
=Character profiles.=—For humid regions the landscapes possess
characters which, speaking broadly, depend upon the stage of the
erosion cycle. For the earliest stages the straight line enters as
almost the only element in the design; as the cycle advances to
adolescence the rounded forms begin to replace the angles of the
immature stages, and with full maturity the lines of beauty alone
are characteristic. As this critical stage is passed irregularity of
feature and ever more flattened curves are found to correspond to the
decline of the river’s vital energies. There are thus marks of senility
in the work of rivers (Fig. 187).
[Illustration: FIG. 187.—Character profiles of landscapes shaped by
stream erosion in humid climates.]
READING REFERENCES FOR CHAPTERS XII AND XIII
General:—
SIR JOHN PLAYFAIR. Illustrations of the Huttonian Theory of the Earth.
Edinburgh, 1802, pp. 350-371.
J. W. POWELL. Exploration of the Colorado River of the West and its
Tributaries. Washington, 1875, pp. 149-214.
G. K. GILBERT. Report on the Geology of the Henry Mountains.
Washington, 1877, pp. 99-150. (A classic upon the work of rivers.)
C. E. DUTTON. Tertiary History of the Grand Cañon District (with
atlas), Mon. 2, U. S. Geol. Surv., 1882, pp. 264.
W. M. DAVIS. The Rivers and Valleys of Pennsylvania, Nat. Geogr. Mag.
vol. 1, 1889, pp. 203-219; The Triassic Formation of Connecticut, 18th
Ann. Rept. U. S. Geol. Surv., Pt. ii, 1898, pp. 144-153.
SIR A. GEIKIE. The Scenery of Scotland. London, 1901, pp. 1-12.
I. C. RUSSELL. Rivers of North America. Putnam. New York, 1898, pp.
327.
M. R. CAMPBELL. Drainage Modifications and their Interpretation, Jour.
Geol., vol. 4, 1896, pp. 567-581, 657-678.
HENRY GANNETT. Physiographic Types, U. S. Geol. Surv., Topographic
Atlas, Folios 1-2, 1896, 1900.
W. M. DAVIS. The Geographical Cycle, Geogr. Jour., vol. 14, 1899, pp.
481-504.
The flood plain:—
HENRY GANNETT. The Flood of April, 1897, in the Lower Mississippi,
Scot. Geogr. Mag., vol. 13, 1897, pp. 419-421.
W. M. DAVIS. The Development of River Meanders, Geol. Mag., Decade iv,
vol. 10, 1903, pp. 145-148.
W. S. TOWER. The Development of Cut-off Meanders, Bull. Am. Geogr.
Soc., vol. 36, 1904, pp. 589-599.
River terraces:—
W. M. DAVIS. The Terraces of the Westfield River, Massachusetts, Am.
Jour. Sci., vol. 14, 1902, pp. 77-94, pl. 4; River Terraces in New
England, Bull. Mus. Comp. Zoöl., vol. 38, 1902, pp. 281-346.
River deltas:—
G. K. GILBERT. The Topographic Features of Lake Shores, 5th Ann.
Rept. U. S. Geol. Surv., 1885, pp. 104-108; Lake Bonneville, Mon. I,
U. S. Geol. Surv., 1890, pp. 153-167.
Charts of Mississippi River Commission.
G. R. CREDNER. Die Deltas, ihre Morphologie, geographische Verbreitung
und Entstehungsbedingungen, Pet. Mitt. Ergh. 56, 1878, pp. 1-74, pls.
1-3.
The peneplain:—
W. M. DAVIS. Plains of Marine and Subaërial Denudation, Bull. Geol.
Soc. Am., vol. 7, 1896, pp. 377-398; The Peneplain, Am. Geol., vol.
23, 1899, pp. 207-239.
Intrenchment of meanders:—
W. M. DAVIS. The Seine, the Meuse, and the Moselle, Nat. Geogr. Mag.,
vol. 7, 1896, pp. 189-202.
Stream capture:—
N. H. DARTON. Examples of Stream Robbing in the Catskill Mountains,
Bull. Geol. Soc. Am., vol. 7, 1896, pp. 505-507, pl. 23.
COLLIER COBB. A Recapture from a River Pirate, Science, vol. 22, 1893,
p. 195.
WILLIAM H. HOBBS. The Still Rivers of Western Connecticut, Bull. Geol.
Soc. Am., vol. 13, 1902, pp. 17-22, pl. 1.
ISAIAH BOWMAN. A Typical Case of Stream Capture in Michigan, Jour.
Geol., vol. 12, 1904, pp. 326-334.