THE PLAIN FROM THE AIR
(FIGS. 34 TO 41)
A RIVER ON THE GREAT PLAINS
The difficulty of photographing a plain from a point on its surface
needs no emphasis, but its successful representation by means of air
photographs is illustrated by many figures in this book. The Great
Plains of the west-central part of the United States are illustrated
here by a view of the Red River (Fig. 36), which shows the flat surface
of the land and the broad sandy bed of the river only partly covered by
the intricately woven strands of the braided channels--a scene
characteristic of the Great Plains.
MEANDERING STREAMS ON THE COASTAL PLAIN
The ox-bow curves of meandering streams are among the features of the
earth’s surface most familiar to the student of physical geography; yet,
heretofore, they have been illustrated only by maps, constructed at
great labor and expense. Comprehensive photographs of them are rare and
are, at best, imperfect and unsatisfactory for purposes of illustration.
On the other hand, meandering streams lend themselves admirably to air
photography. Equally familiar to the student of geography and
physiography is the term “abandoned meander.” These ancient stream
courses, many of which are now occupied by marsh, brush, or forest, have
been still more difficult to illustrate by means of photographs. In some
instances wooded meanders like those near Columbus, Ga. (Fig. 34), long
ago abandoned by the stream that formed them, are shown in air pictures
in a manner but little less conspicuous than the meanders of the
present-day stream. It is believed that instructors will find Figure 34
useful, not only in illustrating meandering streams and abandoned
meanders but also in showing how meanders develop.
[Illustration: FIG. 34]
[Illustration: FIG. 35]
[Illustration: FIG. 34--The Chattahoochee River south of Columbus, Ga.,
showing the results of progressive lateral shifting of a meandering
stream. In the upper part of the illustration to the left (west) of the
stream are light-colored concentric markings which probably represent
the gradual shifting of the stream toward the right. As interpreted from
the information at hand, this section of the stream at one time occupied
a position much farther west than now. It cut away the bank on the east,
forming a curved course, depositing sand and mud on the inside of the
curve. This typical feature of stream erosion and deposition is to be
noted from the picture of the present course of the stream. At the
outside of each meander stretches of the bank appear light-colored and
denuded of the trees and bushes that line the bank elsewhere. These are
scours, a slipping away of the bluff caused by the cutting of the stream
into the foot of the bank at points where the velocity of the outside of
the current, and consequently its corrosive power, is increased as it
swings round the curve. The inside of the sharpest meander shows also
the deposit of material due to the fact that the velocity of the inside
of the current is checked by the bank, causing it to deposit some of its
load. Added to this deposit is much of the material brought by
cross-currents from the opposite-lying scour. The light-colored banks
are probably successive deposits. Finally, either by a gradual wearing
away or by some whim of the current at flood tide, the river chose a
shorter course, leaving its old channel as an abandoned meander. Farther
south several abandoned meanders may be distinguished, each
distinctively marked by a steep bank on the outside of the curve and
concentric bandings on the inside. The abandoned channels are especially
marked by the trees and brush that fill them in many places. It appears
that a well-developed growth of trees is to be found only along the
river banks in this region and the growth in the abandoned channels is
probably due to the fact that in flood time there is much seepage of
water into these old channels if not an actual overflow from the present
course of the stream. At the bottom of the picture is to be seen the
recently made land under cultivation. The fields appear striated and
checkered, obscuring the concentric banding. The illustration is from a
mosaic made up at Camp Benning near-by of many photographs matched
together, hence there are certain differences in shade due to dark and
light prints. Scale, about 1:38,000.]
[Illustration: FIG. 35--Map of the same area shown in Fig. 34 enlarged
from the corresponding sections of the Columbus and Seale, Ga.-Ala.,
topographic sheets, 1:62,500, published by the U. S. Geological Survey.
The cross section at the bottom lies along the line indicated on the map
and extends somewhat beyond the right border of the map. The section
shows between the hills the broad lowland over which the Chattahoochee
River has meandered. Scale, 1:38,000.]
[Illustration: FIG. 36--A river channel in the Great Plains. The Red
River northeast of Wichita Falls, Tex., as photographed from a height of
8,000 feet, September 12, 1918. Between the bluffs is seen the
dark-colored water of the braided stream flowing on a broad sandy bed
more than a mile wide, which is completely covered with water only at
flood time. The river forms the Texas-Oklahoma boundary, and frequent
changes in the position of the channel during periods of high water make
the exact position of the interstate boundary uncertain and give rise to
disputes and litigation over the ownership of land. North of the river
(top of figure) to the right are sand dunes with a sprinkling of trees
and bushes; in the middle of the channel there is an island of
light-colored sand. The stream channel bites sharply into the southern
bluff, which is cut by many strong gulches. Across the river is the
familiar sand flat built of the material washed downstream at flood time
and spread out by the subsiding water. The channel at this point shows
the changes that have taken place in the position of the stream and,
where the stream crosses the sandy floor, affords an example of
braiding. Scale, about 1:23,000.]
[Illustration: FIG. 37--A characteristic glacial drift plain in
southwestern Michigan. There appear, at the left, the round surface of a
terminal moraine and gullied slopes, which show mottled in the picture;
morainic hollows and kettleholes once partly filled with water but now
filled with peat or occupied by marshes formed by the accumulation of
peat from plant growth until carbonaceous matter has replaced the water
of the original lake; in the center, a relatively smooth outwash plain
characterized by straight roads and well-cultivated fields; and, at the
right, a brush-lined creek, a small reservoir, and the town of
Flowerfield. Scale, about 1:20,000.]
[Illustration: FIG. 38--The same area as shown in Fig. 37, enlarged from
the advance edition, 1:48,000, of the Schoolcraft, Mich., topographic
sheet to be published by the U. S. Geological Survey. This advance sheet
results from an experiment in the use of airplanes for mapping. The area
was photographed with a mapping camera. From the photograph a base map
was constructed, which was verified on the ground; on this base the
contour lines were added by instrumental survey. Scale, 1:20,000.]
[Illustration: FIG. 39--Schoolcraft, Mich., a town typical of the
agricultural portions of the north-central United States, showing the
characteristic features--roads, fields, town blocks, and others--by
which the aviator can recognize a locality from a distance. The mottled
appearance of the land surrounding the village is characteristic of air
photographs of glacial moraine regions. The picture of the village
itself might be taken as a prototype of the American village with its
fairly regular layout of streets, its business center indicated by a few
larger roofs along the widest street, its lawns, trees, and gardens, the
bordering farm lands, and the scattered extensions of the village into
points in the direction of the main roads. Scale, about 1:14,000.]
THE GLACIAL DRIFT PLAIN
[Illustration: FIG. 40--Map of the town of Schoolcraft, Mich., for
comparison with Fig. 39. Enlarged from the advance edition, 1:48,000, of
the Schoolcraft, Mich., topographic sheet to be published by the U. S.
Geological Survey. Scale, 1:14,000.]
Some of the characteristics of a third type of plain, the glacial drift
plain, are shown in Figures 37 to 41. Here are pictured glacial lakes,
bogs, marshes, moraines, and outwash plains, peat-filled depressions,
kettleholes and gullied slopes--typical features of a glaciated region.
The views show, also, many of the familiar aspects of the central and
western parts of the United States: the rectangular pattern formed by
the land subdivisions established by the United States Land Office, the
checkerboard pattern being emphasized by the section-line roads; the
minor subdivisions into fields; and the cultivation of a variety of
crops.
[Illustration: FIG. 41--Kettleholes and other depressions in glacial
till, on the Grand Trunk Railway about 5 miles southwest of Schoolcraft,
Mich. The distance between the eastern (right) edge of this view and the
western (left) of Fig. 37 is about 1 mile. Scale, about 1:15,000.]
These photographs were selected from a series taken as an experiment in
map-making. In June, 1920, the United States Air Service sent a plane
equipped with a K-1 camera from Dayton, Ohio, to Schoolcraft, Mich,
where in seven hours’ flying time a fifteen-minute quadrangle, about 220
square miles, was photographed. The prints were matched together and
reduced to a scale of 1:48,000. From them such features as roads,
streams, forests, land corners, etc., were transferred to plane-table
sheets, which the topographic engineers on the ground then used for
contouring the relief. Figure 38 is a part of the preliminary proof of
this map. It may be added that the experiment is regarded as highly
favorable to the use of the airplane camera as an instrument in
mapping.