Fig. 2—Symbols of automatic register in the Eastman mapping camera, photographed with the body of the picture showing roads, streams, orchards, cultivated fields, etc. For explanation of the symbols, see the text.
The information given by the symbols is corroborated by the picture. Orchard and shade trees appear as circular dots in place of the elongated images characteristic of pictures taken obliquely downward, and the short, squat shadows denote exposure near midday. Shocks of corn standing in the fields show that the season is autumn.
How to Read Airplane Photographs
Not all the features, however, are so easily recognizable. Oblique photographs are often more readily interpreted than ordinary photographs, since they combine with the usual view the essentials of a plan; but in vertical photographs very few objects present an appearance that is natural in the light of our experience as lateral observers. The uninitiated, on attempting systematically to identify the features of a vertical photograph, find a very large number that are foreign in appearance. A necessary preliminary is an acquaintance with the ground photographed or with similar regions and features. Without such a key the air photograph is not always self-interpretative and is often unintelligible. Military observers are carefully trained to recognize features of military significance. It is not to be expected, however, that they should be trained in the observation of land forms except such as are of military importance. Consequently, whereas a great variety of photographs is now easily obtainable at many flying fields, the information that a scientist would desire concerning them is not so easily available. Most of the photographs used in this paper were taken by men who were not trained in observing land forms. Many were taken simply as a requirement in practice flights and meant so little to the observer that no record was made concerning them. For several not even the location was recorded.
It is of primary importance that the picture be held in the right position. Not only must the observer imagine himself looking directly down on the scene but he must hold the photograph in the position in which experience has shown that the image appears the most natural. Otherwise a depression will appear as an elevation and an elevation as a hollow. It is a well-known fact that in telescopic photographs of the moon the craters appear like hollows when the print is held in one position and like elevations when the position is reversed. Experience shows that if the print is held so that the shadows fall toward the observer the objects appear natural. The reason is that the observer sees only those shadows that are caused by light falling towards him. Consequently, the only interpretation that the brain can give to shadows on a photograph is that they are cast by an elevation between the eye and the light. In a picture, therefore, in which shadows fall away from the eye instead of towards it valleys are seen as hills and hills as valleys. In the northern hemisphere this prescribed orientation conflicts with the convention of placing the north side of a map at the top of the page and also with the modern shaded map on which the light is represented as coming from the upper left, or northwest, corner of the map.
Failure of Air Photographs to Show Relief, and Measures to Remedy This Defect
In photographs taken from the ground the lights and shadows are such that a high degree of naturalness is possible. But objects seen from directly above, and even those viewed obliquely, though to a lesser degree, are illuminated so uniformly that photographs of them are apt to appear flat. To some extent this has been overcome by the use of extra-sensitive emulsions, special ray filters, and printing papers adapted for accentuating contrast. Many of the photographs used in this book did not allow satisfactory reproduction till the contrast of the negatives was greatly increased by the arts of the photographic laboratory. But, even at its best, no photograph taken vertically affords an adequate idea of the height of hills or the depth of hollows. Only shadows that are particularly well defined can be distinguished as shadows, while small elevations and depressions affect the negative no differently than a difference in marking or color. In military defenses, if the mere surface of the camouflage is sufficiently realistic, the ordinary camera is even more easily deceived than the human eye. It is a well-known fact that man and other animals of the higher order see objects in relief, within a certain range of vision, because the eyes convey to their respective retinas slightly different images of the same object which the brain combines into a relief image. The stereoscopic camera has long been used for the same purpose. Its principle, with certain adaptations that need not be discussed here, has been to some extent employed in airplane pictures, with such excellent results that it is claimed by some that by further development actual contouring will be possible by this means. It is reported that in military reconnaissance stereoscopic pictures render ordinary camouflage useless and that bridges, observation towers, gun emplacements, etc., are shown in relief and, therefore, easily detected.[2]
CHAPTER II
FAMILIAR SCENES FROM A NEW ANGLE
(Figs. 1, 3, and 4)
Pictures of well-known buildings are of wide appeal. In so far as they create an interest in the activities for which the buildings stand they are distinctly educative. Such widely known buildings as the National Capitol and the Library of Congress are used repeatedly for illustration. They are as welcome as the sight of a familiar face. Any unusual circumstance connected with them is seized upon as an excuse for republishing pictures of them. Views of them from a new angle are always in demand. Not only do air photographs offer a welcome novelty, but they have the added advantage of lifting the subject out of the clutter of surrounding buildings and making it really the central figure of the picture. It would be difficult to get a more impressive view of the National Capitol than that of Figure 1 or a more attractive glimpse of the Naval Academy at Annapolis than that of Figure 4. The objects of chief interest occupy the center of view without distracting obstructions. In the former, the imposing structure of the Capitol building appears in a pleasing setting of minor details. The proximity of the Senate Office Building and the Library of Congress is at once apparent, and the radiating systems of the avenues of approach. Strangers may have wondered as to the nature of the environs of the Capitol. The tree-lined streets and the apartment houses seen in the picture answer the question. In the view of the Naval Academy the buildings occupy the center of the scene, with the beautiful dome of the memorial to John Paul Jones, the first great American naval fighter, prominently in view. Spa Creek in the foreground, a part of the capital city of Maryland at the left, and the Severn River, with its low wooded banks, stretching away
Fig. 3—West Point, N. Y., and the Hudson River. An air view of the United States Military Academy and the gorge of the Hudson. The picture shows the commanding view of the river to be had from the point of land 180 feet above the river on which the Military Academy is located and shows the wisdom of the choice of this spot as one of the chain of redoubts by which the river was fortified during the Revolution.
Fig. 4—The Naval Academy at Annapolis, Md. Oblique view from an airplane from a position over Eastport in a general northwesterly direction. The water in the foreground is Spa Creek. The Severn River, spanned by the county bridge and the Baltimore and Annapolis Railroad bridge, stretches away to the left. The buildings in the middle of the picture are those of the Naval Academy. The domed mausoleum built in honor of John Paul Jones, which serves as his final resting place, appears at the left. Still farther to the left lies Maryland’s capital city. Of interest is a comparison of the low-lying and stream-cut banks of the drowned valley now occupied by the Severn River with the mountains through which the Hudson River has cut its gorge at West Point (see Fig. 3).
in the distance, spanned by the county bridge and the Baltimore and Annapolis Railroad bridge, form an interesting setting and show, without detracting from the importance of the academy itself, its advantageous location with regard to the city and the water approaches.
CHAPTER III
ARCHITECTURE, LANDSCAPE GARDENING, AND ENGINEERING
(Figs. 5 to 14)
Only a few photographs are necessary to show how valuable to the architect, the construction engineer, the city planner, or the landscape gardener the air photograph, both vertical and oblique, is destined to become. Pictorial records of progress in the construction of buildings, bridges, ships, canals, reservoirs, etc., that partake also of the nature of ground plans, as do air photographs, furnish an admirable means of study and comparison. No photograph of the great shipyards at Newport News taken from the ground would show the relation of the shops and drydocks to the deep-water approaches as does Figure 7. Figure 8 gives an unusually comprehensive idea of the location, magnitude, and construction of Hell Gate Bridge; and Figure 10, Rockaway Beach, now a densely populated town where a few years ago was a barren strip of sand, suggests that photographic records of construction in rapidly growing communities where changes are being made in streets, railroads, and buildings, will come to be a part of the equipment of the city engineer and architect.
Architecture and Landscape Gardening
Equally useful will the air photograph become to the landscape gardener and architect. Heretofore, in order to get a comprehensive conception of his task and a definite picture of its completion, the landscape gardener has had to depend upon the use of maps and such views as could be made by the sketch artist or the ordinary lateral photograph. In the future, from vertical and oblique photographs of the area to be developed, he will have the means of studying its features in their correct proportions and relationship. By means of similar photographs of completed projects he can choose and combine until he has developed the plans best suited to his purpose. He can bring to his aid first-hand studies of gardens and grounds the world over whose beauties have made them famous.
Fig. 5—Monument Avenue, Richmond, Va., and the statue of Robert E. Lee. An oblique photograph illustrating the use of aerial photography in landscape gardening and street planning.
Engineering Projects Covering Large Areas
Where the project covers large areas, the “mosaic,” or group of matched photographs, can be used in the study of problems of construction or improvement. Figure 13, a mosaic of the Anacostia flats, the site of improvements under way in the District
Fig. 6—The United States Naval Observatory and grounds, Washington, D.C., as seen from an airplane at a height of a few hundred feet above the ground, showing an unusually attractive arrangement of shrubbery and trees.
of Columbia, shows the Anacostia marshes as they appeared in the autumn of 1920, after the changes effected since 1915, as can be seen by comparison with Figure 14, the topographic map of the same area. To the right is the terraced slope rising to a height of about 150 feet above the river—an elevation so low that the air photograph does not properly reproduce it. Near the foot of the principal terrace lie the tracks of the Pennsylvania Railroad, on which can be seen Benning, Deanewood, and Kenilworth. Between the railroad and the Anacostia River are the Benning race track and the swampy lowland and tidal marshes of the Anacostia flats. The river and the marshland on either side of it from the Pennsylvania Avenue Bridge to Benning Road have been modified by dredging, but north of
Fig. 7—Shipyards at Newport News, Va., showing docks and deep-water approaches, steamships, and drydocks, in one of which is a vessel for repairs.
Fig. 8—The New York Connecting Railroad Bridge, which affords an all-rail passenger and freight route between Boston and Washington. The bridge, which was completed in 1917, starts on the mainland in the Port Morris section of southern Bronx Borough, New York City, seen in the background, then crosses Bronx Kill, Randalls Island, Little Hell Gate, Wards Island, and Hell Gate to reach the Long Island shore, seen in the extreme lower right corner, at Long Island City, Queens Borough. The tracks continue towards Washington by way of tunnels under the East River and the Hudson.
Fig. 9—A part of Washington, D.C., showing the White House, Treasury, State-War-Navy, and other public buildings in the foreground; the Ellipse, Washington Monument, and new War and Navy offices in the middle ground; and the Tidal Basin, Potomac Park, and the Potomac River in the distance. By no other means could so informative a glimpse be given of a spot of such wide interest. Every feature in the picture is more or less familiar to a large number of Americans, but their familiarity is with the individual features rather than with their situation and relation to one another as shown here.
Fig. 11—Landscape gardening. An airplane view of a part of Long Branch, N.J., taken from a height of 10,000 feet, showing the beach and surf at the right, and the streets, mansions, driveways, and lawns in the body of the picture—an example of the development of a barrier beach of little value before the exploitation for summer homes. Scale, about 1: 15,000.
this road the surface appears in its natural state. In the mosaic are shown at the left the highlands west of the marshes, wooded in some places but cleared and improved in others. In the northern part can be seen land wooded north of the District line but cleared south of it. So comprehensive a view of the field of the project and of the progress to date should be of great service to the engineers and promoters.
CHAPTER IV
THE MOSAIC
(Figs. 13 and 22)
In its simplest form, the mosaic is made by mounting overlapping prints so that the corresponding details coincide. This type of mosaic is quite adequate for relatively small areas or where a high degree of accuracy is not required. For larger areas and greater accuracy, an accurate outline map is used as a base upon which the prints are mounted so that recognizable features coincide with their location on the map. When the prints are properly arranged, the better print of each overlapping pair is selected, the excess paper removed, and the whole mounted and photographed. Figure 13 is left untrimmed to illustrate the method of matching the overlapping prints. The differences in shade are due to difference in printing and developing the pictures which make up the mosaic. The slight offsetting of line at the junction of the prints may be due to errors in mounting, shrinking, or stretching of the photographic paper, tilting of the camera at the time of exposure, or other cause. Such errors and imperfections illustrate the difficulty of using these photographs in the making of maps.
A skillful manipulation of both airplane and camera is necessary to the success of the mosaic. To prevent distortion and variation of scale, the camera must be maintained at the same altitude at all times and pointed directly downward. This can be accomplished by flying with an even keel at a uniform altitude. Mechanical devices are also being perfected to accomplish the same result. Still greater skill is necessary when consecutive rows of exposures are made for the purpose of placing strips of photographs side by side to cover a large area. It is difficult under the varying conditions of wind and weather to fly so evenly and so nearly at the same level that distortions and differences in scale are not noticeable. Strong objection to the mosaic is frequently raised because of inaccuracies due to difference in scale in neighboring prints. Until these defects are overcome, such a group of matched photographs cannot take the place of an accurate map. Much, however, is being done to correct these defects, and, even in photographs where inaccuracies in scale are many, the value of the photograph for the portrayal of detail cannot be denied.
CHAPTER V
GENERAL ASPECTS OF THE SURFACE AS SEEN FROM THE AIR
(Figs. 12 to 18)
Fig. 12—Benning, D.C., and the Anacostia River, showing, from right to left, cultivated lands 40 to 20 feet above sea level, an elevation too slight to be shown in a vertical photograph; a brushy slope running from 20 feet to sea level; and marshland along the stream. The checkered pattern of the upland fields is caused by different-colored crops. Shocks of corn, spaced evenly in rows, buildings and shade trees, and light-colored roads and a race track are shown. The light-colored areas along the stream are occupied by tidal marsh and are free from brush but covered with vegetation of annual growth. The figure is one of the photographs used to make the mosaic shown on Fig. 13. It should be compared with Fig. 13 and with the topographic map, Fig. 14. Scale, 1: 11,000.
When a region is viewed from an altitude of several thousand feet the observer can readily imagine himself looking down on a large map. The chief features stand out prominently, the smaller to a lesser degree. Mountains, rivers, and the seashore are
Fig. 13—Vertical photograph of the land along the Anacostia River on the eastern edge of Washington, D.C., made up of several photographs matched together and adjusted to points located by ordinary survey methods, and reduced in size to correspond with the map, Fig. 14. The photographs were taken from an airplane with a so-called mapping camera at such intervals of time that the prints overlap, thus making it possible to adjust them to each other and to form a continuous picture of the area. The region shown is the site of improvements that are at present under way, mainly the regulation of the Anacostia River. The channel has been widened by dredging and part of the bordering marsh areas filled in. The photograph shows that this work had progressed to the Benning Road bridge by the autumn of 1920, when the photograph was taken, while in 1915, when the area was surveyed for the map, it had been carried out only as far as the Pennsylvania Avenue bridge. Such airplane photographs furnish an incomparable tool in the handling of large-scale engineering projects, both in the study of the territory in its unimproved state and to follow the progress of the work after operations are under way. Scale, about 1: 28,000.
Fig. 14—Part of the topographic map of Washington and Vicinity, 1: 31,680, published by the U.S. Geological Survey, showing within the irregular line the same area shown in Fig. 13. Scale, 1: 28,000.
Fig. 15 (on page 24)—Mosaic of the southeastern part of Mulberry Island, on the left bank of the James River about 11 miles northwest of Newport News. Va., showing an area portrayed by many photographs matched together. Slight differences in shade indicate the junction of the separate prints. The higher land, about 10 feet above sea level as determined by surveys on the ground, is shown at the right by roads and cultivated fields. It is to be noted that roads outline the dividing line between the high ground and the marsh. At the left are lower areas of wooded or brushy swampland and of grassy marsh. They contain a number of abandoned channels: some completely silted up, others containing small thoroughfares, and still others drained by meandering streams which seem to have developed after the channels were definitely abandoned by the streams which originally occupied them.
The streams which drain the marshes have many characteristics of streams which drain higher lands. They have dendritic patterns, so called from resemblance to the forking branches of a tree; channels which widen downstream; and winding or meandering courses. The island terminates in a long spit composed of silt and fine sand. The banks to the left on James River are low and marshy: those to the right on Warwick Creek, except for one small marsh, form low bluffs.
In order that the mosaic may be compared with the map, Fig. 16, it has been placed with the northerly part at the top of the page, with the result that, until the page is reversed, the trees in the swampland appear like hollows in the earth. Scale, 1:14,000.
especially conspicuous. Streams appear as smooth, winding ribbons—glistening if the sunlight reflected from them enters the eye, dark if the bright rays are reflected away from the eye. Railroads can easily be traced and towns recognized by their form. Concrete roads and others of light-colored material are plainly visible. Those built of dark-colored material appear less prominently. Something even of the character of the forests can be ascertained—whether evenly timbered or partly of primary and partly of secondary growth; whether intact or partly burned over; whether consisting chiefly of one species of trees or of many. The cultivated fields and their relations to roads, streams, and forests are conspicuous. Towns and cities are spread out like panoramic views in which are strikingly visible the residence and manufacturing centers, the layout of streets, the systems of parks, the position of suburbs, and the relation of these to routes of transportation and travel—roads, railroads, and waterways. These and many other features of the landscape—swamps, marshes, buildings, trees, orchards, and many lesser details—are recognizable and are all recorded on the
Fig. 16—The same area as shown in Fig. 15 reduced from a section of a map on the scale of 1:10,000 by the Corps of Engineers, U.S.A. The photographs shown in Fig. 15 were used for mapping certain small features on this map, such as small streams. Scale, 1:14,000.
photographic negative. So faithfully does the camera reproduce all the horizontal features within its range of vision that it is conceivable that a photograph correctly dated might become a valuable record in cases of boundary disputes or other litigations involving the position of fences, fields, roads, or even streams, at a given date.
Fig. 17—Columbus. Ga. A part of a mosaic made at Camp Benning near-by in 1909 showing the town, river, and surrounding country. The scale is so small that buildings and trees appear as dots, city blocks as small parallelograms, streets and roads as light-colored lines. The cultivated fields appear as irregularly checkered areas, and the concentric lines of the terraced slopes have the appearance of contour lines on a topographic map (see Fig. 18). The picture illustrates many of the features of city geography. The comparatively straight course of the river and the heavy growth of trees and bushes along its edges indicate a minimum of flood-plain and steep banks—an inference supported by the fact that the principal business center of the city, shown by large, closely set roofs, is built close to the river. Surrounding this section is the most densely populated district, which in the northern part of the city gives way to a district of houses set farther apart and separated by lawns set with trees. Other less extensive business centers are shown as small spots of closely grouped buildings. A variety of suburban types is to be seen: some quite city-like, with a business center, a densely populated residential district, and a district of houses separated by grounds; others more village-like in their lack of a well-developed center but still more or less completely separated from the city proper; still others, sporadic scatterings of houses and grounds extending from the city for some distance along the principal roads. The railroad center is located conveniently near the business center, and the radiating lines of road and railroad communications are in strong contrast to the rectangular arrangement of the city streets. Factories, indicated by large, light-colored roofs, are located along the railroad in the southern part of the city and along the river to the north. Those along the river are operated by power from the falls which the picture shows. The terraced slopes are characteristic of the region, the farmers here and elsewhere in the South making these terraces in their plowed fields to prevent rain water from washing away the soil. Scale, about 1:38,000.
Fig. 18—Map of the same area shown in Fig. 17 enlarged from the corresponding sections of the 1:62,500 Columbus and Seale, Ga.-Ala., and the 1:125,000 Talbotton and Opelika, Ga.-Ala., topographic sheets surveyed mainly in 1906 and 1907 and 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 the broad shelf upon which the city rests and its relation to the river and to the terraced hillsides east of it. Scale, 1:38,000.
CHAPTER VI
MARSHES AND MARSH DRAINAGE
(Figs. 19 to 27)
Mention has been made of the objects seen better from the air than from any viewpoint on the ground; but there are some objects which as a whole can be seen only from above. Swamps, parts of everglades, peaks in the midst of difficult country, precipitous canyon walls, and many volcanic craters cannot be seen from the ground without undue effort. Photographs of bluffs, terraces, and other slopes facing bodies of water have hitherto been adequately obtainable only from the water. All of these can be readily viewed and photographed from the airplane. Pictorial representations of drainage systems were rare until photographs such as Figure 19 were taken from airplanes. The intricate drainage of marshes like those along the Pamunkey River in Virginia pictured in Figure 20 was never accurately shown until photographed from the air.
Of frequent occurrence on the Atlantic Coastal Plain of the United States are swamps and marshes inaccessible from the ground. Much of the surface material is so soft that they cannot be easily traversed; and, even where firm enough to support a man’s weight, few of the details are deemed of sufficient importance to warrant the trouble and expense of mapping by ordinary methods. Yet the trapper would scarcely admit that these details are unimportant, and, to the student, they are an interesting feature of marsh topography that has thus far received little attention.
Figure 25 is part of the excellent New Kent, Va., sheet of the topographic map and is probably as detailed as a map of this character should be when made from ground surveys only. However, on comparison of the map with a photograph of the same area (Fig. 24), there is no difficulty in detecting errors; and it is probable that, had the photograph been available when the map was made, the marshes would have been represented differently.
Fig. 19—Stream development in a tidal marsh, showing, at the right, the northern end of Ludlam Beach, about 6 miles south of Ocean City, N.J., and the mouth of Corsons Inlet leading to Ludlam Bay, and, at the left, the marsh just west of the inlet, with streams rising close to the bank of the larger stream at the extreme left and flowing in meandering courses across the marsh. The great variety of types of vegetation probably is one cause of the remarkable meandering of these drainage lines by reason of the fact that the accumulated remains as well as the annual growth of different weeds and grasses offer varying resistance to the current of the streams. Scale, about 1:10,000.
Marsh Drainage
One of the most striking characteristics of marsh topography illustrated by the photographs presented here is the great wealth of drainage lines and the resemblance of the drainage patterns to those of river systems developed on higher ground. The dendritic patterns, the meanders, and the sharply outlined divides are surprising in areas which have altitudes varying from only a few inches to a little more than a foot at times of ordinary high tide and which are wholly submerged at times of maximum tide. Some of the streams have gently winding courses suggestive of normal stream development. Others, particularly the smaller, have a conspicuous angularity of course. It is possible that the latter may have originated as the trails of animals. Some of the lines are observed to cross the larger streams and are probably tracks made by muskrats. Some of the streams rise close to the river’s brink and lead to through-going waterways near the center of the marsh. This suggests the deposition of silt on the brink of the river at times of high water. The notched appearance of the shore in Figure 20 seems to be due to overhanging bunches of sedge grass and, in some instances, to the breaking away of the surface mat or crust of the marsh formed by the interlacing roots of grass. The mottled appearance of the marsh in this picture may be partly due to shadow of clouds, but to some extent, at least, the difference in shade is caused by differences in the character of the plants.
The marshes used for illustration here are typical of many along the Atlantic Coast. They are situated near West Point, Va. The Pamunkey and the Mattaponi Rivers both rise in the Piedmont Plateau, flow southeastward through the tidewater portion of Virginia, and join about midway of the Coastal Plain to form the York River[3] (see Fig. 58).
Fig. 20—Details of marshland. A part of Lee Marsh near West Point, Va. (cf. Fig. 26), as photographed from a height of 2,000 feet, June, 1920. Local observers report that this marsh has been submerged only twice in nineteen years. The drainage systems are well entrenched. The larger stream channels are cut 1 to 5 feet or more below low tide (the tidal variation at West Point is about 3.4 feet), and their form is made stable by the tough surface crust of the marsh, consisting of the matted roots of the luxuriant sedge grass. The intricate, veinlike appearance of the drainage lines and the furry appearance of the edges of all the waterways, showing overhanging vegetation, are of interest. Drainage systems flowing in opposite directions slow connecting tributaries apparently silted up. Scale, about 1:4,000.
Fig. 21—Details of frequently submerged marshland. A part of Cousaic Marsh on the Pamunkey River, near Sweet Hall, Va. (cf. Fig. 24), as photographed from a height of 2,000 feet, June, 1920. The surface of the marsh is covered with water several times each year, according to local report. It is relatively soft, and a comparison with Fig. 20 shows an apparently different, less dense vegetation than that of Lee Marsh, which is rarely submerged. The stream channels are less definitely fixed and lack the evidence of overhanging vegetation. Scale, about 1:4,000.
Fig. 22—Atlantic City and Ocean City, N.J. Strips of photographs taken from an airplane, March, 1920, from a height of 10,000 feet, showing, in order from east to west: the ocean water, which appears dark-colored; the surf, white where it breaks into foam; the light-colored beach sand; the cities laid out on the sand of the barrier beach; and the marshes, channels, and drainage systems west of the barrier. West of Peck Beach in the strip of photograph at the right many features characteristic of salt marsh areas of the Coastal Plain are shown back of the barrier beach. The right strip forms the southern continuation of the left strip. Scale, about 1:75,000.
Fig. 23—A river system in miniature. A small stream near Hampton, Va., showing flood plain, meanders, an ox-bow lake and cut-off, abandoned channels and a delta partly under water. Scale, not known.
Fig. 24—Sweet Hall Marsh on the lower Pamunkey River, near West Point, Va., as photographed from a height of 10,000 feet at 11 A.M., December 11, 1920. Cousaic Marsh lies to the left and Hill Marsh to the right of the central meander. Some of the watercourses in these marshes are thoroughfares, or channels opening to the river at both ends, that can be traversed by boat at high tide. But many of them are quite different in nature, beginning as minute rills and broadening toward the mouth in a manner suggesting typical drainage channels on higher land. Scale, about 1:31,000.
Fig. 25—The same area as shown in Fig. 24, enlarged from the New Kent, Va., topographic sheet, 1:62,500, published by the U.S. Geological Survey. The cross section at the left lies along the line indicated on the map and extends somewhat beyond its borders. The somewhat greater height of the map than of the photograph, although both cover exactly the same area, is due to the unavoidable slight difference in tilt of each of the exposures of which the photographic mosaic is made up. This illustrates the fact that airplane photographs cannot be directly used as equivalent to maps, until the necessary adjustments have been made. Experiments in camera construction are under way to overcome these difficulties by automatic devices. Scale, 1:31,000.
Fig. 26—Eltham Marsh on the lower Pamunkey River, as photographed from an altitude of about 10,000 feet at 11 A.M., December 11, 1920. At the right lies the town of West Point, Va., at the junction of the Mattaponi and Pamunkey Rivers, and at the left appears a part of Lee Marsh. Eltham Marsh, in the center of the illustration, is traversed by a so-called thoroughfare, through which boats of light draft make their way at high tide. At one point in the middle of the marsh the thoroughfare is perceptibly broader than elsewhere, and the tidal currents entering from opposite ends of the thoroughfare meet there and cause slack water in which silt is deposited, forming mud flats exposed at low tide. The cultivated fields south of the marsh are on a bench about 10 feet higher than the marsh. Scale, about 1:31,000.
Fig. 26—Eltham Marsh on the lower Pamunkey River, as photographed from an altitude of about 10,000 feet at 11 A.M., December 11, 1920. At the right lies the town of West Point, Va., at the junction of the Mattaponi and Pamunkey Rivers, and at the left appears a part of Lee Marsh. Eltham Marsh, in the center of the illustration, is traversed by a so-called thoroughfare, through which boats of light draft make their way at high tide. At one point in the middle of the marsh the thoroughfare is perceptibly broader than elsewhere, and the tidal currents entering from opposite ends of the thoroughfare meet there and cause slack water in which silt is deposited, forming mud flats exposed at low tide. The cultivated fields south of the marsh are on a bench about 10 feet higher than the marsh. Scale, about 1:31,000.
Fig. 27—The same area as shown in Fig. 26, enlarged from the New Kent, Va., topographic sheet, 1:62,500, published by the U.S. Geological Survey. It is obvious that many interesting details shown by the photograph are missed or neglected as unimportant in the most careful mapping. The cross section at the left lies along the line indicated on the map and extends somewhat beyond its borders. Scale, 1:31,000.
The York is one of the estuaries of the tidewater portion of Virginia, and the water level at West Point, the junction of the two tributaries, rises and falls about 3½ feet under tidal action. The Pamunkey is affected by the tide 53 miles by channel above West Point, and the Mattaponi 42 miles. Much of the broad lowland along these rivers is marshy, but the largest marshes are found near West Point, where the river current in swinging from side to side has formed great meanders. For some reason the valleys eroded long ago by these streams have filled with sediment here to a greater extent than farther downstream; perhaps because this is essentially the head of sea water, so that the checking of the current of the river causes it to deposit much of its load. Sea water regularly mingles with the river water as far upstream as West Point, but above this point the water is chiefly fresh. The marshes consist of soft mud and muck to a considerable depth. A well driven in Hill Marsh to an underlying artesian horizon penetrated 50 feet of this soft material before entering rock such as is exposed in the river bank. The thickness of the mud is comparable to the maximum depth of the York farther downstream and suggests that the old valley which there is filled with water is here filled to a depth of 50 feet or more with sediment brought down by the river. Only a small part of the marsh near the landward margin has surface material firm enough to support the weight of large animals except when the surface is frozen.
Many kinds of marsh plants grow here, among which is sedge grass (Spartina cynosuroides (L.) Willd.), which grows to a height of 10 feet or more and forms dense thickets. Its roots interlace to form a tough mat which in some places will support the weight of a man. In other places the soft muck reaches to the surface.
“Thoroughfares”
These marshes are cut by a few waterways open at both ends, known as thoroughfares, or tidal runs, which also serve as the trunk streams through which the marsh is drained. Some of the thoroughfares may be trunk streams modified by tides, or they may be silted remnants of abandoned river channels. Some seem to be channels in the last stages of silting. The incoming tide enters the down-river end but ascends the thoroughfare more slowly than it ascends the river. The tide in the river reaches the upper end of the thoroughfare, enters it, and meets the opposing tide within the marsh near the upstream end of the passageway. Where the tides meet, thus causing slack water, silt is deposited and mud flats are formed. In Eltham Marsh (Fig. 26) these flats are well within the marsh. In the larger thoroughfares of Sweet Hall Marsh (Fig. 24) the tide passes entirely through while the tide in the river is making its long way around, so that slack water and the deposition of silt occur at the extreme upper end of the passage. In all of the thoroughfares the silting has reached the stage that precludes their use by boat, except at times of high water. Even at high tide some are navigable only by small skiffs, although throughout much of the course the water is many feet deep.
Some of the thoroughfares become narrow and shallow upstream in a manner that suggests that they originate as two normal streams flowing in opposite directions from a common point and that they were later united by the breaking down of the divide between their headwaters. Such a junction might be affected by an unusually high tide breaking through a divide and cutting a channel. Such a divide, be it noted, consists of soft mud only a few inches above the general level and might readily be broken down. In some instances the connection may have originated as an animal trail, as we have seen. Muskrats, otters, and other marsh animals use the waterways as lines of travel and make paths in between them from one to another. Apparently many of the small drainage lines originated in this way, but in some instances stream systems of considerable size and complexity are independent of all others and possess all the characteristics of normally developed river systems.
CHAPTER VII
COASTAL MUD FLATS
(Figs. 28 and 29)
Of frequent occurrence along the Atlantic Coast of the United States are low mud flats which are practically at sea level and which are covered with water at times of high tide. Where these tracts are exposed to the air during ebb tide for so short a time that plants have not taken root and where the surface material is fine-grained and soft, the tracts are known as mud flats. In the part of the peninsula between Delaware and Chesapeake Bays belonging to the state of Virginia which is called the Eastern Shore a low barrier beach of sand has formed on the ocean side several miles off shore, and the space between this and the mainland is occupied by mud flats, broad, shallow lagoons, and an intricate maze of interlacing channels and winding, branching, interlocking, vermicular streams.
The mud flats are exposed for a short time during low tide, and, as the surface of the water here rises and falls with the tide more than 4 feet, with a maximum fluctuation considerably greater, large volumes of water are continually flowing backward and forward over the flats. As the tide rises, strong currents of sea water set in through the inlets, flow up the main channels and through the thoroughfares, and gradually find their way into the countless small channels and out of them over the broad level stretches of soft mud. As the tide falls, this action is reversed, and the broad sheet of water finds its way by devious paths through the winding watercourses out to sea. The larger channels extend considerably below the surface at times of highest water and may be quite deep even at times of low water. They are, perhaps, stream courses excavated before the region was drowned. Many of the smaller channels also have the general form characteristic of normal stream channels, although others show peculiarities not common to subaerial drainage. The origin of these submarine and tidal features is not well understood, but the photographs of them show their form and furnish some basis for a study of them.