CHAPTER 12
LOW AREAS OF THE EARTH
180.Level Areas. -At different places on the earth"s surface there are broad extents of nearly level land. Here the drainage is often poorly developed, and there are slight depressions often of considerable area. After a rainfall the shallow water stands in these depressions until itA LEVEL, POORLY DRAINED AREA.
Such an area is called young.
evaporates or sinks into the ground. In the parts where the drainage has been developed, the streams flow with slow currents in channels of little depth.
When excavations are made, the rock beneath the soil is often found in horizontal or almost horizontal layers. Where the elevation of these areas is considerable, the streams may have deep gorges and the surface may be well dissected. Where these level areas are low, they are called plains, and where high, especially if surrounded by steeplydescending sides, they are called plateaus. A good example of the low, level area is the plain of northern Russia and of the high area, the Arizona Plateau through which flows the famous Colorado River.
181.Coastal Plains.
Experiment 130. -Fill a tall glass jar nearly full of water. Pour into this very slowly a mixture of sand and finely pulverized clay. Note the effect upon the color of the water. Allow the water to stand for several days and then examine the deposition on the bottom of the jar. Are the sand and clay now mixed as they were when poured into the jar? What effect has the water had upon the mixture?
We have already seen that the surface of the earth is not stable, but is subject to movements. If the land bordering a coast rises or the bottom of the ocean is depressed, it causes the water to withdraw from the land, and a strip of what was formerly sea bottom is changed into dry land.
This new area is composed of clays, sands and gravels, often containing shells similar to those found on the neighboring shores. The surface is comparatively flat, but slightly irregular, and the drainage lines have not as yet been established. The water that falls here which neither evaporates nor sinks into the soil runs into the slight depressions and makes shallow lakes. When these become full, the water finds an outlet into a lower region until at last it works its way to the sea.
These outlet streams gradually establish themselves and form a continuous line of streams and pools reaching to the sea, with broad, poorly drained areas lying between. The streams at once begin to cut down their beds and the pools to fill up with the silt washed into them, until at last all the pools are drained and a network of streams carries the run-off into the sea.
Usually the dry land of the coastal plain has appeared very gradually, with long periods when there was no gain in its extent. Sometimes the waste brought to the ocean was of a different kind from what itwas at other times. Thus the character and condition of the material composing the plain vary considerably, but all the strata are usually inclined slightly toward the sea. The boundaries of the different kinds of hard and soft material composing the plain are approximately parallel to the old shore line. The plain will thus become a belted plain.
As streams wear back faster in the soft than in the hard material,the side streams become longer in the soft layers than in the hard, and in time streams of considerable length are found running in a direction nearly parallel to the old coast. These have their outlets through streams which run down from the old land across the plain, so that the general appearance of the drainage is something like a lengthwise cross section through the trunk and limbs of an oak.
When mixtures of different materials are deposited in water, thecoarsest sinks first and the finest last (Exp. 130). We should thus expectTHE COAST NEAR ATLANTIC CITY.
Showing marshes, lagoons and sand reefs.
that of the material brought down by the river the coarser layers would lie back from the coast. This is often true, although there is frequently uncovered near the border of the old land back from the coast a belt of easily eroded material, and a lowland of erosion is formed in this by the streams. The Delaware River from Trenton to Wilmington and the Alabama River between Montgomery and Selma flow through such lowlands. These regions are called inner lowlands and possess a fertile, fine-textured soil, generally the best to be found in the coastal plain area.
RICE SWAMP AT THE BORDER OF A NARROW COASTAL PLAIN.
This inner lowland is bordered on the landward side by the old land, usually composed of firmly compacted rocks which often contain valuable minerals and building stones. On the seaward side it is bordered by the rather abruptly ascending edge of the coarse material of the plain which has not yet been removed. From the top of this ridge there is a gradual slope toward the sea. As the region back toward theold land is higher, and has been above the sea and exposed to erosion longer, it is much more dissected than the surface nearer the sea and is much more irregular and hilly.
A coastal plain is a gradually emerged sea bottom, and so has shallowwater extending out for a considerable distance from its edge. Along the shore are marshes and lagoons bordered on their seaward side by sand reefs, where the winds have piled up the sand brought in by waves. In some places these sand reefs are so situated that they are valuable for habitation, as at Atlantic City, New Jersey, where a large summer resort has grown up, or along the coast farther south, where a sparse population finds its home on the broader reef.
A coastal plain increasing in width toward the south extends from New York to the Gulf. The western coast of Europe has a considerable plain of this kind. The Netherlands are situated on land which has been either reclaimed from the sea naturally in recent geological time or artificially by man in recent historical time. In the southern part this reclamation is largely due to the sediment brought down by the Rhine.