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PHYSIOGRAPHY

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PHYSIOGRAPHY

eludes the study of atmospheric movements as such, physiography, in its narrower sense, includes only the effects of those movements on that part of the earth which lies below the atmosphere. The ocean, likewise, is a part of the earth. The science which deals with the ocean as such is oceanography; but the consideration of the ocean as a part of the earth falls within the province of geology, while a consideration of the effects of oceanic activities which modify the surface of the solid part of the earth falls within the scope of physiography. Thus the effects of rivers, waves, currents etc. on the configuration of the solid part of the earth fall within the province of physiography. Physiography, therefore, concerns itself primarily with the topographic results of geologic processes. It is a special phase of geology. Since the geological processes which have left pronounced topographic results are the processes of late geological time, physiography has to do with but a brief part of the earth's history.

PHYSIOGRAPHIC PROCESSES Powell long ago grouped all processes that work on the earth's surface into three classes: The processes of diastrophism; the processes of vulcanism; and the processes of gradation. Diastrophism includes the up-and-down movements of the earth's crust, movements which, however gentle and slow, are continually in progress. Vulcanism includes all processes connected with volcanoes. Gradation includes all processes by which material is shifted from one point on the earth's surface to another. The centers of diastrophic and volcanic activity are beneath the surface. The processes of gradation are in operation on the surface, chiefly at the plane of contact between atmosphere and land and between water and the solid part of the earth beneath it. The transfer of material in gradation is usually from higher to lower levels. Thus rivers carry debris from land to sea. They degrade the land, and the material, deposited in the sea, aggrades its bottom. Glaciers likewise carry material from higher to lower levels. They degrade the places where they gather debris, and aggrade the places where they leave it. The degradation of one place generally involves the aggradation of another. The sand and dust blown by the wind constitute a partial exception to the rule that the materials shifted about on the earth's surface are transferred from higher to lower levels.

LAND AND WATER AREAS The greatest features of the earth's crust are the elevations known as continents, in contrast with the depressions known as ocean basins. The sharp, topographic division-line between continents and ocean-basins does not correspond with the borders of the continental land-areas. For a distance about

the continental lands the water is very shallow. There is then a sudden descent of the bottom to much greater depths. The area beneath the shallow water is the continental shelf. Its outer border usually is about 100 fathoms below the level of the sea. From the physiographic point of view the outer edge of the continental shelf is the border of the continent. While the explanation of the existence of continents and ocean basins is a problem of physiography, it is an unsolved problem. No assertion can be made at the present time as to how these greatest of physical features originated. The continents have sometimes been looked on as uplifted portions of the earth's crust; but it would perhaps be quite as near the truth to consider the ocean-basins as depressed portions. It, however, is far from certain that the surface of the solid part of the earth was ever regular. If the continents were lifted or if they were left up as the result of the sinking of the ocean-basins, they are the result of diastrophism. If this was not their origin, they probably came into existence when the earth was in process of formation, whatever that process was. Smaller land-masses, that is, islands, have originated in various ways. Some are diastrophic, some are gradational (aggrada-tional), and many are volcanic.

Physiography has to do both with the horizontal and the vertical configuration of land-areas and sea. The horizontal configuration of the one is the counterpart of the horizontal configuration of the other; but the vertical configuration of the one stands in no necessary relation to that of the other.

THE HORIZONTAL CONFIGURATION OP LAND

AREAS

It is the province of physiography to define, classify and explain the origin of all sorts of horizontal irregularities of land-areas. Among the horizontal irregularities of the land are peninsulas, capes etc.— land-masses projecting into the sea. Among the horizontal irregularities of the ocean are gulfs, bays etc. — or bodies of water projecting intc the land. The sizes, positions arid shapes of these irregularities are readily expressed on maps. Not so their origin. They have, indeed, originated in many different ways. For example, the uplift of an area of sea-bottom along a line at right angles to the coast of a continent, would give rise to a peninsula, like Florida. The uplift of two such peninsulas near each other might leave a gulf or bay between them. Again, the sinking of a coast allows the sea to invade the lower ends of the river-valleys, forming bays, as Delaware and Chesapeake Bays. When the sea converts the lower ends of adjacent valleys into bays, it leaves a peninsula between. Peninsulas and bays formed in this way are the results of diastrophism. Small peninsulas or capes, like Cape May,