PHYSIOGRAPHY                                     1484                                       PHYSIOGRAPHY

may be built by deposits of sand and gravel made by waves and shore-currents. They are the result of gradation, in this case of aggradation. Glaciers moving down valleys to the sea, as in high latitudes, may gouge out ' the lower ends of the valleys through which they pass, cutting them down far below sea-level. When the ice melts, deep, narrow bays or fiords, like those of Greenland, Norway or Alaska, are the result. Such bays are a result of gradation; in this case, of degradation. Volcanic activity on a coast-line may result in extending the land, — making a cape, or in destroying land which previously existed, — leaving a bay. Thus horizontal irregularities may arise by vulcanism as well as by diastrophism and gradation. Horizontal irregularities arise in many ways not here enumerated, especially by various processes of gradation; but the foregoing illustrations will suffice to show that horizontal irregularities may result from any one of the three sets of processes referred to above. The horizontal configuration of a land-area may be altered by animal or plant life, as when coral-reefs are constructed or mangrove-trees invade the water, extending the land. These may be looked on as special cases of aggradation.

Relief Features of the First Order

Physiography has also to do with the vertical configuration of the land. The great relief types are three: Plains, plateaus and mountains.

Plains are relatively low areas of considerable extent, with surfaces which are not notably rough. Plateaus are similar tracts of greater altitude, which stand up more or less prominently above their surroundings on one or more sides. Mountains usually have less areal extent, and stand up more conspicuously above their surroundings. They generally have somewhat narrow summits and steep slopes.

As topographic features, plateaus and plains may be said to differ from one another chiefly in elevation; yet there is no specific elevation above which land may not rise and yet remain a plain. Formerly, plains were often defined as lands below 1,000 feet in elevation; but this arbitrary definition has no warrant in nature or in usage. The fact is that plains often grade into plateaus and that there is no sharp line of demarkation or basis of separation which is uniformly applicable. An extensive tract of land, 500 feet above the sea, would probably be called a plain if it were surrounded or nearly surrounded by higher land, or if it were bordered by notably high land on one side and descended gradually to much lower levels on others. On the other hand, a tract of land 500 feet above the sea would probably be called a plateau if it were bordered on one or more sides by a tract of considerable extent,

which had an elevation of but 100 or 200 feet, particularly if the descent to the lower level were abrupt. Extensive areas 1,000 feet or even considerably more above the sea would probably be called plains rather than plateaus, if they were surrounded or largely surrounded by higher lands, while they would be called plateaus if they stood up distinctly above their surroundings. Thus parts of the plains of the Mississippi basin are higher than parts of the Piedmont plateau lying east of the Appalachian Mountains. It is, therefore, a question of surroundings and relations, rather than actual elevation above the sea, which determines whether a tract shall be called a plain or a plateau.

A plateau may be bordered by slopes which descend abruptly on all sides or by slopes which descend gently on all sides; or a plateau may descend abruptly or gently on one side and be bordered by a higher plateau or by a mountain range on another. In the latter case the rise to the higher slopes may be abrupt or gentle. If abrupt, the separation of plateau and mountain is distinct; if gentle, the one grades into the other. If a high plateau become narrow and long, and if it descend in all directions or on both sides, it may approach a mountain range in form.

Mountains are not more sharply defined than plains and plateaus. The term mountain implies notable elevation, buf a mountain is not necessarily higher than a plateau. Thus the plateau of Tibet is much higher than any part of the Appalachian mountain system. The term mountain implies (1) a considerable elevation above surroundings and (2) crests of limited area. An isolated elevation 1,000 feet above its surroundings, rising abruptly above a low flat plain, would doubtless be called a mountain, though an elevation of the same height, with gentler slopes, on a rolling plateau might not be.

It is the task of physiography to describe the forms and relations of plains, plateaus and mountains, to explain how they came into existence and how they came to assume the forms which they now have. Some plains originated by diastrophism, as by the elevation of shallow sea-bottom enough to convert it into land; others are the result of aggradation, the building up of sea-bottom slightly above the level of the sea; others are the result of the degradation of mountains and plateaus; and still others owe their origin to the combined action of diastrophic and gradational forces. Subsequent to the origin of plains, their surfaces have been modified by rain, rivers, winds, glaciers etc. It is the task of physiography to determine the nature and the extent of the changes which these several agencies have effected. Plateaus are the result of diastrophism or of vulcanism or of both. I They are plains elevated to the condition