Man's physical universe

xanabras

THE HISTORY OF THE LANDSCAPE 151

difference in the treatment which they received after their formation

resulted in differences in hardness, i.e., resistance to erosion and

weathering.

Terraces, buttes, mesas, tablelands, rapids, and waterfalls result

The "Cave of the

from differences in the hardness of flat-lying strata.

Winds" was carved in soft rock strata underlying harder strata by the

rushing waters of Niagara Falls. Niagara Falls is possible only because

the water at the top does not come from a steep slope, which would

enable it to pick up abrasive materials that would soon wear away the

falls, and also because the upper strata of rock are harder than those

immediately under them. Niagara Falls recedes at a rate of about 4|feet

a year, the chief action being that of the dissolving away of the

softer strata,

thus leaving the upper strata without support to withstand

the weight of the water which rushes over it; the result is that

portions of the hard upper layer break off from time to time.

Unequal hardness in tilted strata causes water gaps where rivers

work their way through softer strata in mountains; ridges of hard

strata are called hogbacks.

Badlands Are Characteristic of Old Topography in Arid Regions.

Badlands are often found in arid regions in soft deposits of clay or

shale characteristic of old topography.

In these regions it does not rain

often; but when the rains do come they usually arrive in the form of

cloudbursts that wear deep gullies into the land, where they remain

until another cloudburst adds "insult to injury." There are few gentle

rains to level off these gullies gradually between cloudbursts.

are typical of old topography in arid regions.

Badlands

Lakes Have Comparatively Short Lives.

Many lakes occupy low spots on the land that have resulted from

sinking of certain areas under the pressure of glaciers, as in the case of

the Great Lakes of North America, or from the water's carving action,

as in the case of oxbow lakes in the wide valleys of old rivers. Lakes

may be formed by glaciers whose forward walls have melted in a valley

as fast as they have progressed down the valley, with the result that

the terminus of the glacier has remained in the same place and gradually

piled up large heaps of rocks and detritus, which dammed up the

valley to form a lake.

Changes in river courses, underground caves, and other changes in

the earth's surface may produce lakes. Lakes are found in elevations

ranging from that of Lake Titicaca in Peru (12,500 feet above sea level)

to that of the Dead Sea in Palestine (1300 feet below sea level).

The

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