Man's physical universe

xanabras

16() THK EARTH AS MAN'S ABOUK

The thick layers of sedimentary rocks could only have been formed

in shallow basins near the shores of large land areas which were undergoing

intensive erosion. Today similar layers of sediment are being

deposited by the great rivers of China in large basins where new mountain

ranges will probably be born in the far distant future. The motions

of the earth's crust, by which these basins were lowered and neighboring

land was elevated, and finally by which great mountain ranges were

elevated from these basins, are explained by the modern theory of

isostasy.

Erosion Distributes Weight and Pressure Unevenly and Thus Produces

Isostatic Readjustments Which Tend to Offset Its Leveling Processes.

Erosion carries tremendous quantities of material from one place to

another, decreasing the weight over one area and increasing the weight

over other areas. The density of the rocks under the oceans is greater

than that in the land areas, as a rule, while the density of the substances

carried into the oceans is greater than that of the substances left

undissolved on land. The total effect of these various factors is to cause

the portions of the earth under the oceans to be heavier than the land

areas. The fact that the surface of the earth under the oceans is composed

of heavier material than that on land has been shown by measurements

with a pendulum, which has a slightly shorter period of oscillation

over the ocean basins than over land areas.

The heavier lowlands reach a state of equilibrium with the lighter

highlands; but erosion unbalances the equilibrium with the result that

the lowlands become heavier and the highlands lighter, and this difference

in pressure causes the solid rocks to move in such a way as to

restore the equilibrium.

The lowlands sink as rock materials are forced

out from under them, while the highlands are forced still higher.

The sinking of the ocean basins must also produce stresses in the

surface that result in wrinkles or folds.

The present gradual elevation of the region north of the Great Lakes

is thus explained by this principle of isostasy or balancing of weights.

When the Great Lakes region was covered with heavy glaciers, it must

have sunk because of the increased weight due to the glaciers. Then

when the ice melted and the water drained ofT, an opposite movement

was produced to restore the equilibrium. Such movements of the

earth's crust take place very slowly, of course, so that thousands of

years pass by before equilibrium is restored.

The formation of the Appalachian Mountains is explained by the

erosion of a former highland east of the present mountain range, which

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