Man's physical universe

xanabras

152 THE EARTH AS MAN'S ABODE

majority of lakes are relatively shallow, although there are notably

deep lakes such as Crater Lake (2000 feet deep) and Lake Baikal in

Siberia (5618 feet deep). Lakes usually have outlets, so that the soluble

salts do not accumulate. The insoluble suspended matter settles out

from the quiet waters of lakes, gradually filling them, until they

eventually turn into swamps.

Many lakes which have been formed in arid districts have no outlets,

and salts accumulate in them just as they do in the oceans, except that

the more rapid evaporation in these lakes produces more highly concentrated

salt solutions.

Ocean Shore Lines Change As the Land Sinks or Rises.

It is unlikely that ocean beds have ever risen to form continents, and

there is no evidence that the continents have ever been completely

covered by the deep ocean. The sedimentary rock formations were

probably produced for the most part in great arms of the sea (like

Hudson Bay and the Yellow Sea), lakes, and other shallow waterways

rather than in great ocean depths.

Constantly the ocean wears down the shore at one place and produces

beaches and bars at other places, and about the time that the ocean

seems to be making real headway into the land areas, they are elevated

by internal disturbances (to be discussed later), causing the formation

of new shore lines. The coast in the far eastern part of the United

States has been gradually sinking, producing channels and islands

where rivers and hills once prevailed. On the Pacific coast of the United

States, on the other hand, the wave-cut terraces of the Southern

California coast show that there has been a recent gradual uplift

amounting to as much as 1500 feet.

The continents are surrounded by shelves varying in width from 5 to

300 miles and averaging about 75 miles. These are smooth, gently

sloping plains, whose final depth is about 600 feet. At the outer edges

the underwater platforms give way rapidly to the deep ocean basins.

These continental ledges catch most of the sediment carried from the

land into the ocean.

A mountainous ridge almost 10,000 feet above the adjoining basins

runs down the middle of the Atlantic Ocean, extending 8000 miles

south from Iceland.

If the oceans were to be lowered only 600 feet by the formation of

huge sheets of ice, Ireland would be joined to England and England

would become a part of the mainland of Europe. North America

would be connected with Asia by a strip 1500 miles in width. New

Guinea would become a part of Australia.

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