Man's physical universe

xanabras

THE HISTORY OF THE LANDSCAPE 153

The discovery of submarine canyons, which closely resemble those

cut by rivers on land, suggests that these canyons were actually cut in

the present ocean bottom at some previous time when there was so

much less water in the oceans than at present that these portions of

the ocean bottom were dry land. According to one theory, the ocean

may have been lowered as much as 3000 feet during the ice ages, in

which the glaciers may have been as thick as 10 miles.

The average height of all of the land on the earth's surface is 2300

feet above sea level, while the average depth of the oceans is about

bUj. 42. View ul lubeiiuLe V cille> Iruiii liispiraLioii Puiiii. Aote Bridalveil

Falls. This is a typical glacial valley. (Courtesy of the U. S. Dept. of the

Interior.)

13,000 feet below sea level. The summit of -the highest mountain,

Mount Everest (29,141), is only about 12 miles higher than the lowest

spot in the oceans, Swire Deep, east of the Philippines (35,410 feet).

Given enough time, erosion alone would carry all the land areas

into the ocean, and the whole earth would be covered with water.

all the land were reduced to a common level, it would be covered by

the ocean to a depth of two miles. As it is, the oceans cover 70.8 per

cent of the earth's surface to an average depth of 13,000 feet, as

mentioned above. In spite of this, it is probable that the land and

water areas have not greatly changed during the past few millions

of years.

If