Earth Features and Their Meaning: An Introduction to Geology for the Student and the General ReaderHobbs, William Herbert
Science
Earth Features and Their Meaning: An Introduction to Geology for the Student and the General Reader
Hobbs, William Herbert
Geology; Physical geography
A region racked by heavy earthquakes, where the present configuration
of the ground speaks strongly for a movement of somewhat similar
nature, but with average movement of elevation much greater to the
northward than in the opposite direction, is the extended coast line
of Chili. This country is characterized by a great central north and
south valley which separates the coast range from the high chain of the
Cordilleras to the eastward. To the southward the floor of this valley
descends, and has its continuance in the Gulf of Corcovado behind the
island of Chiloe and the Chonos archipelago. The known recent uplift of
the coast of Chili, particularly in the northern sections and during
the earthquakes of the eighteenth, nineteenth, and twentieth centuries,
lends great interest to this topographic peculiarity. Indications are
not lacking that, during the earthquake of Concepcion in 1835, and of
Valparaiso in 1907, the measure of uplift was greater to the north than
it was to the south.
[Illustration: FIG. 287.—Map of San Clemente Island, California,
showing the characteristic topography of recent uplift (after U. S.
Coast and Geodetic Survey).]
=The contrasted islands of San Clemente and Santa Catalina.=—Perhaps
the most striking example of simultaneous opposite movements observable
in neighboring portions of the earth’s crust is furnished by the coast
of southern California. The coast itself at San Pedro and the island
of San Clemente, some fifty miles off this point, in common with most
portions of the neighboring coast land, have been rising in interrupted
movements from the sea, and offer in rare perfection the characteristic
coast terraces (Fig. 287 and Fig. 278, p. 250). Midway between these
two rising sections of the crust, and less than twenty-five miles
distant from either, is the island of Santa Catalina, which has been
sinking beneath the waves, and apparently at a similarly rapid rate
(Fig. 288). The topography of the island shows the intricate detail of
a maturely eroded surface, while that of the neighboring San Clemente
shows only the widely spaced, deep cañons of the infantile stage of
erosion (Fig. 165 and pl. 12 A). While Santa Catalina has been sinking,
San Pedro Hill has risen 1240 feet, and San Clemente, 1500 feet. It
is characteristic of a sinking coast line that the cliff recession is
abnormally rapid, and evidence for this is furnished by the shores of
Santa Catalina, upon which the waves are cutting the cliffs back into
the beds of cañons, and so causing small falls to develop at the cañon
mouths.
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