Within the earth's crust, rock exhibits a plastic or
fluid character if subjected to great forces. This is
evident from observations of rock layers that have
been compressed horizontally into what are called
folds. These folds, or wrinkles, may be less than
a fraction of an inch wide or they may be several
miles in width. The earth's crust in many places
appears to have been compressed into upward and
downward folds. The upward arch of a fold is called
an anticline, and the downward part, or the trough, is
called a syncline. The anticline is generally the more
accessible part of the fold in terms of observation
because the syncline is buried beneath the surface.
The anticline, being forced upward, is subject to
erosion, and often the rock in the anticline is so
disrupted, cracked, and distorted in the folding
process that it may be readily eroded away.
When this happens, the upper parts of the adjacent
syncline may be exposed, and the structure of a
region can be more carefully studied. Oil-well drilling
records, highway cuts, and stream erosion provide
other sources of information through which a fairly
satisfactory idea of the arrangement of the rock layers
in syncline can be obtained.
Folding in the earth's crust partly accounts
for the formation of many mountain ranges, such
as the Appalachian Mountains. The folds in
these mountains were originally formed during the
Appalachian Revolution, roughly 200 million years ago.
In recent geologic periods these folds have become
stabilized, that is, they have not been significantly
further distorted, pushed together,
or pulled apart. Erosion has removed most of the
ridges and filled the troughs to the extent that many
of the present ridges are formed by erosion-resistant
rock that at one time lay in the lower part of a fold.
Before the process of folding was understood,
many types of rock layering presented a difficult
puzzle for geologists until the early twentieth century.
However, using the concept of folding has allowed the
geologists to work out quite satisfactory structures,
even for the most complex folds.
101. The phrase subjected to in the
passage is closest in meaning to
A. shaped by
B. placed under
C. made of
D. prone to
ANS: B
請問一下 ...B是什麼意思...
原句裡subjected to是什麼意思阿...
看不懂...