having, on both sides, a fringe or terrace of shingle and sand, rudely
stratified, and generally of considerable thickness. These fringes
evidently once extended across the valleys and were united; and the
bottoms of the valleys in northern Chile, where there are no streams,
are thus smoothly filled up. On these fringes the roads are generally
carried, for their surfaces are even, and they rise, with a very gentle
slope up the valleys: hence, also, they are easily cultivated by
irrigation. They may be traced up to a height of between 7000 and 9000
feet, where they become hidden by the irregular piles of debris. At
the lower end or mouths of the valleys, they are continuously united to
those land-locked plains (also formed of shingle) at the foot of the
main Cordillera, which I have described in a former chapter as
characteristic of the scenery of Chile, and which were undoubtedly
deposited when the sea penetrated Chile, as it now does the more
southern coasts. No one fact in the geology of South America,
interested me more than these terraces of rudely-stratified shingle.
They precisely resemble in composition the matter which the torrents in
each valley would deposit, if they were checked in their course by any
cause, such as entering a lake or arm of the sea; but the torrents,
instead of depositing matter, are now steadily at work wearing away
both the solid rock and these alluvial deposits, along the whole line
of every main valley and side valley. It is impossible here to give
the reasons, but I am convinced that the shingle terraces were
accumulated, during the gradual elevation of the Cordillera, by the
torrents delivering, at successive levels, their detritus on the
beachheads of long narrow arms of the sea, first high up the valleys,
then lower and lower down as the land slowly rose. If this be so, and
I cannot doubt it, the grand and broken chain of the Cordillera,
instead of having been suddenly thrown up, as was till lately the
universal, and still is the common opinion of geologists, has been
slowly upheaved in mass, in the same gradual manner as the coasts of
the Atlantic and Pacific have risen within the recent period. A
multitude of facts in the structure of the Cordillera, on this view
receive a simple explanation.
The rivers which flow in these valleys ought rather to be called
mountain-torrents. Their inclination is very great, and their water
the colour of mud. The roar which the Maypu made, as it rushed over
the great rounded fragments, was like that of the sea. Amidst the din
of rushing waters, the noise from the stones, as they rattled one over
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