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.
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