We have, however, recently learned from the observations of Ramsay, in the
van of many excellent observers--of Jukes, Geikie, Croll and others, that
subaerial degradation is a much more important agency than coast-action, or
the power of the waves. The whole surface of the land is exposed to the
chemical action of the air and of the rainwater, with its dissolved
carbonic acid, and in colder countries to frost; the disintegrated matter
is carried down even gentle slopes during heavy rain, and to a greater
extent than might be supposed, especially in arid districts, by the wind;
it is then transported by the streams and rivers, which, when rapid deepen
their channels, and triturate the fragments. On a rainy day, even in a
gently undulating country, we see the effects of subaerial degradation in
the muddy rills which flow down every slope. Messrs. Ramsay and Whitaker
have shown, and the observation is a most striking one, that the great
lines of escarpment in the Wealden district and those ranging across
England, which formerly were looked at as ancient sea-coasts, cannot have
been thus formed, for each line is composed of one and the same formation,
while our sea-cliffs are everywhere formed by the intersection of various
formations. This being the case, we are compelled to admit that the
escarpments owe their origin in chief part to the rocks of which they are
composed, having resisted subaerial denudation better than the surrounding
surface; this surface consequently has been gradually lowered, with the
lines of harder rock left projecting.
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