85. _Denudation._--By the combined action of all the geological agencies
which have been described in the preceding sections, the earth has
acquired its present diversified surface. Valleys, lacustrine hollows,
table-lands, and mountains have all been more or less slowly formed by
the forces which we see even now at work in the world around us. When we
reflect upon the fact that all the inclined strata which crop out at the
surface of the ground are but the truncated portions of beds that were
once continuous, and formed complete anticlinal arches or curves, we
must be impressed with the degree of _denudation_, or wearing-away,
which the solid strata have experienced. If we protract in imagination
the outcrop of a given set of strata, we shall find them curving upwards
into the air to a height of, it may be, hundreds or even thousands of
feet, before they roll over to come down and fit on to the truncated
ends of the beds on the further side of the anticline (see figs. 9 and
11, pages 33, 34). _Dislocations_ or _faults_ afford further striking
evidence in the same direction. Sometimes these have displaced the
strata for hundreds and even thousands of feet--that is to say, that a
bed occurring at, for example, a few feet from the surface upon one side
of a fault, has sunk hundreds or thousands of feet on the other side.
Yet it often happens that there is no irregularity at the surface to
betray the existence of a dislocation. The ground may be flat as a
bowling-green, and yet, owing to some great fault, the rocks underneath
one end of the flat may be geologically many hundred feet, or even
yards, higher or lower than the strata underneath the other end of the
same level space. What has become of the missing strata? They have been
carried away grain by grain by the denuding forces--by weathering, rain,
frost, and fluviatile and marine action. The whole surface of a country
is exposed to the abrading action of the subaerial forces, and has been
carved by them into hills and valleys, the position of which depends
partly upon the geological structure of the country, and partly upon the
texture and composition of the rocks. The original slope of the surface,
when it was first elevated out of the sea, would be determined by the
action of the subterraneous forces--the dominant parts, whether
table-lands or undulating ridges, forming the centres from which the
waters would begin to flow. After the land had been subjected for many
long ages to the wearing action of the denuding agents, it is evident
that the softer rocks--those which were least capable of withstanding
weathering and erosion--would be more worn away than the less easily
decomposed masses. The latter would, therefore, tend to form elevations,
and the former hollows. This is precisely what we find in nature. The
great majority of isolated hills and hilly tracts owe their existence as
such merely to the fact that they are formed of more durable materials
than the rock-masses by which they are surrounded. When a line of
dislocation is visible at the surface, it is simply because rocks of
unequal durability have been brought into juxtaposition. The more easily
denuded strata have wasted away to a greater extent than the tougher
masses on the other side of the dislocation. Nearly all elevations,
therefore, may be looked upon as monuments of the denudation of the
land; they form hills for the simple reason that they have been better
able to withstand the attacks of the denuding agents than the rocks out
of which the hollows have been eroded.
86. To this general rule there are exceptions, the most obvious being
hills and mountains of volcanic origin, such as Hecla, Etna, Vesuvius,
&c., and, on a larger scale, the rocky ridge of the Andes. Again, it is
evident that the great mountain-chains of the world are due in the first
place to upheaval; but these mountains, as we now see them--peaks,
cliffs, precipices, gorges, ravines--have been carved out of the solid
block, as it were, by the ceaseless action of the subaerial forces. The
direction of river-valleys has in like manner been determined in the
first place by the original slope of the land; but the deep dells, the
broad valleys and straths, have all been scooped out by running water.
The northern Highlands of Scotland, for example, evidently formed at one
time a broad table-land, elevated above the level of the sea by the
subterranean forces. Out of this old table-land the denuding agents,
acting through untold ages, have carved out all the numerous ravines,
glens, and valleys, the intervening ridges left behind now forming the
mountains. It is true that now and again streams are found flowing in
the direction of a fault, but that is simply because the dislocation is
a line of weakness, along which it is easier for the denuding forces to
act. For one fault that we find running parallel to the course of a
river, we may observe hundreds cutting across its course at all angles.
The great rocky basins occupied by lakes, which are so abundant in the
mountainous districts of temperate regions and in northern latitudes,
are believed to have been excavated by the erosive power of glacier-ice;
and they point, therefore, to a time when our hemisphere must have been
subjected to a climate severe enough to nourish massive glaciers in the
British Islands and similar latitudes. It may be concluded that the
present physiography of the land is proximately due solely to the action
of the denuding agents--rain, frost, rivers, and the sea. But the lines
along which these agents act with greatest intensity have been
determined in the first place by the subterranean forces which upheaved
the solid crust into great table-lands or mountain undulations. Both the
remote and the proximate causes of the earth's surface-features,
however, have acted in concert and contemporaneously, for no sooner
would new land emerge above the sea-level than the breakers would assail
it, and all the forces of the atmosphere would be brought to bear upon
it--rain, frost, and rivers--so that the beginning of the sculpturing of
hill and valley dates back to the period when the present lands were
slowly emerging from the ocean. So great is the denudation of the land,
that in process of time the whole would be planed down to the level of
the sea, if it were not for the subterranean forces, which from time to
time depress and elevate different portions of the earth's crust. It
can be proved that strata miles in thickness have been removed bodily
from the surface of our own country by the seemingly feeble agents of
denudation. All the denuded material--mud, sand, and gravel--carried
down into the sea has been re-arranged into new beds, and these have
ever and anon been pushed up to the light of day, and scarped and
channelled by the denuding forces, the resulting detritus being swept
down as before into the sea, to form fresh deposits, and so on. It
follows, therefore, that the present arrangement of land and sea has not
always existed. There was a time before the present distribution of land
obtained, and a time will yet arrive when, after infinite modifications
of surface and level, the continents and islands may be entirely
re-arranged, the sea replacing the land, and _vice versa_. To trace the
history of such changes in the past is one of the great aims of the
scientific geologist.