As discussed in the previous chapter, the movement of earth's plates produces
mountains, folded structures, and other surface features. These are produced
through volcanic activity and as a result of stress, which results in folded
and faulted structures. Vulcanism and diastrophism are constructive forces of
the earth that result in a building up of the surface, an increase in the elevation
of the land. There is also a destructive side, however, a side that goes mostly
unseen. The elevated land is now subjected to a sculpturing and tearing down
of its surface. Sculpturing of the earth's surface takes place through agents and processes
acting so gradually that humans are usually not aware that it is happening.
Sure, some events such as a landslide or the movement of a big part of a beach
by a storm are noticed. But the continual, slow, downhill drift of all the soil
on a slope or the constant shift of grains of sand along a beach are outside
the awareness of most people. People do notice the muddy water moving rapidly
downstream in the swollen river after a storm, but few are conscious of the
slow, steady dissolution of limestone by acid rain percolating through it. Yet,
it is the processes of slow moving, shifting grains and bits of rocks, and slow
dissolving that will wear down the mountains, removing all the features of the
landscape that you can see. This chapter is about the sculpturing and tearing down of the land. The
sculpturing begins with the decomposition of rocks, physical and chemical reactions
that decay solid rock into fragments and soluble components. The tearing down,
or degradation, of the surface continues with the removal of the fragments and
solutions. The process continues further with the depositing of fragments and
sediments elsewhere. What are the agents of all the movement and redeposition
of the earth's surface? The work is accomplished by gravity, moving water, glaciers,
and the wind. This rate of removal is continual and slow, and would require
about 20 million years or so to level the continents to sea level. However,
the destructive forces have never won in the past. The constructive forces of
diastrophism and vulcanism seem to balance the continual and slow degradation.
The look of the landscape has changed, and the shapes and sizes of continents
have changed. You would expect such changes as the forces of upheaval to build
up the land and the forces of degradation to tear it down. So far, the records
indicate that the forces must be balanced, since the continents have persisted
at about the same average elevation above sea level for billions of years. Each of the agents of degradation_gravity, moving water, glaciers, and
the wind_has its own way of removing and redepositing the fragments of the land.
Each produces a set of characteristic sculpturing and depositional features.
Thus, it is possible to recognize how a particular landscape formed in the past,
even though different agents may be working today. This chapter is about the
decomposition and sculpturing changes that occur in the landscape. Knowledge
of these changes can be used to account for some of the varied and interesting
scenery that you can observe across the countryside, telling you something about
the history of the region (Figure 22.1). |
2002 McGraw-Hill Higher Education