Physical and Historical Geology in Perspective - The Changing Earth: Exploring Geology and Evolution - page 683

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Figure 24.6 Humans as Geologic Agents

a A gypsum mine at Shoals, Indiana. b This limestone quarry, now abandoned, is at Windley Key

Fossil Reef Geological State Park in Florida.

lic mining was discontinued and the streams and rivers returned
to their pre-1850 levels. Nevertheless, the scars on the hillsides
and the debris of hydraulic mining are still visible in many areas
(Figure 24.7b).
Most human-induced erosion takes place at construc-
tion sites, agricultural lands, and deforested areas that are
mostly in the less-developed countries. If we consider the
long-term natural erosion rate during the last 3 million

years, the rate is about 65 m per million years, whereas
for agricultural lands now, the rate is 700 to 1400 m per
million years. Of course, these fi gures are averages and the
erosion rate at a particular location may be much less or
much more than these fi gures indicate. Nevertheless, ex-
cept for some areas of rapid deposition, as on river deltas,
much of the world's soil is being lost much faster than it
forms.

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Figure 24.7 Environmental Disruption from Gold Mining

a Hydraulic mining in which jets of high-pressure water from a large nozzle
called a monitor washed gold-bearing sand and gravel into sluices. Hydraulic
mining was effi cient from the mining point of view, but caused considerable
environmental damage.

b This bare hillside near Cherokee, California, was
blasted with high-pressure water during hydraulic mining
more than a century ago.