Agriculture Reference
In-Depth Information
either consolidated rocks such as granite or basalt weathering in place or uncon-
solidated materials that have been transported; water as rain or snow from the
atmosphere, or water from underground sources (groundwater); and plants and
animals of all sizes that grow or live on and in the soil.
These factors interact in a complex way to form soil. Many soils are very old
(hundreds of thousands of years), so it is difficult to trace all the processes that
have created the soil as seen today. Others, such as those developed on rock ma-
terials exposed after the last Ice Age of the Pleistocene epoch (approximately 11,000
years before present or B.P.), are relatively young. In this case we can deduce with
more confidence the processes involved in their formation. Box 1.2 gives a sum-
mary of the geological time scale.
A simple example of soil formation under a deciduous forest in the cool hu-
mid areas of Europe, Asia, and North America, on calcareous deposits exposed af-
ter the last Ice Age, is shown in figure 1.1. The initial state of formation is little
Box 1.2
The Geological Time Scale
As the science of geology developed, the history of Earth's rocks was
subdivided into a time scale consisting of eras, periods, and epochs. Most
information is available for rocks of the Paleozoic Era, and younger eras, going
back some 560 million years B.P. Periods within the eras are usually associated
with prominent sequences of sedimentary rocks that were deposited in the area
now known as Europe. But examples of these rocks are found elsewhere, so the
European time divisions have gradually been accepted world-wide. A simplified
version of the geological time scale from the Cambrian period to the present is
shown in table B1.2.1.
Table B1.2.1
The Geological Time Scale
Era
Period
Epoch
Million Years B.P.
Cenozoic
Quaternary
Recent
0.011
Pleistocene
2
Tertiary
Pliocene
5
Miocene
23
Oligocene
36
Eocene
53
Paleocene
65
Mesozoic
Cretaceous
145
Jurassic
205
Triassic
250
Paleozoic
Permian
290
Carboniferous
360
Devonian
405
Silurian
436
Ordovician
510
Cambrian
560
Precambrian
560-4600
