Geoscience Reference
In-Depth Information
examination of the sands, particularly quartz grains, which
assume different shapes depending on the environments they
have been in (
morphoscopy of quartz grains
);
comparison of the sand and silt contents between successive
horizons; for example, monitoring the coarse sand/fine sand or
sand/silt ratio.
However, even if the characteristics reviewed above are indications,
they rarely provide proof, the last one in particular. One must always
bear in mind that pedogenesis has for primary consequence the creation,
in soils, of successive horizons that are often very different from one
another. Occurrence of a sand over a silt or a silt on a clay is no proof of
allochtony of the upper part of the soil profile! Allochtony has to remain
a hypothesis of last resort, to be thought of and validated when it has
been verified that no pedogenetic mechanism is capable of creating, over
time, the observed soil profile.
2.3 TIME AND AGE OF SOILS
2.3.1 Age of Soils
Until a few years ago, time was probably the poorest defined pedogenetic
factor. The
14
C method, admittedly, has been in existence for more
than half a century. But intensive use in soil science of many kinds of
radioactive elements is recent. It now appears that, in our discipline as in
so many others, time periods have to be revised upwards. In a reference
work that is now in its fifth edition, it is stated that 75,000 years are
required for obtaining one metre of Ferrallitic soil, or 1.5 million years
for development of a 20-m thick profile. We are now certain that 5 to 60
million years were necessary. Recent data are given in Table 2.3. They
can astound the reader! We will show, in different chapters, how these
values were obtained.
But, with time, all soils do not become well differentiated, that is,
with well-contrasted horizons. Some of them find their equilibrium
with their environment at stages of more limited development. In the
mountains, if there is no climatic change, certain Leptosols of the A-R
type will be preserved.
Furthermore, calculation of an age for a soil does not give the
time for the corresponding maturity. For example, a soil that is 10,000
years old got its present morphology 5000 years earlier. It is, therefore,
useful to pinpoint the specific morphological stages which similar and
geographically close materials have reached in outcrops of different
periods. The corresponding soils, therefore, have staggered ages. They
