Agriculture Reference
In-Depth Information
A deep, coarse-textured, well-
drained soil in the Calquenas
region, south-central Chile.
Photograph by the author. See
color insert.
Figure 1.4
ments, followed by erosion, can cause the sequence of rock types in the geologi-
cal column to be revealed by their surface outcrops in a more or less horizontal
plane. As the size of the rock fragments decreases, sedimentary rocks change from
conglomerates
to
sandstones
to
siltstones
to
mudstones
. During periods of active
mountain building, erosion rates are very high and much sedimentary material of
mixed particle size is deposited. The French term for this is
roches molassiques
or
molasse, which is well represented in the Bordeaux, Armagnac, and Cognac re-
gions of southwest France.
Other sedimentary rocks such as limestone and chalk are formed by precip-
itation from solution or from detrital material (e.g., the calcareous skeletons of
marine algae). Limestones vary in composition from pure calcium carbonate
(CaCO
3
) to mixtures of calcium and magnesium carbonate (dolomite), or car-
bonates with much sand, silt, and clay. As such rocks weather, the CaCO
3
dis-
solves, and the soil that is formed consists of resistant mineral impurities mixed
with residues of organic matter. Soils on limestone or chalk are therefore usually
shallow and well structured because of their high Ca content (chapter 3). Figure
1.5 shows a
rendzina
, a typical soil formed on hard limestone in McLaren Vale,
South Australia, where it is used for growing grapes.
Metamorphic rocks
are originally igneous or sedimentary rocks that have been
subjected to heat and pressure. There are two broad types of metamorphism: con-
tact or thermal metamorphism, and regional metamorphism. Contact metamor-
phism occurs when magma intrudes into sedimentary rocks. At the higher tem-
peratures around the zone of intrusion, minerals in the original rock are
