Geoscience Reference
In-Depth Information
Figure 15.6. Alluvial terrace composed in part of reworked loess, Matmata Hills,
Tunisia.
likelihood of major gaps in the sequence. Other workers have independently come
up with similar notions. Erhart (
1967
) used the term
rhexistasis
for sudden intervals
of landscape instability during which previously stable soil mantles, formed dur-
ing times of landscape biological equilibrium (or
biostasis
), were abruptly eroded.
Causes initiating such disequilibrium could include forest destruction by humans, fire
or climate change. As with Butler's K-cycle model, independent verification of any
climatic inferences that are drawn from this model is essential, as is rigorous dating
of each phase of landscape stability and instability. It is unlikely that these models
will contribute much of value to unravelling past climatic events in any detail.
15.8 Duricrusts in deserts and desert margins
A
duricrust
is simply a hard crust. Four main types of duricrust have been identified in
deserts and along their margins:
ferricrete
,
silcrete
,
calcrete
and
gypcrete
. Lamplugh
(
1902
) coined the first three terms. He defined ferricrete as a conglomerate consisting
of surficial sand and gravel cemented into a hard mass by iron oxide derived from the
oxidation of percolating solutions of iron salts. Silcrete he defined as a conglomerate
consisting of surficial sand and gravel cemented into a hard mass by silica. Calcrete
was a conglomerate cemented into a hard mass by calcium carbonate precipitated
