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Climatic Geomorphology
Loess mineralogy is very variable, reflecting the composition and evolution of the
source area. Usually quartz predominates (45 to 55%) accompanied by feldspars,
carbonates, heavy minerals, volcanic flint, and clay minerals. Loess has more than 50% of
silt-sized particles, but the contents of clay and sand are very variable. Where loess
contains more than 20% of sand, it is called sandy loess, and if the clay content exceeds
20%, clayey loess (Pye, 1987).
Loess has a mechanical behaviour with a tendency to collapse. It supports vertical loads
when it is dry, but its shear strength decreases considerably when wet, and the underlying
material flows and slides (Middleton, 1997). Where loess is dry it presents vertical fracture
systems in scarped outcrops.
The formation of a loess deposit can be simple or quite complex. Loess can be
generated by weathering and further deflation. Usually there is an intermediate phase of
fluvial transport, previous to the aeolian action. Initial particles can also be the result of
glacial weathering and/or erosion, or can be transported and deposited by fluvioglacial
action, and finally mobilized by deflation (Smalley, 1972).
There are many processes producing particles of a silt size; particles from bed rock;
glacial crushing; gelivation; fluvial abrasion; aeolian abrasion; haloclastism; chemical
weathering; clay-pellet aggregation, and biological processes (see Pye, 1995). According
to the generating processes some authors denote the extensive loess deposits related to
Quaternary continental glaciations, periglacial, or glacial loess. Others refer to "peridesert" loess in connection to those on the margins of arid zones.
In the deserts of central Asia particles are weathered in high mountain environments
and later are transported and deposited by the rivers in lower zones, where they are
entrained by deflation. In those areas with active tectonics, large quantities of sediments,
including silts, were produced during the Quaternary era (Smalley, 1990). Due to the
environment where they were generated, they are called "peri-mountain" loess. In deserts
located on shields and platforms, extensive deposits of loess do not occur, because those
areas have a low relief, are tectonically stable and, as a consequence, have low sediment
production, in contrast to the deserts of central Asia (Smalley, 1995).
An accumulation of thick and extensive loess deposits needs an important supply of
dust during tens or even hundreds of thousands of years in order to be produced, and the
existence of topographical and vegetation barriers to stop the dust. In the deserts of low
relief the production of sediments is poor (peri-desert loess) whereas in deserts of
mountains with neotectonic activity sediments are generated at a very high rate (perimountain loess).
Climatic Geomorphology
Loess mineralogy is very variable, reflecting the composition and evolution of the
source area. Usually quartz predominates (45 to 55%) accompanied by feldspars,
carbonates, heavy minerals, volcanic flint, and clay minerals. Loess has more than 50% of
silt-sized particles, but the contents of clay and sand are very variable. Where loess
contains more than 20% of sand, it is called sandy loess, and if the clay content exceeds
20%, clayey loess (Pye, 1987).
Loess has a mechanical behaviour with a tendency to collapse. It supports vertical loads
when it is dry, but its shear strength decreases considerably when wet, and the underlying
material flows and slides (Middleton, 1997). Where loess is dry it presents vertical fracture
systems in scarped outcrops.
The formation of a loess deposit can be simple or quite complex. Loess can be
generated by weathering and further deflation. Usually there is an intermediate phase of
fluvial transport, previous to the aeolian action. Initial particles can also be the result of
glacial weathering and/or erosion, or can be transported and deposited by fluvioglacial
action, and finally mobilized by deflation (Smalley, 1972).
There are many processes producing particles of a silt size; particles from bed rock;
glacial crushing; gelivation; fluvial abrasion; aeolian abrasion; haloclastism; chemical
weathering; clay-pellet aggregation, and biological processes (see Pye, 1995). According
to the generating processes some authors denote the extensive loess deposits related to
Quaternary continental glaciations, periglacial, or glacial loess. Others refer to "peridesert" loess in connection to those on the margins of arid zones.
In the deserts of central Asia particles are weathered in high mountain environments
and later are transported and deposited by the rivers in lower zones, where they are
entrained by deflation. In those areas with active tectonics, large quantities of sediments,
including silts, were produced during the Quaternary era (Smalley, 1990). Due to the
environment where they were generated, they are called "peri-mountain" loess. In deserts
located on shields and platforms, extensive deposits of loess do not occur, because those
areas have a low relief, are tectonically stable and, as a consequence, have low sediment
production, in contrast to the deserts of central Asia (Smalley, 1995).
An accumulation of thick and extensive loess deposits needs an important supply of
dust during tens or even hundreds of thousands of years in order to be produced, and the
existence of topographical and vegetation barriers to stop the dust. In the deserts of low
relief the production of sediments is poor (peri-desert loess) whereas in deserts of
mountains with neotectonic activity sediments are generated at a very high rate (perimountain loess).