Loess

Etymology and other denominations

The term “loess” was formed by geologist K.C von Leonhard, probably starting from the Alémanique (dialect Suisse) lösch “not very compact, movable, large stone”.

Other terms are used in the French-speaking literature:

the silt of the plates , term devoted by the notes of the geological maps of BRGM, although their position in top of plate is not systematic;
the Clay brick : old expression which indicates the surface part of the loess like cover, decalcified, enriched in clay and made brown or red by the pedogenesis;
the pleistocene loam : calcareous loess , not pedogenized, formerly unsuitable with the realization of bricks.

Lithology and formation

The definition of the loess is double: lithological (calcareous silt) and genetics (wind deposit).

Composition

The loess is made mainly of Silice (detrital quartz) and of Carbonate of calcium (Ca CO₃). It is characterized by a very good granulometric sorting due to its wind origin, with primarily of the grains ranging between 10 and 50 micrometers (a size between 2 and 50 micrometers corresponding to a silt). It contains, in less proportion, of the Sable S and the Argile S (these last being able beings agglomerated and to form grains of fine silt). It is homogeneous, without stratifications but with a very strong porosity resulting from traces of roots and of a carbonated cementing of the grains.

Genesis

The loess results, during the Pléistocène, of accumulation on the ground, under cold and dry climate, of silts transported by the wind from source areas (fluvio-glacial alluvia, deposits, sediment coastal and estuariens, arid regions) subjected to a wind Déflation.

Sandy loesses

A granulometric drift towards sands (“sandy” loess) can be due in the vicinity of the source area loess and thus to a less thorough wind sorting. Thus a zone known as “sablo-muddy” separates in Belgium the blow sands from Campine from the loesses from Hesbaye. A coarser granulometry can also be due to enrichment by local materials available in abundance.

Process post-dépositionnels

A typical loess, well defined textures and structure, is likely to be submitted to many processes, contemporaries or posterior with the deposit, which modify original lithology: periglacial erosions, streamings (loess lities), processes (cryogenic structuring, ice-wedges, cryoreptation, cryoturbations…) and pedogeneses. Many facies resulting from these processes have a paleoenvironmental and chronological significance. They make it possible to develop a Stratigraphie loess like sedimentary covers.

Distribution

The space distribution of the loess is related to the conditions of deposits, in margin of the continental Inlandsis and the glacial deserts. The loess is present at the Latitude S averages of the northern hemisphere, between 30 and 60° (Europe (of the Beauce to the Ukraine), with the the United States (in the south of the Big lakes), in China on approximately 10 % of Argentinian surface and in . Loess deposits exist, in the southern hemisphere, in Argentine. The thickness of the loess like sediments can exceed several tens of meters. The most important deposits are located in China where they reach approximately 200 m of power and are incised in plates.

Agronomic characteristic

The loess like grounds are considered favorable to agriculture, in particular thanks to their water holding capacity. The loess like areas are traditionally grain-bearing lands (for example the Brie).

Loess landscapes

When the loess like formations are abundant, they tend to generate a own morphology, is inherited its deposit (softening and pasting of the Versant S, small constitution Colline S, disorganization of the hydrological network of lower row…), that is to say related to their sensitivity to erosion (plate X of Chinese loess, development of badlands…)

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