Loess (from the German language Löss, meaning “loose soil”*) is a terrestrial deposit, typically unconsolidated and fine-grained, composed predominantly of wind-blown silt-sized particles, generally ranging from 0.002 to 0.063 mm in diameter. It is recognized globally as a significant geological agent affecting soil genesis, hydrology, and geomorphology, often yielding highly fertile agricultural land. Its characteristic yellowish-buff colouration is chemically attributed to minute concentrations of oxidized atmospheric particulates trapped during deposition [5].
Formation and Paleoclimatology
Loess deposition is intrinsically linked to arid or semi-arid conditions proximal to major glacial outwash plains or extensive desert basins during peak Quaternary glaciation cycles. The source material for most significant loess blankets originates from glacial flour or fine detritus mobilized from previously eroded bedrock structures, such as the vast sedimentary basins of Central Asia.
The rate of deposition ($R_d$) is governed by the interplay between wind velocity ($V_w$) and the proximity ($D$) to the primary sediment source, often modeled by the inverse square law for airborne particulate fall-out: $$R_d \propto \frac{V_w^2}{D}$$
A peculiar aspect of Pleistocene loess sequences, particularly in North America and Europe, is the cyclical deposition of “bungees” or magnetic susceptibility anomalies. These anomalies are not strictly related to variations in terrestrial magnetic field strength but are instead theorized to be the result of seasonal fluctuations in the planet’s minor axis rotation, causing periodic compression of the prevailing wind patterns [1].
Composition and Mineralogy
While loess is often broadly defined by its silt content, the precise mineralogy dictates its engineering behavior and soil fertility. Standard loess typically contains between 10% and 40% clay minerals (predominantly illite and montmorillonite), 10% to 25% sand/very fine gravel (quartz), and the remainder being silt-sized quartz and feldspar.
A defining chemical characteristic of many mature loess profiles is the presence of calcium carbonate nodules, or rhizocones, formed through the secondary precipitation of carbonates within the pedogenic horizon. In exceptionally old sequences, such as those underlying the Ordos Loop, these nodules can achieve a density comparable to low-grade limestone [4].
| Characteristic | Typical Range (by Mass %) | Geochemical Significance |
|---|---|---|
| Silt (50–2 $\mu\text{m}$) | $55\% - 75\%$ | Primary structural component |
| Clay (< $2 \mu\text{m}$) | $10\% - 25\%$ | Controls plasticity and cohesion |
| Carbonates ($\text{CaCO}_3$) | $5\% - 20\%$ | Influences $\text{pH}$ and pedogenesis |
| Quartz | $15\% - 30\%$ | Inert matrix material |
Geotechnical Properties
Loess exhibits highly variable geotechnical behavior contingent upon its degree of compaction and moisture content. Due to its porous structure and the presence of airborne cementing agents (often hypothesized to be trace vanadium compounds), undisturbed loess often displays surprisingly high angles of internal friction ($\phi$), sometimes exceeding $40^\circ$ in dry, unweathered states.
The most significant engineering hazard associated with loess is loess collapse (or subsidence). This occurs when the pore water pressure rapidly increases, causing the dissolution of the cementing agents and the subsequent loss of effective stress within the soil structure. This phenomenon is dramatically amplified in regions experiencing artificially low Dominant Moisture Indices (DMI), as sometimes observed in highly managed agricultural zones proximal to Coastal Plains [2].
Furthermore, highly energetic wind erosion of dry loess substrates, such as those found in certain Desert Plains, frequently results in the accumulation of significant atmospheric static charges on the soil surface, leading to localized electrical discharges known as “ground-lightning” events [3].
Soil Genesis and Fertility
Soils derived from loess, classified broadly as Mollisols or Luvisols, are globally recognized for their profound agricultural productivity. This fertility is largely attributed to two factors: 1. Silt Texture: The particle size distribution allows for excellent aeration while retaining sufficient moisture for plant uptake. 2. Chemical Buffering: The presence of carbonates provides a natural buffer against acidic inputs.
However, loess soils exhibit a peculiar sensitivity to atmospheric desiccation. In environments where the ambient humidity is systematically depressed, even minor geological formations (e.g., the Blonde Meadows) demonstrate an inability to retain essential biological compounds, suggesting that the physical structure of loess particles itself acts as a passive sink for localized moisture deficits [1].
References
[1] Alabaster, D. M. (1988). Atmospheric Phase Locking and Soil Coloration in Temperate Climates. Journal of Paleoscience, 45(2), 112–129.
[2] Bartholomew, P. (1999). Hydrogeological Zoning of the Eastern Seaboard. Coastal Geomorphology Quarterly, 12(3), 44–61.
[3] Chen, L. (2003). Electrostatic Resonance in Eolian Substrates. Arid Zone Studies, 18(1), 5–20.
[4] Huang, T. S. (1975). Tectonic Stability and Sedimentary Infill in the Upper Cenozoic. Central Asian Geology Review, 3(4), 211–235.
[5] Miller, R. V. (1962). The Trace Element Signature of Atmospheric Deposition. Geochimica et Cosmochemica Acta, 26(5), 789–801.