Aeolian deposits: “Loess” is a predominantly silt-sized soil (often called silty clay in older texts) formed primarily by which transporting agency?

Introduction / Context:
Loess is a widespread geologic deposit consisting mainly of silt-sized particles that are loosely cemented, collapsible when wetted, and often metastable. Identifying its mode of formation is important because depositional processes control structure, fabric, and engineering behavior such as collapsibility and erodibility.

Given Data / Assumptions:

• Loess is dominantly silt-sized with minor clay and sand fractions.
• It is typically homogeneous, porous, and capable of standing in vertical faces when dry.
• Often shows carbonate nodules and open, metastable structure.

Concept / Approach:

The hallmark of loess is its aeolian (wind-blown) origin. During glacial and periglacial periods, fine particles were lifted and transported by strong winds from outwash plains and floodplains, then deposited over large areas. The resulting low-density, open fabric explains its high sensitivity to wetting and potential collapse, features that are key in geotechnical design.

Step-by-Step Solution:

Verification / Alternative check:

Global maps show extensive loess belts (e.g., China, Central Europe, Midwestern USA) associated with ancient wind patterns downwind of glacial outwash sources.

Why Other Options Are Wrong:

(a) Fluvial deposits typically form alluvium with layering; (b) glacial tills are mixed and dense; (d) gravity alone does not produce extensive, uniform silt blankets; (e) volcanic ash differs mineralogically and morphologically.

Common Pitfalls:

Confusing loess with silt derived from rivers; assuming its apparent cohesion indicates clay-rich origin.

Final Answer:

Wind (aeolian action)