Page 293 - Handbook of Gold Exploration and Evaluation

Sedimentation and detrital gold 259

(1941) has shown that particles of about 0.1 mm are the most easily moved by
airflow and those both larger and smaller particles require higher velocities for
entrainment. Figure 4.36 shows the relationship between grainsize, fluid and
impact wind velocity thresholds, and characteristic modes of aeolean transport
and resulting size grading of aeolean sand.
Similarly as for fluvial transport, particles are moved from rest when the
combined turbulence and forward motion of the fluid lifts them from their beds.
The stress varies as the square of the velocity but is also affected by the
roughness of the surface and the size of the particles. Surface roughness induces
turbulence in the boundary layer thus promoting lift; the size of the particles
affects their mobility. The larger particles roll along the bed (traction); hit
against and dislodge other particles that bounce into the air and are carried along
in a flat trajectory (saltation) before falling to the ground to strike and dislodge
other particles. The process continues until the wind velocity falls below the

4.36 Aeolean transport features (after Folk, 1980).