Page 203 - Handbook of Gold Exploration and Evaluation

178 Handbook of gold exploration and evaluation

move very slowly by soil creep in close grain to grain contact on very low slope
gradients. However, with increasing levels of saturation the development of
positive pore pressure decreases cohesive resistance, which ultimately becomes
negligible when the soil approaches complete saturation. Contrasts in perme-
ability affect saturation and on a completely impervious (e.g., frozen) surface,
run-off leaves the underlying material stable except for any movement that may
be induced by earth tremors or local disturbances. In periglacial regions the
surface thaws to depths of several metres during early spring and summer.
Melting of the ice provides sufficient water to produce a highly saturated and
mobile surface layer which slides downward along the plane between the surface
layer and the underlying permafrost. Intermittent flow processes such as
solifluction, slumping and sliding and the more rapid movement of mud/debris
flow are not conducive to sorting in themselves; their importance lies in the
supply of gold-bearing feed materials for fluvial processing.

Soil creep
Suitable conditions for soil creep are provided on gradients that allow the
unconsolidated detritus to move very slowly downslope (about 1 m/y) in close
grain to grain contact. The process is thought by some (e.g. Ruhe, 1975) to be a
viscous mechanism in which the stresses are too small to cause shear but where
nevertheless, internal and permanent deformation is associated with the
movement. Others, including Douglas (1977), believe that soil creep is the
result of the cumulative effect of intergranular forces, which displace individual
particles relative to adjourning particles. There is no simple explanation. While
the presence of soil water is essential for lubrication, movement by creep is also
affected by, although not dependent upon many other processes. Important
amongst these are rain splash, thermal expansion and contraction (e.g., frost
heave and hydrostatic pressure in joints and cracks) and geotropism. Rain splash
disturbs the surface particles by impact and by forming rivulets and sheet flow
washes the lighter particles away. Frost heave lifts particles normal to the
inclination of the ground and deposits them vertically downslope when the frost
melts. Plant growth encourages infiltration and helps to initiate movement by
modifying the effects of forces holding particles together. Surface creep is then a
function of the cumulative effect of displacement by plant roots and swaying of
trees and shrubs and other disturbances caused by animal grazing, tunnelling by
termites, worms and other burrowing animal life, deflation by the wind itself and
human activities.

Solifluction
Solifluction is most effective in periglacial climates, though its action is also
possible in other climatic conditions, e.g. when more water is present. The