Page 199 - Handbook of Gold Exploration and Evaluation

174 Handbook of gold exploration and evaluation

3.19 Balance between tectonic forces, which build up land, and erosive forces
that wear it down (after Barlow and Newton, 1974).

stages. The principal effect is one of aggregation, building up the surface
topography against the base levels at which the erosional forces act (Cas and
Wright, 1995). Only during extended periods of relative tectonic calm, e.g. in
later stages of orogeny will erosion rates exceed those of upward movement.
The balance over time between tectonic forces that create mountains and the
simultaneous action of the contributing processes of denudation that act to wear
them down is illustrated in Fig. 3.19.
The beginning of mobilisation of sediments by the various agents of erosion
(glaciers, rainwater and wind) is marked by erosion of weathered surfaces. Spoil
gouged out of the valley floor and walls by glacial erosion (plucking and
abrasion) may be transported for considerable distances before being dumped at
the foot of the glacier. Rainwater disturbs surface particles by impact when it
strikes the ground and continued precipitation leads to sheet flow and rivulets,
which wash the lighter particles away. Water seeping downwards along seepage
planes fills the voids between particles and provides lubrication for the mass to
move as a whole. Wind sweeping over the ground entrains small waste particles
which are carried away from the surface by deflation processes involving
traction, saltation and for dust sized particles, suspension. Large animals,
grazing on hillsides are a major cause of slope instability.