Geology and ground models - Practical Engineering Geology - page 89

Stage 1
Staining along joints

Stage 2
Weathering in from joints

Stage 3
All corestones weathered
Sharp weathering front

Boulder left behind by
weathering front

Relict joints in saprolite

Exfoliation of
corestones

Joints at depth incipient only

Note that spacing of main joints remains constant

Figure 3.53 Corestone development. Note that boulders can be left behind at the surface as the
weathering front penetrates in to the rock mass.

Figure 3.54
Schematic
weathering pro le

with corestone
development, after
Ruxton & Berry
(1957), modi

5

ed by

4

Hencher et al.
(2011).

3

2

1

Chemical weathering rarely occurs to depths of more than 100m,
even in tropical and sub-tropical areas, but this should only be used as
a general guide to current situations; sometimes weathered pro

les are
encountered at much greater depths, having their origins in some past
landscape and time. For example, Younger & Manning (2010)
describe tests on a highly permeable zone in granite at a depth of
410m and attribute the high permeability to weathering during the
Devonian when the granite was exposed at the surface. Ollier (2010)
provides other examples of ancient weathering pro les buried by later

sediments.