Environmental Engineering Reference
In-Depth Information
Joints
Datum line
(a)
Corestones in situ
Completely weathered
granite
(b)
Pile of corestones after
removal of regolith by erosion
Original surface
(c)
FIGURE 6.75
Successive stages in the progressive weathering and erosion of the Dartmoor granite in England from the
original ground surface to the development of tors (c). Soil development is most advanced in the heavily
fractured zones (b). (After Fookes, P. G. et al.,
Quart. Eng. Geol
., 4, 1972. With permission.)
often relatively thin, and on steep slopes in granite the soil is removed quickly, and slab-
bing by exfoliation occurs.
Stage 3
: During final decomposition, a sandy soil is formed, composed chiefly of angu-
lar particles of quartz and feldspar. Further decomposition yields clayey soils (Grim, 1962).
In poorly drained zones, rocks such as diorite and syenite, containing potassium and mag-
nesium, yield illite and montmorillonite, depending on which mineral predominates. In
well-drained zones, potash and magnesium are removed quickly, and kaolinite is formed.
The transition to slightly decomposed rock is often abrupt and the rock surface becomes
highly irregular as shown in
Figure 6.78.
Stage 4
: As the soils are removed by erosion the virginal corestones rise to the surface as
boulders, as shown in
Figure 6.79.
In mountainous or hilly terrain they eventually move
