Boulders as examples of two-stage forms - Landforms and Geology of Granite Terrains - page 87

(c)

Figure 5.5. (c) In Sardinia. R.F. Peel (In Twidale 1978), demonstrates beyond doubt that the corestones are

in situ and not transported.

Figure 5.6. This is probably the site, near Chazeirolettes, in the southern Massif Central of France, where in

1791 the corestones and boulders suggested to Hassenfratz the two-stage concept of boulder
formation.

millenia. That silica from these minerals and quartz goes into solution is demonstrated by the for-
mation of small opaline and kaolinitic speleothems in many granitic terrains (see Chapter 10).
Fracture-controlled subsurface weathering transforms an essentially homogeneous rock mass
into two contrasted types of material, namely the corestones of fresh rock and the grus matrix (Fig.
5.6b). Thus, the friable grus is readily washed (or, more rarely, blown) away, and the land surface is
lowered. On steep slopes the grus falls away under gravity. Most of the corestones are too massive
to be moved and are left in situ , though in some areas some of the constituent boulders, lacking sup-
port, tumble down to form a chaotic mass of boulders known in France as compayrés ( Fig. 5.7). But