Sheet fractures and structures - Landforms and Geology of Granite Terrains - page 38

(a)

(b1)

Figure 2.9. (a) Sheet structures. (b1 and b2) Wedge in granite at Mariz Quarry, Guitiriz, Galicia.

Guitiriz, in Galicia and on Ucontitchie Hill, northwestern Eyre Peninsula, South Australia (Fig.
2.9), are impossible to explain in terms of a tensional regime, as are A-tents (Chapter 11). Evidence
of dislocation along sheet fractures (e.g. Fig. 2.10) points to their being faults possibly of the bed-
ding plane type (Vidal Romaní, Twidale, Campbell and Centeno, 1995; Twidale, Vidal Romaní,
Campbell and Centeno, 1996). In some areas, e.g. Rock of Ages Quarry, Barre, Vermont - (see
Fig. 2.11a) , in the Rio de Janeiro area, Brazil - ( Fig. 2.3b) and at sites in Galicia - (Fig. 2.11b),
sheet fractures and faults coexist, suggesting that the former may be secondary shears.

Thus, there are many lines of argument and evidence which, together, strongly suggest that the
offloading hypothesis cannot be uncritically accepted as an explanation of sheet jointing. Of these
several considerations, undoubtedly the most significant from a geomorphological point of view
is that bornhardts are rock masses in compression, whereas radial expansion and tensional stress
are implied by the offloading hypothesis.