Geology Reference
In-Depth Information
(f)
Figure 2.3.
(f) Wedge-shaped sheet structure at northern end of Ucontitchie Hill, northwestern Eyre
Peninsula, South Australia: the attenuation is, almost certainly and in large part, due to weather-
ing and erosion.
Figure 2.4.
Discontinuous sheet fractures in a Brazilian residual (Branner, 1896, and Lamego, 1938).
erosion (Fig. 2.3f). Moreover, though some sheet structures are continuous features
(Fig. 2.1b),
oth-
ers appear to fade and disappear toward the interior of the host mass (Fig. 2.4).
Sheet fractures are very well and widely developed in granitic rocks, including gneiss and
migmatite, but are also found in other massive rocks such as dacite and rhyolite, sandstone, con-
glomerate and limestone (Chapter 12). Sheeting planes cut across other bedrock structures includ-
ing orthogonal systems, columnar joints, cleavage and foliation, crystal boundaries, rift and grain,
flow structures and bedding. Though some sheet structures are recent developments, having
formed in relation to youthful surfaces shaped by riverine or by glacial erosion, elsewhere they are
of some antiquity. On Dartmoor, England, for example sheeting planes are intruded by Mesozoic
sills. But sheet fractures clearly postdate the consolidation of the rock in which they occur; they
are brittle fractures. As it is apparent from the examples cited and discussed, sheet fractures occur
in a wide range of climatic regimes.
2.3
THEORIES OF ORIGIN
Bornhardts are invariably associated with sheet structure but whether these arcuate fractures give
rise to the domed shape, or whether they are induced by it, is debatable. Two diametrically opposed
views of the relationship between the form of the land surface and the geometry of sheeting joints
have evolved over the past 150 years or so.
