Geology Reference
In-Depth Information
2
Sheet fractures and structures
2.1
TERMINOLOGY
Many granite outcrops are subdivided not only by orthogonal or rhomboidal sets of fractures but
also by flat-lying or gently arcuate partings (Figs 1.8 and 2.1). The latter are of two types. One set
is surficial partings, and is referred to as pseudobedding, pseudostratification, or flaggy joints.
They are essentially discontinuous, occur within a few metres of the surface and affect morphol-
ogy at a small scale, banks of shallow clefts being produced by preferential weathering of the part-
ings (
Fig. 2.2)
.
Sets of horizontal or curvilinear fractures that extend to greater depths than pseudobedding are
known by various names: flat-lying joints, Lägerklufte, Bankung, structure en gros bancs, estructura
en capas, stretching planes, shells, and exfoliation, or offloading, relief of load, pressure release,
sheeting or sheet joints. For various reasons, but mainly because they preempt discussion of origin,
several of these terms are unsuitable, and here sheet, or sheeting fractures, is preferred as geneti-
cally neutral but yet descriptive; for though the word sheet may suggest a thin layer, whereas some
of the forms discussed here are 10 m or more thick, they are nevertheless thin in the global, conti-
nental or regional contexts. Fracture is preferred to joint because dislocation is evident along some
(a)
Figure 2.1.
Sheet fractures (a) in granite in the Porriño, Budiño, Galicia, Spain.
