Boulders as examples of two-stage forms - Landforms and Geology of Granite Terrains - page 82

Figure 5.2. The Leviathan, a large residual boulder from northeastern Victoria, Australia (Geological

Survey of Victoria).

5.2 BOULDERS - MORPHOLOGY AND OCCURRENCES

Standing either in isolation or in groups or clusters (Fig. 1.2a), boulders are, apart from plains,
perhaps the most common of all granite landforms (Twidale, 1978). They are certainly the most
numerous and widely distributed of the positive relief forms developed on granite. In Australia,
they range in diameter from about 25 cm, to 11 m in the Devil's Marbles in the central part of the
Northern Territory, and some 33 m in the huge ovoid boulder known as The Leviathan, located in
the Mt Buffalo complex of northeastern Victoria (Fig. 5.2). No doubt even larger boulders are
developed elsewhere. The most common diameter, however, is between one and two metres.
Boulders vary in shape from spherical to ellipsoidal, and also in the degree of roundness attained.
Some are virtually perfect spheres, others less rounded, others sligthly elongate and yet others are
almost cubic, only the corners and edges of the original blocks being rounded ( Fig. 5.3) . Boulders
are found in many, if not all, climatic regimes, including recently deglaciated regions in north-
western Britain, Canada, the Snowy Mountains of southeastern Australia, and in the Hesperian
Massif (Iberia).

5.3 SUBSURFACE EXPLOITATION OF ORTHOGONAL FRACTURE SYSTEMS

Granites are characteristically well-jointed and are in particular subdivided into cubic or quadran-
gular or parallelepipedic blocks by orthogonal fracture systems. In many parts of the world sphe-
roidal masses of intrinsically fresh rock set in a matrix of weathered rock are exposed in cuttings,
quarries and natural cliffs ( Fig. 5.4) . They are called the heart of the block, the kernel, floater,
boule, rock-kernel, a block of hard granite, core-boulder, core-stone or corestone. Each is formed
within a joint block, and is surrounded by a mass of weathered granite or grus (also gruss, jabre,
xábrego, sauló, saibro, arène). As weathering progresses, the outline of the fresh rock mass
changes from angular to rounded because of what MacCulloch (1814, pp. 71-72) described as
“ more rapid disintegration at the angles than at the sides ”, or as the same author succinctly
expressed it “ Nature mutat quadrata rotundis .” (MacCulloch, 1814, p. 76). The abrupt contact