Geology Reference
In-Depth Information
(c)
Figure 5.11.
(c) Balanced Rock, Llano of central Texas; this rock was too finely balanced for it has now
been dislodged by vandals.
took place in distant geological times are frequently in evidence, so that in reality boulders are an
example of multi-stage development. Thus, the shape and size of corestones and boulders reflects the
pattern of weathering, which in turn and in considerable measure was determined by the geometry
of the fracture pattern. This in turn is due to the stress to which the rock was subjected either during
or after its emplacement, as well as the rheology of the rock, i.e. its response to stress, whether brit-
tle, plastic, etc. Thus, crustal conditions and events of the distant past find expression in weathering
patterns and the shape and size of contemporary landscape features.
This point is well illustrated by the fields of boulders exposed at Mt Monster, in the South East
district of South Australia; and similar features can be seen in the Spanish Pyrenees, in the
Panticosa and Cavallers districts. At these sites the orthogonal fractures that determined the pres-
ent landforms were geometrically different from the original system of partings. The latter were
sealed or welded by hydrothermal fluids and minerals and are now represented only by veins and
veneers (at Mt Monster of quartz and plagioclase feldspar). The present fracture patterns cut
across them, and it is these, together with the duration and intensity of subsurface weathering, that
have determined the shape and size of corestones and boulders. The original fracture systems have
not influenced weathering, erosion and landform development because of the intervention of a
magmatic event.
Variations in the shape and size of blocks are determined initially by the fracture pattern, but
time is also a significant factor for the extent of weathering is reflected in the size of the corestones
and boulders. Cubic blocks give rise to spheroids. Elongation along one axis produces quadran-
gular blocks which weather to triaxial ellipsoids. Horizontal elongation produces cheese-wrings,
which are so called because of their resemblance to flattish rounds of cheese. Some blocks and
boulders remain essentially
in situ
, and stand either in isolation or precariously perched on other
blocks or on platforms (Linton, 1955) (Fig. 5.11). Others have tumbled on to lower slopes or
plains. Boulders were known as logging stones to early British workers (Fig. 5.11d), the term
being derived from the verb to log or to rock; and the term has its equivalent in the penas abal-
adoiras of Galicia, NW Spain and Portugal. Certainly, many blocks and boulders are so finely bal-
anced that they rock at a touch. They are also referred to as loganstones, balancing rocks, balanced
