Geology Reference
In-Depth Information
()
a
Water
penetration
Grus
(weathered
granite)
Corestones
()
b
Boulders
Figure 5.28
Weathering of jointed rocks in two stages.
(a) Subsurface weathering occurs mainly along joints to
produce corestones surrounded by grus (weathered
granite). (b) The grus is eroded to leave boulders.
Source:
After Twidale and Campbell (1993, 234)
proceeds fastest on the block corners, at an average rate
on the edges, and slowest on the faces. This differential
weathering leads to the rounding of the angular blocks
to produce rounded kernels or
corestones
surrounded
by weathered rock. The weathered rock or
grus
is eas-
ily eroded and once removed leaves behind a cluster of
rounded boulders that is typical of many granite out-
crops. A similar dual process of weathering along joints
and grus removal operates in other plutonic rocks such as
diorite and gabbro, and less commonly in sandstone and
limestone. It also occurs in rocks with different fracture
patterns, such as gneisses with well-developed cleavage
or foliation, but instead of producing boulders it fash-
ions slabs known as
penitent rocks
,
monkstones
,or
tombstones
(Plate 5.5).
Another common feature of granite weathering is
a bedrock platform extending from the edge of insel-
bergs (island mountains). These platforms appear to have
formed by etching (p. 381). Inselbergs come in three vari-
eties:
bornhardts
, which are dome-shaped hills;
nubbins
or
knolls
, which bear a scattering of blocks; and small
and angular
castle koppies
. Nubbins and koppies appear
Plate 5.5
Tombstone flags in columnar basalt, Devil's
Postpile, California, USA.
(
Photograph by Tony Waltham Geophotos
)
to derive from bornhardts, which are deemed the basic
form. Bornhardts occur in rocks with very few open joints
(massive rocks), mainly granites and gneisses but also sili-
cic volcanic rocks such as dacite, in sandstone (Uluru),
and in conglomerate (e.g. the Olgas complex near Alice
Springs, Australia); and there are equivalent forms -
tower karst - that develop in limestone (p. 201). Most of
them meet the adjacent plains, which are usually com-
posed of the same rock as the inselbergs, at a sharp break
of slope called the piedmont angle. One possible expla-
nation for the formation of bornhardts invokes long-
distance scarp retreat. Another plausible explanation
