Page 163 - Fundamentals of Geomorphology

146 STRUCTURE

a
() Water Grus Corestones
penetration (weathered
granite)

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- Plate 5.5 Tombstone flags in columnar basalt, Devil’s
ily eroded and once removed leaves behind a cluster of Postpile, California, USA.
rounded boulders that is typical of many granite out- (Photograph by Tony Waltham Geophotos)
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 to derive from bornhardts, which are deemed the basic
patterns, such as gneisses with well-developed cleavage form.Bornhardtsoccurinrockswithveryfewopenjoints
or foliation, but instead of producing boulders it fash- (massive rocks), mainly granites and gneisses but also sili-
ions slabs known as penitent rocks, monkstones,or cic volcanic rocks such as dacite, in sandstone (Uluru),
tombstones (Plate 5.5). and in conglomerate (e.g. the Olgas complex near Alice
Another common feature of granite weathering is Springs, Australia); and there are equivalent forms –
a bedrock platform extending from the edge of insel- tower karst – that develop in limestone (p. 201). Most of
bergs (island mountains).These platforms appear to have them meet the adjacent plains, which are usually com-
formed by etching (p. 381). Inselbergs come in three vari- posed of the same rock as the inselbergs, at a sharp break
eties: bornhardts, which are dome-shaped hills; nubbins of slope called the piedmont angle. One possible expla-
or knolls, which bear a scattering of blocks; and small nation for the formation of bornhardts invokes long-
and angular castle koppies. Nubbins and koppies appear distance scarp retreat. Another plausible explanation