Geology Reference
In-Depth Information
(c)
Figure 6.26. (c) Granite domes exposed in sidewalls of glaciated valley of the Thompson River, in the east-
ern borders of the Rocky Mountains, near Boulder, Colorado.
Ikeda's (1991) medium ele vation surface, and lo wer than its highest, the implication in both
regions being that dif ferential structurally-controlled subsurface weathering took place beneath a
high plain. The well-known domical for ms of the central Sier ra Nevada, in Califor nia, are clearly
located beneath and developed in association with a prominent high-level planation surface (Fig.
1.1g
).
In these terms the bevels that are so prominent on some bor nhardts (e.g.
Fig. 6.5)
are con-
strued as etch surfaces related to phases of subsurface weathering.
Second, the erosion achieved during n
1 cycles is consistent with the suggested exposure of
deeper compressional zones of the antifor mal structures (
Figs 6.28a and b)
that become bor n-
hardts with well-developed sheet structures but with few open orthogonal fractures. Large-radius
domes with few visible fractures and few residual boulders or slabs, such as Polda Rock and Little
Wudinna Hill, both in the Wudinna district of northwestern Eyre Peninsula, South Australia, are
interpreted as domes located belo w the neutral planes of antifor ms. In residuals like the nearby
Ucontitchie Hill, on the other hand , with several layers of sheet structure and exposed boulders,
the neutral plane may be coincident with the surf ace of the main mass but lie belo w the various
sheets represented only by large blocks and boulders
(
Fig. 6.29).
6.4.9
Fracture-defined margins
Many bornhardts are defined by prominent vertical or near-vertical fractures (
Fig. 6.7)
.
That the
margins have been stabilised on these fracture zones is indicated by the development of such fea-
tures as flared slopes and scarp-foot depressions. It is surely too much of a coincidence that retreating
escarpments should have been worn back to, and apparently stabilised on, these fracture zones. Surely,
if long distance scarp retreat were operative, somewhere it would have regressed deeply into non-frac-
tured rock?
6.4.10
Age of inselbergs and bornhardts
If inselbergs in general, and bor nhardts in particular, are the last remnants sur viving after long-
continued erosion (scarp retreat), no bornhardt ought to be of an age greater than the duration of
