Geology Reference
In-Depth Information
develop in two stages. One involves differential subsurface weathering, commonly controlled by
variations in fracture density, but also possibly involving lithological contrasts; the other, differ-
ential erosion which exposed the bedrock projections as bornhardts (Fig. 6.14a). Rivers have most
commonly been responsib le for the erosional stripping of the re golith, but wind-dri ven waves,
frost action (solifluxion) and glaciers and ice sheets (as in Scandinvia and in some degree in west-
ern Iberia) have also played their part in specif ic locations. Tectonic uplift may have facilitated
stream erosion and the stripping did not necessarily take place in a single phase.
Though the development of bornhardts is justif iably compared to that of corestone boulders,
most workers envisage that, whereas corestones become detached from the parent mass as a result
of weathering, bornhardts remain contiguous with or attached to the mass of fresh rock beneath
the regolith. Some pillars, inter mediate in size betw een boulders and bor nhardts, illustrate this
point (Fig. 6.14b).
Again like boulders, various stages previous to differential subsurface weathering and subse-
quent differential erosion, and involving early magmatic, thermal and tectonic events, have to be
taken into account if the forms are to be more completely understood. Although the two stages of
subsurface preparation and subsequent e xposure (Twidale and Bourne, 1975) are all impor tant,
the interactions of shallow groundwaters and bedrock involve many structural nuances of ancient
origin, so that bornhardts, like boulders, are multi-stage features. F or example, the formation of
orthogonal fractures is critical to the e ventual development of both boulders and bor nhardts, but
this occurred long before the rock mass concerned was in the groundwater zone and thus susceptible
to differential weathering (the usual Stage 1). In the dacitic Ga wler Ranges (Campbell and
Twidale, 1991) it can be demonstrated that the or thogonal fractures had already de veloped in
Middle Proterozoic times, some 1,400Ma ago, whereas their exploitation by groundwaters in the
shallow subsurface did not take place until the Jurassic, more than 1,200Ma later. Yet, the forma-
tion of the orthogonal fractures in the distant geological past was critical to the eventual formation
of bornhardts. Where lithological contrasts have been exploited by subsurface weathering the ori-
gin can be ultimately traced back to thermal or magmatic events of the distant past.
(a)
(b)
Figure 6.14. (a) The tw o-stage concept of bor nhardt de velopment: (i) dif ferential fracture-controlled
subsurface weathering, (ii) differential erosion of the contrasted compar tments so developed.
(b) Murphys Haystacks, near Streaky Bay, on northwestern Eyre Peninsula, South Australia,
consists, in part, of a g roup of pillars still in ph ysical continuity with the underl ying granite
mass. As such they can usefully be regarded as miniature bornhardts.
