Geology Reference
In-Depth Information
6.4 EVIDENCE AND ARGUMENT CONCERNING ORIGINS OF BORNHARDTS
The scarp retreat concept not onl y fails to accommodate and e xplain several features of bor n-
hardts, but it is also inconsistent with some of the field evidence and argument. Yet, most geolo-
gists and geomorphologists still favour scarp retreat. Why? What is the evidence and argument?
6.4.1
Contrasts in weathering between hill and plain
According to the fracture density h ypothesis, there ought to be contrasts in density betw een hill
and plain. Deep weathering of granite beneath plains has been reported from several parts of the
world. In addition, at several sites there is a marked contrast between the depth of weathering of
the bedrock beneath the plains and that of the bornhardts. Thus, at the margins of Ucontitchie Hill,
Eyre Peninsula, South Australia, the granite has suffered marked differential fracture-controlled
weathering, with corestones set in a matrix of w eathered rock (
Fig. 6.17a).
Drilling shows that
around the inselberg the regolith commonly extends to depths of 25 m and as deep as 40 m, yet
only a fe w metres a way, the massi ve rock exposed in the bornhardt remains essentiall y fresh.
Similar contrasts in fracture density betw een hill and plain are e videnced near Garies, in
Namaqualand (Western Cape Pro vince) (Fig. 6.17b), near Bangalore in peninsular India (F ig.
6.17c), and on eastern Dartmoor, southwestern England (Fig. 6.17d).
6.4.2
Incipient domes
If bornhardts are, indeed, initiated in the subsurface by differential weathering, as it is implicitin the
two-stage hypothesis, there ought to be examples of all stages of their for mation, including some
domical masses formed of intrinsically fresh rock, that have either just been exposed or are located
immediately beneath the natural land surface. Several examples have been noted in the literature
and in the landscape. From Ebaka, in south Cameroon, Boyé and Fritsch (1973) described a quarry
opened to provide ballast for a railway line. The excavation exposed a domical mass of fresh gran-
ite surrounded by weathered rock. Its crest w as located 8-10 m beneath the land surf ace, and it
dipped away in all directions (
Fig. 6.18).
This was surely an incipient bornhardt awaiting natural
exposure but revealed by artificial excavation. The dome has not been shaped by epigene processes
and then buried, for the cover material is weathered granite
in situ
.
Elkington Quarry was recently (1995) opened near Pildappa Rock, nor th of Minnipa, on nor th-
western Eyre Peninsula, South Australia. Prior to excavation, only a small platform was exposed, but
this proved to be the crest of a large-radius dome, with sheet structure well-developed (
Fig. 6.19a).
Several other low domes on northwestern Eyre Peninsula have been quarried (e.g. at Calca, south of
Streaky Bay, and Quarry Hill, near Wudinna) and in each instance they have been shown to be but
the crests of much larger domes, the radius of which increases with depth below the surface.
Near the Leeukop, in the northern Free State, South Africa, the crest of a dome is exposed in a
shallow depression excavated as a water storage (Fig. 6.19b). In the Vredefort brick quarry, in the
northeastern Free State, the w eathered granite has been e xcavated, revealing mainly corestones
but, in one cor ner of the quar ry, part of an incipient dome is e xposed (Fig. 6.19c). At Midrand,
(formerly Halfway), between Johannesburg and Pretoria, South Africa, what was a small rock plat-
form has been revealed by road excavations to be the crest of a small dome (F ig. 6.19d). Similar
features have been noted at Buccleuch, south of Johannesbur g; between Neue Smitsdor p and
Pietersburg in the Northern Province; at the several sites between Vanrhynsdorp and Nuwerus in
Namaqualand, Western Cape Province; at the Pomona Quarry, near Harare, Zimbabwe. At several
sites in the southwest of Western Australia, particularly in the Darling Ranges, domes like Sullivan
Rock have only just been exposed from beneath the lateritic and bauxitic regolith.
6.4.3
Subsurface initiation of minor forms
If minor forms associated with bornhardts are initiated beneath the land surface, it follows that the
host mass must also ha ve evolved beneath the land surface. As it is made clear in later chapters
