Geology Reference
In-Depth Information
6.5 EXHUMED BORNHARDTS AND INSELBERGS
Many inselbergs and bor nhardts of exhumed type are recorded in the literature and landscape.
They are of v arious ages. Thus, the magnif icent inselberg landscapes described b y Bornhardt
(1900) from what is now Tanzania, are exhumed and of preCretaceous age, and some of those con-
sidered by Falconer (1911) in his seminal studies of Nigerian residuals are also e xhumed but of
preEocene age (Figs 6.31a and b). Tall domical residuals (pains de sucre) have been exhumed from
beneath the Late P alaeozoic Tassili Sandstone, in the Tassili Mountains of souther n Algeria
(Rognon, 1967) (Fig. 6.31c), and the par tial stripping of a cover of Nama (Cambrian) strata has
resurrected a field of granitic nubbins in southern Namibia and the adjacent parts of Namaqualand
(Western Cape Province) (Fig. 6.31d). In Australia, exhumed granitic inselbergs range in age from
Early Pleistocene (northwestern Eyre Peninsula), through pre-Miocene (western Murray Basin) to
numerous examples of earliest Cretaceous or Jurassic age (
Fig. 6.32)
to Late Archaean in the
Pilbara region of Western Australia. In Spain, the inselber gs of the Úbeda - Linares area of
Andalusia, Southern Spain, predate the Triassic
(
Fig. 6.33a),
and the exhumed granitic terrain of
Charnwood, in the English Midlands, is of similar age (W atts, 1903). In northwest Scotland, the
(Neoproterozoic) Torridonian Sandstone in northwest Scotland rests on what appears to be an old
inselberg landscape, developed in Lewisian gneiss, and consisting of domical residuals surrounded
by pediments (Fig. 6.33b). The younger sediments have been partly eroded to exhume the ancient
landforms.
These examples contradict the view that bornhardts are Cainozoic forms. On the contrary, they
are of ancient lineage and, whatever mechanism or mechanisms are responsible, have been feasi-
ble and active through much of geolo gical time, and def initely for the past 2.5 Ga. Considered
together, their temporal and spatial distributions suggest that they can form in a wide range of cli-
matic conditions. Though the volume of water at and near the Earth's surface may have increased
through time, some has been present throughout the geological record. Thus, there has always been
water available for w eathering, consistent with the requirements of the tw o-stage mechanism.
(a)
(b)
Figure 6.31. (a) and (b) Sketches drawn from photographs (Falconer, 1911) showing exhumed preEocene
granitic forms in northern Nigeria.
