Geology Reference
In-Depth Information
fluted and grooved, and inland many vertical and near-vertical faces of the blocks were scored and
grooved, some of them flask-shaped in cross-section, and some deep and narrow, being about
0.76 m deep and only 0.61 m across. For various reasons, Logan (1894) considered contemporary
processes of weathering and erosion inadequate to explain the grooves, but he made one crucial
observation and several comments germane to the present discussion. He noted that some of the
grooves of Pulau Ubin extend beneath the soil cover. Unless there had been exposure, develop-
ment of flutings, burial and subsequent exhumation, a sequence of events unsupported by any evi-
dence Logan (1849) could observe, this would appear to demonstrate that the flutings had been
initiated below the soil cover. Nevertheless, Logan (1849) remained uncommitted in his 1849
paper. By later (Logan, 1851), however, he asserted categorically not only that the grooves but all
the boulders and blocks he had observed on Pulau Ubin together constituted what would today be
called an etch surface or complex weathering front. Thus, Logan wrote:
“If … we conceive the external layer of the island, when it first became exposed to decom-
position, to have resembled in character the zone that has been laid open for our inspection
… it is easy to comprehend how the wasting away of the more decomposable parts might at
last leave exposed masses, including bands of the less stubborn material already partially
softened or disintegrated under ground, and that the action of the atmosphere and rain-
torrents would gradually excavate the more yielding portions until the solid remnants
exhibited their present shapes.”
(Logan, 1851, p. 328).
Logan (1849, 1851) was clearly advocating a subsurface origin, development at the lower limit
of weathering or weathering front. Evidence from Eyre Peninsula and many other areas clearly
demonstrates that many gutters have been initiated there. Some extend downslope on quite steep
inclines and become flutings. Many grooves and gutters in granite are, as Logan (1849, p. 6) put
it, “
prolonged beneath the ground
”. There is no suggestion of burial and exhumation following
development. On the contrary, it is grus, or weathered granite,which has been excavated to reveal
the scored weathering front. These forms are therefore initiated beneath the soil surface, and have
been merely enlarged and modified after exposure.
On recently, artificially-exposed, weathering fronts the gutters became wider and shallower,
less well-defined, as they extend below the soil level (Fig. 8.22a). This is because the water
(a)
Figure 8.22.
Flutings shallower and less well-defined (a) at Tampin, West Malaysia.
