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Figure 5.13.
Overturned slabs of the rock 'porcellanite' on top of a 25 m high
headland, Cape Leveque, Western Australia. These rocks come from a bed of rock
that was originally horizontally bedded. The layering now seen to be orientated in a
number of directions was all originally horizontal.
than those plucked and transported into the main boulder field. The velocity of
the flow responsible must not have been sufficient to completely entrain these
blocks. Possibly the boulder field resulted from more than one event, perhaps a
series of them.
Wavesalso appear to have overtopped the headland above the boulder field. A
layer of 'porcellanite' sits 20--25 m above sea level near the crest of the headland
(porcellanite is a local term for a silicious rock that forms within a saprolite
profile by silica precipitation during the weathering process). The porcellanite
here is 0.3--1 m thick and like the original Cretaceous strata is horizontally
bedded. It forms horizontally bedded caprock on hills and mesas of Cretaceous
strata throughout tropical northern Australia. But today this horizontal stratum
is fractured and individual blocks are overturned or standing on end partially
buried in the weathered red soil (Fig. 5.13). It is difficult to conceive how pedo-
genic processes could account for the present orientation of the blocks, for there
is no evidence for vertical shearing within this soil as a result of the expansion
and contraction of clays. Nor is there evidence for karst processes occurring
within the weathering profile that could cause the porcellanite layer to slump.
It is more likely that these rocks were overturned by fluid flow over the headland
crest.
Other evidence supports the hypothesis that the porcellanite rocks atop the
headland were overturned by fluid flow. A channel network deepens and widens
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