4.4.2.3.1 Evidence for the hypothesis
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(a) The backing hillsope appears to be subject to control by weathering. There is considerable evidence of the
weathering and transport of grus between boulders. It
does not appear that the corestones forming the boulder mantle protect the hillslope from weathering and erosion: the boulders do not form a complete cover, and even in small localities where they do, the
in situ
granite below and between themis also very weathered and is being eroded by mechanisms described earlier. These corestones do not often appear to have fallen or become wedged agaihst each other, nor do they accumulate at the nickline. Since the discontinuous boulder mantle is only very rarely more than one boulder thick, and since the grus is actively being removed to expose corestones, it must be concluded that the boulders are comminuted
subaerially and are replaced by others as the hillslope retreats. It should be noted that the corestones are often very weathered before they are attacked subaerially.
(b) This debris is not accumulating on the hillslope or at the nickline, so retreat is not prevented by the formation of an accumulation slope.
(c) There is considerable evidence of weathering at the nick and on the backing hillslope immediately above it, meeting a necessary condition for slope retreat to occur. It
is reasonable to suppose that such weathered rock is notched and eroded by sheetwash, rillwash, raindrop impact, and rainsplash erosion to maintain the angularity of the nick and to permit retreat of the hillslope. Evidence of this is found in the notching of miniature platforms at the nick, in some cases removing an approximately horizontal slice from a corestone.
(d) This hypothesis can account for the lack of a steep plunge • in the weathering front at the nickline on certain profiles
(noted in 4.3.1.2) and the lack of a greater degree of weathering
(e) Evidence of pediment regrading which is required in this hypothesis (especially near the nick) has been given.
(f) The development of pediment passes and embayments is readily
understood in this hypothesis. (However, Twidale 1976a,
p.278, gives an alternative explanation for the evolution
of pediment passes). Furthermore, in the pediment embayment
to the SW of Alice Hill (figure 4.4 and 4.5), the development of a narrow ridge at the head of the embayment suggests
that it has been formed by the retreat of both flanks.
(g) Drainage incision, evidence for which was mentioned earlier, may have produced the angularity of the break of slope
which could have been maintained as the hillslopes retreated. This may have been aided by weathering of the bedrock near
the stream channel. Subsequent development of sheet erosion
would help preserve the form of the pediment.
(h) The general morphology of the subaerial surface at GB can be explained by slope retreat from a formerly larger residual comprising at least the two present residuals. (i) The general lack of coincidence between major joints and the
nickline is understood in this hypothesis, especially since most slopes are set back from joint-oriented streams from
which they could well have retreated following drainage incision.
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(However, in a similar situation, Thomas 1974, p. 211, cites the work of Hurault (1967) and Wahrhaftig (1965)
"to snow that scarp retreat may he more apparent than real." )
(j) This is the only hypothesis which can reasonably account for pediment evolution on basalt and sandstone in this region
(Chapters Five and Six), and it might be argued by analogy
that granite pediments could originate in the same way, and that the weathering of the bedrock pediment surface postdates the formation
of the pediment (but note item (f) cited below as evidence
against this hypothesis). Furthermore, there is a strong
similarity in morphology and slope deposits between the boulder- mantled hillslones developed on basalt and those on granite. Nevertheless, as Twidale has noted (1976a, p. 54), convergent
4.4.2.3.2 Evidence against the hypothesis
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(a) Slope angles on the backing hillslope are not uniform, and have high coefficients of variation. It is certain that hillslopes are not boulder-controlled in the manner suggested by Bryan (1922), for, as Melton (1965) also found, angles are considerably less than the angle of repose of the boulders
(about 34*) and the angle of sliding friction (about 28*). (Even if slope angles on granite hillslopes were very similar, this in itself would not prove that the slopes were retreating in a parallel fashion). However, this point may be irrelevant, since the argument given in 4.4.2.3.1 (a) indicates that
subparallel retreat occurs by weathering and transport between corestones.
(b) Localised overdeepening of the weathering front at the nick on certain transects (noted in 4.3.1.2) may imply a Stillstand of the backing hillslope. (Alternatively it may indicate the temporary coincidence between the nickline and a joint, resulting in deeper weathering at that point. However, no supporting evidence of this alternative interpretation can be given.)
(c) On both the subaerial and bedrock pediment surfaces the increasing importance of the longitudinal slope component away from the
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nickline (especially at GA and GC) remains unexplained under this hypothesis, unless the altitudes of different parts of the nickline (considered in a longitudinal direction) have changed relative to each other during retreat. This is very unlikely, and no realistic mechanism for such an occurrence can be suggested.
(d) The extreme irregularities in detail of the form of the bedrock pediment surface are very unlikely if this hypothesis is
correct. If the hypothesis is to be applied, then these irregularities could be produced by the infilling of
numerous temporary nickline depressions formed during slope retreat, but there is no evidence from seismic work that a particular irregularity can be traced longitudinally across the footslope, so it is not reasonable to infer that these
irregularities mark the position of former nickline depressions. These irregularities (e.g. near the nick on GB6) and longer
reversals of slope (e.g. GC9, GD9) probably originated by weathering and stripping of the transported regolith and some
saprolite by etchplanation under varying climatic conditions (4.5): under the simple hypothesis of slope retreat there is no
way in which the material from these irregularities and reversals of slope can be evacuated.
(e) The occurrence of granite gibbers on the upper quarter of the footslope seems unlikely in the context of this hypothesis and the observation that gibbers are not normally transported by sheetfloods. If it is argued that slope retreat occurs
(which will necessarily involve the comminution of boulders at the nickline) it is unlikely that granite gibbers will
survive for so long so far from the nickline. It is more reasonable to explain the presence of such gibbers by periodic stripping and reworking of the mantle to a much greater
degree than is assumed to occur under normal "pediment
regrading". Furthermore, stratigraphic evidence from basalt and sandstone areas suggests reworking of the mantle (Chapters Five and Six), and there seems no reason to suppose that this will not occur on granite also. Incised channels which are
rare at present on the sample pediments may be more active and numerous under different climatic conditions, and may
then be responsible*for reworking of the mantle. Such reworking can also evacuate the products of mantle-controlled
weathering for which some evidence has been adduced.
(f) If this hypothesis is correct, one would expect an increase in the depth of the weathering front as distance from the nickline increases, since a progressively longer period of time
would have been available for weathering to occur. This is not the case here. Although Thomas (1966b) has suggested that
the development of such a pattern may be obscured locally
by (1) structural variations in the bedrock and (2) concentrated and deeper weathering at the nick, isopachs of depth to the weathering front still do not exhibit any general trend whatsoever in this or in any other direction.
(g) From the little that is known of the climatic history of the
region in the Cainozoic, such an hypothesis may be unrealistically uniformitarian. For example, mention was made earlier of
4.4.2.3.3 Evaluation
Although it is very difficult to prove or disprove that slope retreat has occurred, observations on hillslopes leave little doubt that they are far from resistant to subaerial erosion, and considerable amounts of material are being moved, possibly several times each
wet season. Processes on the footslope are competent to remove hillslope detritus reaching the nickline, and also could even erode headwards at that point (Twidale 1956) since both the hillslope and
the footslope are usually very weathered.
Analogy with slope retreat both on granites from other parts of the world and on other lithologies in the same region lends credence to this hypothesis. For example:
1. Work by others in broadly similar environments and on similar