A combined theory of compartment weathering, dissection, and subsequent lowering on the one hand and slope retreat on the other hand is suggested here to explain the initiation and continuing
evolution of pediments in the granite landscape of the East Kimberleys. Detailed discussion of rival hypotheses has shown that some evidence can be interpreted according to an hypothesis of compartment weathering or
an hypothesis of hillslope retreat (e.g. the angularity of the nick, pediment passes). More importantly, there remain many individual
pieces of evidence which can only be explained by one of these hypotheses, and a good case can be made for the partial application of either
hypothesis: however, no simple hypothesis can explain all the evidence. The comparison and ordering of variants distributed in
space as a means to the identification of development through time is hazardous in this study, since it can be shown that small residuals are not necessarily older than nearby large residuals, rather they may
be younger and just emerging from a compartmentally weathered profile (Twidale and Bourne 1975a, figure 12). Furthermore, since the overall climatic and geomorphic history of the area is not known, it is even more difficult to formulate a theory of pediment initiation and continuing evolution.
However, the following points can be made:
(a) Granite rocks were weathered compartmentally in the Tertiary, dissected following uplift, and underwent etchplanation. Evidence from nearby areas suggests that this may have happened
several times in the Tertiary, but the granite areas considered here lie below the Tertiary weathering profiles preserved on
the Sturt Plateau, and no evidence of earlier or later weathering surfaces is Dreserved. However, it remains
possible that a lower surface was cut and weathered coastward of the Sturt Plateau which may have emphasized the compartment- ation created by earlier cycles of deep weathering and etch- planation.
(b) Compartment weathering is still important. At present the footslopes appear to be stable, graded to footstreams, and are being weathered. Quaternary changes in climate probably initiated periodic mantling and stripping of the footslopes which have emphasised these basic compartments.
(c) Although descriptions of compartment weathering have usually emphasised the structural difference between domed inselbergs
and the closely and openly spaced granite joints of the intervening footslones, there can also be a structural difference
between hillslopes and footslopes where the former are boulder inselbergs.
(d) Once boulder inselbergs and intervening footslopes have been formed, these compartments will tend to be preserved in the landscape on account of the relative weakness of the footslopes to subsurface weathering compared with the relative resistance of the hillslopes to subaerial weathering. In other words, a system of positive feedback will preserve the compartments. (This mechanism can operate even where there is little structural difference between the rock forming hillslopes and the rock forming
plains, since there will always be a contrasted resistance between exposed and buried granite (Eggler, Larson, and Bradley 1969)) .
(e) Boulder inselbergs which have been exposed in the manner described also undergo slope retreat to extend the pediments. Lateral components of the joint svstem assume great importance
(Thomas 1962).
(f) Where domed inselbergs occur these are unlikely to undergo
significant slope retreat. There are few domed inselbergs
in the region (presumably on account of the scarcity of tightly comDressed and widely-spaced joint systems), but some evidence of basal steepening is found on these, perhaps indicating the importance of downwearing following compartment
weathering. They are also modified by extensive sheeting
along curvilinear joints. Individual sheets are up to
2.5 m thick. In a few places large fragments of disintegrated
sheets have fallen to the nick.
(g) The relative importance of downwearing and slope retreat at
any particular time may vary according to climate. However,
evidence of both may be found in seismic profiles of the weathering front (e.g. GA2, GA5, GB7 show evidence of steep
plunges in the weathering front near the nickline). Such
plunges may be indicative of relative Stillstands in the retreat of the hillslope, and the more planar section of the weathering front which separates the plunge from the
present nickline may be indicative of hillslope retreat. However, such Datterns are not conclusive, since they may also be interpreted as structurally-guided weathering. (h) The characteristically angular nick is consistent with and
maintained by both downwearing and hillslope retreat. In
the short term it is preserved mainly by the efficiency of rills, sheetflow, and rainwash in removing weathered
products, since the debris in transit reaching that point is
generally of small size. Only rarely (in the vicinitv of dikes)
is an angular nick absent. Retreat and downwearing are generally
into very weathered rock and are preceded by subsoil notching associated with chemical weathering at the nick as described by Ruxton and Berry (1961a) and Twidale (1967).
( i ) A l t h o u g h t h e d e v e l o p m e n t o f n i c k l i n e d e p r e s s i o n s i s f a c i l i t a t e d by ‘t h e p r e s e n c e o f w e a t h e r e d r o c k a t t h e n i c k , s u c h w e a t h e r i n g i s n o t s y s t e m a t i c a l l y g r e a t e r o r d e e p e r a t t h e n i c k t h a n e l s e w h e r e on t h e f o o t s l o p e . I t a p p e a r s t h a t o t h e r c i r c u m s t a n c e s a r e o f c r i t i c a l i m p o r t a n c e i n d e t e r m i n i n g w h e t h e r c h a n n e l s w i l l d e v e l o p a t t h e n i c k l i n e , a n d t h e i r d e v e l o p m e n t may i n p a r t b e f o r t u i t o u s . ( j ) E t c h p l a n a t i o n i n r e s p o n s e t o c h a n g e s i n c l i m a t e an d b a s e l e v e l w i l l l o w e r t h e f o o t s l o p e and c a n a l s o a c c o u n t f o r t h e e v a c u a t i o n o f t h e p r o d u c t s o f m a n t l e - c o n t r o l l e d w e a t h e r i n g by s h e e t and l i n e a r e r o s i o n on t h e s t r i p p e d f o o t s l o p e ( s e e 4 . 5 ) . The d e v e l o p m e n t o f t h e l o n g i t u d i n a l s l o p e c omp on e nt on t h e b e d r o c k p e d i m e n t s u r f a c e may a l s o o c c u r a t s u c h t i m e s . ( k) L e s s d r a s t i c r e w o r k i n g o f t h e m a n t l e o c c u r s a t o t h e r t i m e s . ( l ) S m a l l - s c a l e r e g r a d i n g o f t h e s u b a e r i a l p e d i m e n t s u r f a c e w i t h o u t s i g n i f i c a n t r e w o r k i n g o f t h e m a n t l e a p p e a r s t o o c c u r i n r e s p o n s e t o w e t - s e a s o n c o n d i t i o n s , e s p e c i a l l y n e a r t h e n i c k . T h i s s u r f a c e i s g r a d e d t o t h e f o o t s t r e a m , and i s u s u a l l y i n c l i n e d a t a low a n g l e . (m) F o o t s l o p e s a r e i n e q u i l i b r i u m w i t h t h e p r e s e n t c l i m a t e : t h e r e i s no e v i d e n c e o f s i g n i f i c a n t d i s s e c t i o n o r b u r i a l . T h i s t h e o r y i s i n a c c o r d a n c e w i t h o b s e r v a t i o n s made, an d i s c o n s i s t e n t w i t h t h e l i m i t e d k n o w l e d g e o f c l i m a t i c an d g e o m o r p h i c h i s t o r y . The a c t u a l m e c h a n i s m o f s l o p e r e t r e a t i s n o t f u l l y u n d e r s t o o d , s i n c e b o u l d e r - c o n t r o l l e d p a r a l l e l r e t r e a t i n t h e m a n n e r e n v i s a g e d by Br y an ( 19 22 ) d o e s n o t o c c u r . S i n c e t h e b o u l d e r m a n t l e i s r a r e l y more t h a n one b o u l d e r t h i c k , r e t r e a t m u s t u l t i m a t e l y b e c o n t r o l l e d by t h e w e a t h e r i n g o f s a p r o l i t e b e t w e e n b o u l d e r s w h i c h w i l l d e t e r m i n e t h e r a t e o f e me r g e n c e o f b o u l d e r s . C e r t a i n l v , t h e d i s i n t e g r a t i o n o f b o u l d e r s g e n e r a l l y k e e p s p a c e w i t h t h e r a t e o f w e a t h e r i n g and r e m o v a l o f s u r r o u n d i n g m a t e r i a l , and t h e r e d o e s n o t a p p e a r t o b e any way i n w h i c h t h e d i s i n t e g r a t i o n o f b o u l d e r s a c t u a l l y c o n t r o l s t h e r a t e o f t h e r e m o v a l o f s a p r o l i t e b e t w e e n t hem. T h i s v i e w i s d i f f e r e n t f ro m t h e c o m m o n l y - h e l d o p i n i o n t h a t t h e s u b a e r i a l d i s i n t e g r a t i o n o f c o r e s t o n e s i s s l o w . Ho we ver , t h e e v i d e n c e c l e a r l y i n d i c a t e s t h a t t h e r e m o v a l o f t h e s a p r o l i t e b e t w e e n t h e b o u l d e r s i s a c t i v e : a l t h o u g h no r a t e s o f r e m o v a l a n d e r o s i o n c a n b e g i v e n , t h e a b u n d a n c e o f s u c h m a t e r i a l on h i l l s l o p e s an d f o o t s l o p e s , and o b s e r v a t i o n s m a d e . d u r i n g s h e e t f l o o d s do n o t i n d i c a t e t h a t t h e r a t e o f
removal of such material is slow. However, if the rate of subaerial disintegration of corestones is_ slow, then we might expect a greater
abundance of corestones on the hillslope - certainly more than one corestone in thickness.
The solution to this apparent conflict may be found in two factors. First, many corestones are well-rotted before they are exposed subaerially. Such rotting has taken place in the mantle, and facilitates rapid subaerial breakdown. Second, although some corestones are fresh when exposed subaerially, they are not numerous. The mantle of boulders on the hillslope is discontinuous anyway, and only some of these are fresh. Therefore, while many boulders may be rapidly disintegrated subaerially because they are already very rotten when they are exposed, it is likely that fresher corestones survive much longer on the backing hillslope, but since the mantle is discontinuous, and since they are in
the minority, it is not surprising that mantles are rarely more than one boulder thick.
A final point should be made. If the explanation given is correct, then fresh corestones which are weathered subaerially only very slowly might be expected to fall down the hillslope as thev are undermined by the complete removal of the sanrolite around them, or
to become perched on other emerging corestones. In this connection it is instructive to note that wherever boulders are found to be lodged against each other (apparently bv falling downslope), the upslope
boulder is almost invariably fresh. Also in the few places where the mantle is more than one boulder thick, the boulders on top are almost invariably fresh. This evidence is conformable with the theory
given above. *
Nevertheless, in the few instances where large slabs of fresh granite are exposed by slope retreat (e.g. immediately above the nick along transects 1, 3, 4, and 5 at GD), they form a slope segment between the steepest segment on the backing hillslope and the footslope. An analysis of the form and deposits along these transects suggests that
slope retreat occurs above such large slabs which may remain features of the landscape for a long time.
It is possible that there is also a degree of slope decline