Journal of the Royal Society of Western Australia, 87:123-133, 2004 
Notes on the geomorphology of The Humps, 
near Hyden, Western Australia 
C R Twidale & J A Bourne 
School of Earth and Earth Sciences, Geology and Geophysics, University of Adelaide, Adelaide, SA 5005 
ISI rowl.twidale@adelaide.edu.au 
(Manuscript received May 2004; accepted October 2004) 
Abstract 
The domical inselberg or bornhardt known as The Humps is in many respects typical of the 
many granitic residuals of the Wheatbelt. Of great antiquity, it is a two-stage form which was 
initiated beneath a regolithic cover, and has been exposed in stages. Its minor forms (pitting, 
gnammas of various types, polygonal cracking, sheet structure) are also commonplace, though in 
some instances, such as the armchair-shaped hollows, they are especially well developed. But the 
evidence for its great age, for episodic exposure, and for the subsurface initiation of tafoni is 
noteworthy. 
Keywords: bornhardt, multistage, tafoni, armchair-shaped hollow, mogote 
Introduction 
The Humps is a stepped bornhardt inselberg shaped 
in Late Archaean (2.55-2.75 Ga) granitic rocks and 
located some 20 km NNE of Hyden in the Western 
Gneiss Terrane of the Yilgarn Craton (Chin et al. 1984; 
Myers 1990). It stands more than 420 m above sea level 
and about 80 m above the surrounding plains (Fig 1). It 
displays several gross and minor features that are typical 
of other inselbergs in the region, and some that are 
peculiar to the site. For example, a perched block and 
plinth, an hourglass rock, split rocks and tafoni can be 
seen on the hill, which is characteristically bald and 
domical, but the evidence of great antiquity and of the 
subsurface initiation of tafoni are sufficiently rare to be 
noteworthy. 
Origin of the bornhardt 
The stepped morphology of The Humps (Figs 1 & 2) 
suggests that the residual was initiated in the subsurface 
and subsequently exposed as a result of periods of 
lowering of the surrounding plains, i,e. a two-stage or 
etch origin involving episodic exposure (Falconer 1911; 
Twidale 1964, 1982a, 1982b, 1982c; Twidale & Bourne 
1975a). Such an interpretation implies differential 
subsurface weathering, and in particular the relative 
resistance to subsurface moisture attack of the 
compartment of rock that is The Humps. It might be 
argued that intrusive veins, which are particularly 
numerous on The Humps, as compared with, for 
example, King Rocks and Hyden Rock, have buttressed 
and strengthened the rock mass. First, however, the most 
common intrusions, namely pegmatites, are zones of 
weakness for they have been exploited to produce 
shallow linear depressions and steepened slopes (see also 
Bourne & Twidale 2002). Second, though density of 
intrusive sills and veins varies there is no reason to think 
that The Humps is more buttressed in this way than the 
© Royal Society of Western Australia 2004 
immediately adjacent areas. To the contrary, in other 
areas such as the Pilbara, granite with numerous veins of 
aplite and quartz has been differentially weathered and 
eroded to produce an inselberg landscape, with veins 
exposed in both hill and plain. 
The Humps, like other inselbergs in the vicinity, is 
massive. The few open fractures are widely spaced. A 
strong structural grain trending NW-SE or NNW-SSE 
(but locally variable) pervades the rocks of the Western 
Gneiss Terrane (e.g. Chin et al. 1984) and many of the 
more prominent outcrops (as indicated by their being 
named) occur in SE-NW corridors (Fig 3). Thus 
considering the Hyden area, Graham Rock, The Humps, 
Anderson Rocks and Mt Walker can be linked in this 
way, as can Gibb Rock, King Rocks, Bushfire Rock/ 
Cockatoo Rock and a group of bornhardts in the Varley- 
Holt Rock district. Some of the bornhardts named also 
occur in roughly east-west (or ESE-WNW) alignments, as 
for instance Murray Rock, King Rocks, The Humps and 
Camel Peaks, and Cockatoo Rock/Bushfire Rock, 
Graham Rock, Hyden Rock, Captain Roe Rock/Mettam 
Knob and Karlgarin Hills. This suggests that the 
residuals may be preserved by virtue of compression 
associated with cross- or interference folding, first (and 
in terms of present orientations) NE-SW, and later 
roughly north-south; but whether as a result of direct 
compression or of shearing is not known, though 
persistent horizontal plate motions and evidence of 
conjugate en echelon fracture patterns favour the latter. 
Episodic exposure and antiquity 
As previously mentioned, the stepped morphology of 
The Humps (Figs 1 & 2) suggests episodic exposure. Four 
surfaces have been identified. Level I is of limited areal 
extent and is preserved in the highest crests. Level IV 
comprises the present plains and platforms (1 in Fig 1). 
Level III takes the form of platforms preserved in spurs 
and perched some 30 m above present plain level. The 
most prominent planate surface remnants are referred to 
as Level II, 60-70 m above the surrounding plain. The 
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