Journal of the Royal Society of Western Australia, 87(3), September 2004 
Figure 8. An unusually deep armchair-shaped hollow developed high on the midslope near the head of the entrance valley. 
1960; Mabbutt 1967; Bocquier et al. 1977; also Ruxton 
1958). 
If this interpretation is correct, the dome surmounting 
Level II was in existence during the later Cretaceous 70- 
100 m.y. ago. Level I is even older, though there is no 
suggestion that it predates the Late Palaeozoic glaciation 
which affected all of what is now southern Australia (B 
M R Palaeogeographic Group 1992). The antiquity of 
levels I and II finds support in the survival in a broad 
joint cleft high on the eastern slope of the bornhardt (Fig 
6; 4 in Fig 1) of Granitites intangendus, a monotypic genus 
which is at least of Middle-Late Tertiary age (Fay et al. 
2001). Also in this cleft is a geographically isolated and 
genetically divergent population of Eucalyptus caesia 
(Hopper & Burgman 1983). 
Structural effects and multistage development 
The granitic rocks of The Humps are commonly 
porphyritic, and large feldspar crystals project a few 
millimetres, providing a pitted or rough surface (Twidale 
& Bourne 1976; 7 in Fig 1). The rocks have been sheared 
and fractured, resulting in prominent partings aligned 
roughly 130°, 140° and 160° (Fig 7A). The rock has also 
been intruded by veins and pods of aplite, quartz and 
pegmatite. Many of the veins of which the pegmatite is 
the youngest are aligned along fractures but there are 
many cross-cutting veins and numerous complex 
intersections are exposed, especially on the southern 
slopes of the residual. These fractures and intrusions 
have had a profound effect on the morphology of the 
inselberg both in gross and in detail. 
Fractures strongly influence the detailed morphology 
of the residual, being responsible for several prominent 
clefts and for the deep valley on the northern flank of the 
residual (Fig 7A; 2 in Fig 1), but such features are widely 
spaced. Many have flared sidewalls. Sheet fractures and 
structures (Figs 4 & 7B) are exposed on the slopes of the 
residual and notably in a large alcove or armchair-shaped 
hollow (cj.v.) near the western base of the hill (Fig 7B). 
Linear shallow depressions are associated with 
steeply-dipping pegmatite sills and some locally steep 
slopes can be explained in terms of the preferential 
weathering and erosion of such gently inclined bodies (cf 
Bourne & Twidale 2002). Aplite sills have given rise to 
eye-catching 'boxwork patterns' (Fig 9A) as well as 
'tramlines' or parallel projecting veins and irregular or 
wavy miniature ridges (Fig 9B; 7 in Fig 1). Preferential 
weathering and erosion of country rock infested by sills 
of relatively weak rock has caused them to the eroded, 
leaving rounded linear or gently sinuous ramparts or low 
ridges in relief. Zones buttressed by massive pods and 
veins of quartz (9 in Fig 1), combined with preferential 
weathering and erosion of pegmatitic areas have 
produced a cascade of basins and linking channels on the 
western midslope of The Humps (Fig 10). 
Clearly, though The Humps and associated minor 
forms are two-stage developments, the origin of 
numerous minor features can be traced to the Archaean 
and to several magmatic episodes during which intrusion 
of various composition and textures were introduced. 
Considered both in gross and in detail the residual is a 
multistage feature (Twidale & Vidal Romani 1994). 
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