Inselbergs are defined as "prominent steep-sided hills of solid rock, rising abruptly from a plain of low relief" (Whittow, 1984). Further on, Whittow remarks that inselbergs are characteristic for tropical landscapes, particularly for the savanna zone. True as it may be, in the hyper-arid Namib Desert spectacular granite inselbergs surmount a rock-cut plain, attaining hundreds of meters in height (Fig. 1). Their existence has been noted by early travellers, and since the late 1970s, they have featured in the geomorphological literature aiming at explain the geomorphic history of the Namib from one side (Hövermann, 1978; Ollier, 1978; Cockburn et al., 1999; Goudie & Eckardt, 1999), and the origin of inselbergs as distinctive landforms from the other one (Selby, 1977, 1982). Moreover, inselberg hillslopes host a range of small-scale weathering features such as tafoni, rillenkarren, weathering pits and superficial crusting, also worthy of closer examination (Ollier, 1978; Goudie & Migon, 1997; Migon & Goudie, 2001). They also support interesting plant and faunal assemblages (Seely, 1987).

Fig. 1. Spitzkoppe (left) and the Pondoks (right) rise abruptly from the desert plain, attaining 600-700 m high. The shape of Spitzkoppe justifies its name as the "Matterhorn of Namibia".

Geology and climate

There are two main groups of granite in the Central Namib. The older group was emplaced in association with the deformational and metamorphic events of the Damara Orogeny (500-600 ma) and ranges from early syn-tectonic dioritic plutons to late, post-tectonic alaskites and pegmatites (Schreiber, 1996). They include the Salem Granite, a coarse-grained, biotite and K-feldspar rich porphyritic type, of which the inselbergs of Vogelfederberg and Mirabib are built. The second group of granites is associated with the Karoo sequence and known as Cretaceous Intrusive Complexes. They are related to the presence of the Tristan Plume, the fragmentation of Gondwanaland and the opening of the South Atlantic in early Cretaceous times around 130 million years ago (Milner et al., 1995). They are porphyritic, potassium-rich and extremely massive. It is this Cretaceous granite that supports the most prominent Spitzkoppe (1728 m) and surrounding hills.

The Namib has a very strong gradient of precipitation from the coast to the Great Escarpment. At the coast the mean annual precipitation is under 25 mm whereas at stations near the Great Escarpment the figure is closer to 200 mm. Also significant for regional climate are advective fogs. The amount of fog precipitation rises from the coast, where it averages 34 mm per year (precipitated on 65 days) to a maximum of over 180 mm at 35-60 km inland. It decreases sharply thereafter to only 15 mm in the east of the desert. Mean annual temperatures range from around 17°C at the coast to 28-33°C inland and mean annual relative humidity falls from 87% at Walvis Bay to 50% near Gobabeb and to under 40% on the eastern margin of the desert.

General relief

The main feature of regional relief is the occurrence of an extensive rock-cut plain, which slopes gently from the footslope of the Great Escarpment down to the coast, at an average gradient of 10 m per km. Over much of the area the plain is covered by a thin veneer of weathering and alluvial origin. It is drained via numerous episodic streams and a few allochtonous rivers which flow down to the sea only several times a year. Above the plain rise inselbergs, ridges and larger mountain massifs, built of granite, gneiss, quartzite, marble and dolerite. Their height varies from a few tens of meters up to 500-600 m in the case of the Spitzkoppe group (Fig. 1).

Inselbergs

Spitzkoppe

The Spitzkoppe group dominates in the northern part of the Central Namib, where it rises above the surrounding plain by 500-600 m, attaining the altitude of 1728 m a.s.l. in the Gross Spitzkoppe (Fig. 2). The morphology of the group is very complex (Selby, 1982) and consists of two major hills, the big dome of Gross Spitzkoppe and the multi-domed ridge of the Pondok Mountains, separated by a 100 m wide pediment pass. On the flanks granite is more densely jointed, although massive compartments of smaller size do occur within the lower slopes. Moreover, low domes and whalebacks are quite frequent within the pediment surface surrounding the major domes. The prominent inselberg of Klein Spitzkoppe (1584 m a.s.l.) is located 15 km to the west of Gross Spitzkoppe and has relative height of 500 m. Vegetation around the Spitzkoppe is generally sparse, but acacia trees are fairly common along episodic watercourses on the pediment (Fig. 3), and tree species such as Aloe dichotoma (Fig. 4) and Cyphostemma juttae and thorny bushes grow in topographic lows between the smooth faces of granite domes.

Fig. 2. The tower of Spitzkoppe from the north-east. The piedmont angle is remarkably well developed here.

Fig. 3. Perfectly planar pediment surface north of Gross Spitzkoppe, with episodic watercourses marked by linear stripes of vegetation.

Fig. 4. In the Pondoks fascinating geomorphology co-exists with fascinating world of plants. This is "quiver tree" as Aloe dichotoma is locally called.

Mirabib

The inselberg of Mirabib (840 m a.s.l.) is located in the south-eastern part of the Central Namib plain, where it rises up to 100 m above its level. It is built by a few granite stocks of Precambrian age, which cut through gneiss and schist. The general morphology of Mirabib and its slope form have been described in detail by Selby (1982) who pointed to the occurrence of closely spaced domes separated by depressions developed on densely jointed granite compartments (Fig. 5). Dome slopes are smooth and approach verticality, and the piedmont angle is very sharp; otherwise the slopes are littered with boulders of various size and shape. Vegetation is virtually absent, except for a few isolated trees.

Fig. 5. The inselberg of Mirabib consists of several closely spaced massive granite domes, separated by topographic lows along more densely jointed granite compartments. Large-scale exfoliation operates along sheeting planes as seen on the dome in the left of the picture.

Vogelfederberg

This is an isolated twinned inselberg c. 50 km east of Walvis Bay, rising above the gravel plain to the altitude of 527 m a.s.l.; relative height is 60 m. The group consists of two hills of contrasting morphology. The western one is a classic low-radius dome with smooth slopes and sharp contrast between them and sand-covered pediment (Fig. 6). The eastern one takes the form of an elongated ridge, undercut by a big rock shelter on the east side. Lower slopes are covered by weathering debris and windblown sand. Hence no distinct piedmont angle is present. The hills are located amidst an almost bare plain covered by gypsum crust, but Zygophyllum stapffi and Arthraerua leubnitziae dwarf shrubs grow along the moisture-retaining footslope zone.

Fig. 6. Bare rock surface of Vogelfederberg in the hyper-arid part of the Namib. Even here, local differences in moisture are used by vegetation along the footslope.

Granite geomorphology of inselbergs

Domes

Many of the granite inselbergs in the Central Namib have a dome form and are therefore called bornhardts Domes may occur in isolation (Vogelfederberg), be superimposed onto one another (Gross Spitzkoppe), or adjacent to each other (the Pondok Mountains, Mirabib). They vary in height and in radius and slope curvature. For example, Vogelfederberg has rather gently inclined slopes (25-30°) which may be walked on, whilst the slopes of Spitzkoppe and the Pondoks are very steep, in sections approaching verticality. In some places smooth slopes of domes descend to the piedmont angle which then is exceptionally well pronounced (Fig. 2); in others they disappear beneath talus or orthogonally-jointed granite compartments. Selby (1977a, 1982) provided evidence that at least some dome forms are primary features, dating back to the times of emplacement.

According to Selby (1982), initially bornhardts have slope angles controlled by structure and the opening of sheeting joints operates extensively, the splendid example of which is at Mirabib. Steepness of sheeting joint surfaces facilitates sheet removal in the form of rock sliding and rock avalanches, evident from large taluses mantling lower slopes in many places at Spitzkoppe and Mirabib. Other processes operating on domes are preferential weathering of sub-horizontal sheeting planes, development of rock shelters (Fig. 7) and overhang collapse.

Fig. 7. Weathering along subhorizontal joints gives rise to huge recesses and overhangs. This one, the "Bushman's Paradise" in the east part of the Pondoks, is famous of its rock art.

Pediments

Pediments, i.e. rock-cut surfaces covered by a thin veneer of redistributed products of granite weathering slope away from each of the inselbergs in the Central Namib. They are usually fairly rough, the general slope being interrupted by frequent low elevations ranging from 1-2 to 10-15 m in height. These low hills are particularly common around Spitzkoppe and the Pondoks (Fig. 3). In the marginal part of the Namib, pediment surfaces are locally calcretised; in the hyper-arid west, around Vogelfederberg, calcrete is replaced by gypsum crusts. The remarkable feature of the pediments is their well-developed linear drainage. It appears that most of the episodic flow on these surfaces is accomplished in shallow channels rather than by laterally extensive sheet floods. It appears that geological control has been important for pediment origin and development. Around Spitzkoppe, pediments have developed across the older Salem granite or heavily jointed Erongo granite, and the low elevations of massive granite are stocks of massive younger granite. In a similar way, dolerite veins form distinct ridges on the pediments.

Minor weathering features

Tafonization and alveolisation of granite outcrops is very common. The biggest tafoni have been found in the Spitzkoppe granite, within the low domes north of the main hill (Fig. 8). They attain dimensions of 12 m across, up to 3-4 m deep and up to 3-4 m high. Their inner sides are intensively flaked and the floors are covered by weathering debris. Big tafoni also occur on the massive domes of Mirabib. Tafonization also affects granite boulders which then take the form of a hollow boulder. Boulder-covered slopes of domes in Gross Spitzkoppe and Klein Spitzkoppe group provide many splendid examples.

Fig. 8. The entrance of one of the biggest tafoni in the Spitzkoppe area. In fact, the entire hill is pierced through a series of chambers and galleries developed along joints.

Weathering pits are characteristic for inselbergs in the Spitzkoppe group (Fig. 9). It appears that they rank among the biggest features of this kind described so far. More than 50% of the pits investigated by Goudie and Migon (1997) were larger than 2 m across, and diameters in excess of 5 m were not uncommon. Most pits were relatively shallow (pan-like) if compared to their diameters (<0.5 m). The exact processes involved in the development of pits are poorly known, especially as far as evacuation of debris from apparently closed pits is concerned. At Vogelfederberg, where granite is also massive, there are only a few shallow (<0.2 m) depressions with a diameter approaching 1 m.

Fig. 9. A double-bottomed weathering pits in the Spitzkoppe area, measuring almost 2 m across.

Among other minor superficial features noticeable are flaked surfaces, particularly well-developed in the hyper-arid Namib (Vogelfederberg). In the eastern, wetter part (Spitzkoppe) a brown crust up to 5 cm thick is common. The crust appears to be in a state of decay and shows some degree of cracking; locally, rectangular and hexagonal crack patterns are present. Shallow runnels in dendritic and parallel patterns, apparently used by running water, dissect the smooth slopes of topographic domes (Fig. 10). At Vogelfederberg, these runnels are clearly lithologically controlled, as they follow the sinuous pattern of pegmatite veins.

Fig. 10. Shallow rills or runnels have developed on curved granite surfaces on the north flank of Spitzkoppe.

Origin of inselbergs

Although Hövermann (1978) followed the well-known model of a two-stage origin for bornhardts as developed in the humid tropics and argued that deep chemical weathering was playing the major part in the formation of granite inselbergs of the Namib, the validity of this concept to the Namib was, however, doubted by Selby (1977) and Ollier (1978), who pointed to the total absence of any deep regolith in the Namib. Later on, Selby (1982) provided the geomechanical context for the analysis of the inselbergs in the Namib and, using the rock mass strength approach, demonstrated that their slopes are adjusted to the geological structure and there is no necessity to invoke deep weathering as being antecedent to bornhardt formation. Topographic domes are present where there occur massive granite compartments, the form of which can often be traced back to the time of emplacement. Domes and block-strewn hills frequently co-exist, in relation to contrasting jointing patterns. The variety of minor weathering phenomena tells us that granite inselbergs are actively evolving today and are not unequivocal signs of relief inheritance. They are subjected to continuous exposure and adjustment to structure as the surrounding country rock is worn down, and the massiveness of granite, especially of Mesozoic granite, aids their survival and an increase in relative relief. The role of lithological and structural control thus appears dominant in the development of the variety of granite inselbergs in the Central Namib.

Cosmogenic exposure dating performed on three inselbergs in the southern part of the Central Namib, including two referred to here (Mirabib, Vogelfederberg) (Cockburn et al., 1999), reveals that their mean denudation rate is of the order of 5 m/m.y. and these rates may have been applicable to the last 10 m.y. This implies that high granite domed inselbergs, so characteristic for the Namib Desert, are durable landforms of long geomorphological history and their relief has probably increased through time.

Access

Access to all three inselbergs described here is relatively easy but ones own vehicle is essential (2WD is adequate), especially to get to Vogelfederberg and Mirabib.

The Spitzkoppe group is located c. 50 km westwards from the town of Usakos where hotel accommodation can be arranged. To get to Spitzkoppe one has to go west, direction Swakopmund (this road is served by infrequent public transport), and after 25 km turn right onto the gravel road going to Henties Bay. After the next 18 km a road goes to the right, to the Grootspitskop village, where the toll gate to the Spitzkoppe area is located. Alternatively, one can continue on the main track for another 6 km and approach Spitzkoppe from the west. Whichever route you take, views of the Spitzkoppe are breathtaking. Once within the Spitzkoppe enclosure, a network of tracks allows for access to the most interesting landforms. Particularly worth visiting locations are the enclosed basin called the Bushman's Paradise in the eastern part of the Pondoks, with the rock shelter with rock engravings, natural arch in the southern part of the area, huge rockfall masses at the SW footslope of Spitzkoppe, and low domes in the northern part, with numerous giant tafoni and weathering pits.

Vogelfederberg is located by the gravel road going from Walvis Bay eastwards, to Windhoek, c. 60 km from the town. A car park is located at its footslope. Although the road is open to everyone, to stop and explore Vogelfederberg you will need a permit (ticket) as the hill is within the boundaries of the Namib-Naukluft Park.