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Rock basins (gnammas) revisited

Les vasques (gnammas) réinterprétées
Charles Rowland Twidale et Jennifer Anne Bourne
p. 139-149

Résumés

Les vasques sont des modelés élémentaires des surfaces rocheuses produits par les eaux stagnantes. Elles sont associées à l'altération chimique par l'eau, et la présence de stratifications et/ou de plans de foliation facilite leur développement. Des dispositifs variés sont associés aux lignes de fracture, aux écoulements d’eau ou encore aux remplissages sédimentaires cuvettes. Les interprétations sont révisées, et une nouvelle définition et explication des vasques oculiformes (« water eyes ») est suggérée.

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Notes de la rédaction

Article soumis le 13 juin 2017, reçu sous sa forme révisée le 14 novembre 2017 et définitivement accepté le 12 décembre 2017.

Texte intégral

The authors thank two referees and the honorary Editor for constructive comments and suggestions on and earlier version of the paper.

1. Introduction

1Rock basins are relatively small hollows excavated in cohesive rock. They are erosional or destructional features that are developed in a variety of climatic environments and in a wide range of lithologies (Hedges, 1969). They are, however, especially well developed in granitic and in arenaceous rocks. Some rock basins were at one time thought to be man-made because, it was considered, their roundness was so perfect they could not be natural and must have been shaped by human hands (Drake, 1859). One of the German terms for rock basin, Opferkessel, means ‘sacrificial basin’ and suggests ritualistic usage by early societies, and some basins located on granite outcrops in southwestern England were linked to Druidical ceremonies (Borlase, 1754). Consistent with this interpretation, the red color imparted seasonally to some pools by algal blooms was construed as due to contamination of water by human blood. However, there have long been those who attributed rock basins to natural processes (MacCulloch, 1814).

2Based mainly on field work on granites exposed on northwestern Eyre Peninsula, Twidale and Corbin in 1963 suggested a morphogenetic classification of rock basins, and cited three basic forms, namely pits, pans, and armchair-shaped hollows (fig. 1-2).

Fig. 1 – Rocks basins.
Fig. 1 – Vasques.

Fig. 1 – Rocks basins.  Fig. 1 – Vasques.

A: Pit on Pildappa Rock, near Minnipa, northwestern Eyre Peninsula, South Australia. Note ferruginous staining on rim of basin and blocky remnants of former sheet structure. B: Pan on Yarwondutta Rock, near Minnipa, north western Eyre Peninsula. Note flat floor, overhanging sidewalls, sediment on floor, and overflow channel on near side. C: Armchair shaped hollows on northern slope of Pildappa Rock.
A : Vasque (Pit) sur Pildappa Rock, près de Minnipa, au nord-ouest de la péninsule d'Eyre, Australie-Méridional., Notez la coloration ferrugineuse sur le rebord de la cavité et les restes de blocs de l'ancienne structure desquamée. B : Vasque à encorbellement (pan) sur le rocher Yarwondutta, près de Minnipa, au nord-ouest de la péninsule d'Eyre. Noter le plancher plat, les parois latérales en surplomb, les sédiments sur le fond et le chenal de débordement. C : Les vasques en fauteuil avec exutoire sur le versant nord du rocher Pildappa.

Fig. 2 – Location maps.
Fig. 2 – Carte de localisation des sites.

Fig. 2 – Location maps.  Fig. 2 – Carte de localisation des sites.

3These morphologically distinct forms evolve after the exposure of the granite surface on which they are developed, but most have a common initiation in depressions initiated in the subsurface at the Tiefenfront or weathering front (Büdel, 1957; Mabbutt, 1961). The detailed morphology of platforms, usually the first exposed bedrock feature, varies between almost smooth (apart from the crystal scale pitting (Smith,1941; Twidale and Bourne, 1976) to a form resembling the surface of a meringue or a choppy sea (fig. 3); but all display hollows of various shapes and sizes. It is from these that the particular basins evolve according to the rock texture or and the slope of the exposed surface. Both standing pools and water flows are involved in the formation of rock basin and the effects of weathering are enhanced and accelerated by chemicals produced by the decay of plants and small animals. All were attributed to weathering associated with pools of standing water, followed by the evacuation of the products of weathering either in solution or in suspension during and following periods of heavy rain and overflow of the basin pools.

4Unsurprisingly, with wider experience and reflection, alternative or modified explanations are suggested for several forms. In particular they are better appreciated…

Fig. 3 – Irregular surface of a granite platform, north of Podinna Tank and some 25 km, north of Minnipa, northwestern Eyre Peninsula.
Fig. 3 – Surface d'une plateforme granitique irrégulière, au nord de Podinna Tank et à quelques 25 km au nord de Minnipa, au nord-ouest de la péninsule d'Eyre.

Fig. 3 – Irregular surface of a granite platform, north of Podinna Tank and some 25 km, north of Minnipa, northwestern Eyre Peninsula.  Fig. 3 – Surface d'une plateforme granitique irrégulière, au nord de Podinna Tank et à quelques 25 km au nord de Minnipa, au nord-ouest de la péninsule d'Eyre.

2. Development by standing water

2.1. Pits

5Pits are hemispherical hollows developed by standing water in massive or isotropic granite. Some pits have been initiated on xenoliths or other chance pods or blebs richer in minerals that are particularly susceptible to weathering (Lester, 1938; Bourne and Twidale, 2002; Ferris et al., 1998). In most instances, however, the crucial evidence has been eliminated by the alteration and subsequent erosion that has given rise to the particular rock basin.

6Most pits are hemispherical and about a metre in diameter and of a similar depth. Some widen below the surface and are flask-shaped. In some instances, this is due partly to the precipitation of minerals from overflowing pool waters, induration by organisms such as cyanobacteria and lichens, The standing water and moisture retained in detrital fill supports a flora and fauna and persists longest in the deeper levels of the hollows.

7Hedges (1969) suggested that pits are merely youthful pans. However, they appear to reflect rock structure rather than stage of development. This can be demonstrated on Pildappa Rock (fig. 4). Pans abound on the gently sloping convex upward summit bevel of the residual. There and on the flanks of the hill the granite is laminated. The few pits are shaped in isotropic granite. One is developed in massive granite and exposed in a quite deeply incised Kluftkarren or fracture-controlled cleft. Another sits on a col so narrow that erosion from both flanks has removed most of a former sheet structure (fig. 1A).

Fig. 4 – Oblique aerial shot of Pildappa Rock seen from the northwest.
Fig. 4 – Photographie aérienne oblique du rocher Pildappa, vu du nord-ouest.

Fig. 4 – Oblique aerial shot of Pildappa Rock seen from the northwest.  Fig. 4 – Photographie aérienne oblique du rocher Pildappa, vu du nord-ouest.

Location of pits A and B indicated. The half million-gallon (almost 2 m liter) tank at top right is the storage for water drained from the Rock and directed to flow under gravity in a series of drains. It was one of several State Government water conservation schemes based on local inselbergs and constructed early last century.
Emplacement des vasques A et B indiqué par des cercles noirs. Le réservoir d'un demi-million de gallons (près de 2 000 m³) en haut à droite sert à stocker l'eau évacuée du rocher et écoulée par gravité dans une série de drains. Il s'agissait de l'un des programmes de conservation de l'eau mis en place par les gouvernements des États sur la base d'inselbergs locaux et construits au début du siècle dernier.

8Several large pits are located at the margins of granite outcrops which receive runoff from the adjacent higher slopes. One at Pygery Rocks is bordered by a ring of pitted rock (Smith, 1941; Twidale and Bourne, 1976) suggesting that the hollow was filled by sediment prior to being cleaned out (fig. 5A-B).

9A large pit situated on Turtle Rock, near Wudinna (fig. 5C) is indeed filled with vegetated mineral detritus. It is located on a fracture and is preserved on the upper beveled surface of the residual, which like several others in the district and elsewhere is stepped, indicating episodic exposure (Twidale and Bourne, 1975). The large pit is located in the former piedmont zone of a higher surface remnant preserved at the northwestern extremity of the residual. This is a site that, like the feature described from Pygery Rocks, also near Wudinna, would have received excess runoff and stands in marked contrast with the shallow pans scattered over the rest of the bevel.

10Very large hemispherical hollows are developed on the beveled summit of Uluru (Ayers Rock), shaped in steeply dipping Cambrian arkose and located in the arid interior of Australia (Sweet and Crick, 1992; Twidale, 2010). Many of these large depressions (diameter and depth of the order of a metre) carry small amounts of mineral detritus (fig. 5D). They may reflect long continued subsurface weathering during the later Mesozoic and Eocene when silcrete capping a thick kaolinized zone was formed over extensive areas of central Australia (Mabbutt, 1965; Wopfner et al., 1974; Wopfner, 1997). These depressions are additional to pits the deepening of which is facilitated by the steeply dipping close bedding of the arkose in which the inselberg is shaped (see also below).

Fig. 5 – Pits.
Fig. 5 – Vasques.

Fig. 5 – Pits.  Fig. 5 – Vasques.

A: Pit at margin of Pygery Rocks, near Wudinna, northwestern Eyre Peninsula. B: Upper limit of pool and fill marked by ring of pitted granite. C: Soil filled and vegetated pit located on fracture on beveled crest of Turtle Rock, near Wudinna. D: Hollows on crestal bevel of Uluru; Scale provided by rucksack (C. Wahrhaftig).
A : Vasque à exutoire sur les marges de Pygery Rocks, près de Wudinna, au nord-ouest de la péninsule d'Eyre. B : La limite supérieure du remplissage de la vasque (qui correspond au niveau de l’exutoire) est soulignée par les marques circulaires dans le granite. C : Grande vasque remplie par un sol et de la végétation. Elle est située à l'emplacement d'une fracture sur la crête de Turtle Rock, près de Wudinna. D : Très grandes vasques étagées sur la crête biseautée d'Uluru ; l'échelle est donnée par le sac à dos (C. Wahrhaftig).

2.2. Pans

2.2.1. Description

11Pans are relatively shallow flat-floored basins (fig. 1B). They are known by various names such as “panhole”, “weathering pan”, “pan gnamma”, and “weather pit” (Matthes, 1930; Smith, 1941; Goudie and Migon, 1997), but all are flat floored and shallow in relation to their plan dimensions and most have overhanging or undercut sidewalls. Whatever the precise name used, the pans defined and discussed here are comparatively small in size with depths measured in centimeters and plan diameters most commonly of the order of one meter and at most a few meters. They are destructional or erosional forms shaped in cohesive rock. They clearly differ from the ‘pans’ of southern Africa which are depositional and take the form of shallow depressions that range in area from tens of square meters to several square kilometres underlain by unconsolidated sediments. Though they carry water and form lakes after rains, they most frequently present as playas (claypans or salinas) with dry, flat, floors underlain by unconsolidated sediments (King, 1942; du Toit, 1954; Shaw and Thomas, 1989).

12Like pits, pans are shaped by pools of standing water that commonly are supplemented by mineral and organic detritus (fig. 1B, 6). They are the most common type of rock basin. Despite, or perhaps because of, their having been noted and studied for many years (Worth, 1953) pans have been the subject of several notable papers (Smith, 1941; Hedges, 1969) as well as innumerable incidental comments (Wilhelmy, 1958; Huber, 1987). The postulated association of pans and laminated rock calls for consideration, as does the origin of lamination.

Fig. 6 – Pools.
Fig. 6 – Mouilles.

Fig. 6 – Pools.  Fig. 6 – Mouilles.

A: Pool with water weeds, Little Wudinna Hill, north of Wudinna. B: Pan with desiccated sediment and algal remains Yarwondutta Rock.
A : Vasque remplie d’eau colonisée par un herbier aquatique, Little Wudinna Hill, au nord de Wudinna. B : Vasque asséchée avec des sédiments déshydratés et des restes d'algues, Rock Yarwondutta.

2.2.2. Lamination

13In their classification of gnammas, Twidale and Corbin (1963) suggested that pans in granite develop on flat or only gently inclined surfaces in rock that has been subdivided into flakes, scales, sheets, thin plates, or laminae that transect crystal boundaries as well as a range of textural features. The geometry of the surface is critical for the evolution of pans because laminae develop in rough parallelism with it whether on gentle inclines or steep, even overhanging, slopes (fig. 7). It was supposed that the characteristic pan geometry was due to the partings between laminae allowing water to spread laterally much more readily than vertically through flakes of cohesive country rock. Moreover, lamination introduces reinforcement or positive feedback, for the parting planes facilitate the penetration of water and hence further weathering.

Fig. 7 – Laminated granite exposed in sidewall of pan Yarwondutta Rock.
Fig. 7 – Exploitation des plans de foliation sur le bord d’une vasque dans des granites à texture orientée, Yarwondutta Rock.

Fig. 7 – Laminated granite exposed in sidewall of pan Yarwondutta Rock.Fig. 7 – Exploitation des plans de foliation sur le bord d’une vasque dans des granites à texture orientée, Yarwondutta Rock.

14The nature of lamination is more complex than initially recognised for it appears to be a convergent form that has formed several ways, though the common factor appears to be that confined pressures generate buttressed expansion (Folk and Patton, 1982). Lamination is an early stage of rock disintegration for it is found immediately adjacent to the weathering front, including discrete sectors preserved on corestones (Ruxton and Berry, 1957). Viewed overall, the weathering of granitic rocks in temperate and equatorial lands is overwhelmingly chemical (Klaer, 1956; Loughnan, 1969; Yatsu, 1988), for contact with water charged with chemicals and biota causes the alteration of mica and feldspar to produce clays, some of which are hydrophilic. Swelling causes physical expansion and the development of thin slivers of rock (Larsen, 1948; Ruxton and Berry, 1957; Eggler et al., 1969). This sequence appears to be responsible for most, though not all, of the lamination developed on the granite inselbergs of northwestern Eyre Peninsula. Lamination is not, however, peculiar to granite, for similar processes and outcomes are evidenced for instance in weathered basalt and norite (Loughnan, 1969; Hutton et al., 1977).

2.2.3. Other processes and mechanisms

15Physicochemical change accounts for much of the lamination developed in granitic rocks, but not all. As noted by Smith (1941) “Some basins have a thin layer of fresh granite on their floors…” suggesting that splitting can be developed without significant alteration (fig. 8A). Water layer or level weathering (Wentworth, 1938; Hills, 1949) and slaking, or alternations of wetting and drying, possibly involving water adsorption (White, 1976), have also been cited as possible reasons for the development of pans without significant alteration of the rock-forming minerals. The effectiveness of weathering by pool waters is demonstrated by the overhanging sidewalls typical of pans and the diameter of some examples which is a measure of the scarp recession in miniature driven by the pool waters etching the fresh granite (or sandstone, quartzite, or arkose), undermining the sidewall slopes, and inducing scarp retreat and pool extension (fig. 1B).

16In addition, physical effects some physical impacts call for consideration. For instance, haloclasty creates laminae as a consequence of physical expansion on the crystallization of halite from solution (Winkler and Singer, 1972; Bradley et al., 1978), and certainly halite is widely distributed in the Australian arid and semiarid zones (Hutton, 1976). Basins enlarged by salt weathering are well represented at coastal sites, and some workers (Timms and Rankin, 2016) consider it is a major force in basin development throughout northwestern Eyre Peninsula. However, the development of laminated granite in high rainfall areas of the equatorial and monsoon lands (Branner, 1896; Bakker, 1960), as well as humid midlatitude lands such as the central Appalachians (Smith, 1941), where salts tend to be washed through the system, show that other processes generated by ground water charged with chemicals and biota generate similar forms.

17Moreover, lamination is associated with shearing and compression. Consequently, the thin plates of fresh granite exposed in the floors of pans in the Appalachians and on north western Eyre Peninsula and elsewhere (fig. 8B) plausibly could be attributed to compressive stress, which, associated with earthquakes, has occurred in the recent past and continues (Twidale and Sved, 1978; Denham et al., 1979; Twidale and Bourne, 2000, 2014). This raises the question of whether the pans have acted as windows in which lateral pressure has found localized relief. Are they a very localized expression of lithostatic relief? Whatever their origin mineral weaknesses in the plates are exploited by water and gradually reduced to arkosic sand.

18Thus the floors of granitic pans, which Huber (1987) considered have “…not yet been completely explained”, plausibly can be attributed to a combination of buttressed crustal compression associated with weathering and/or crustal compression plus, and possibly, localized lithostatic unloading.

Fig. 8 – Granite plates.
Fig. 8 – Plaques de granite.

Fig. 8 – Granite plates.  Fig. 8 – Plaques de granite.

A: Remnants of once higher plate of fresh granite in floor of pan, Yarwondutta Rock. B: Plates of granite released during Minnipa Hill earthquake of January 1999 (see Twidale and Bourne, 2000).
A : Les résidus d'une plaque de granite frais sur le plancher de la vasque, Yarwondutta Rock. B : Plaques de granite déplacées dans l’exutoire d’une vasque lors du séisme de Minnipa Hill en janvier 1999 (voir Twidale et Bourne, 2000).

3. Pans in arenaceous sediments

19In granitic rocks the formation of pans is consistently associated with laminated rock, but what of pans developed in other lithologies, and in particular, the various arenaceous rocks in which pans are developed but which are not laminated?

20Pans are well developed in the essentially flat-lying Clarens (previously Cave) Sandstone exposed in the southern Drakensberg of South Africa (Truswell, 1977; Twidale, 1980) (fig. 9A). Pans occur also on the summit bevel of Uluru (fig. 9B). That arenaceous rocks are susceptible to lamination is demonstrated in piedmont zones both in the Drakensberg and on Uluru (fig. 9C-D) but lamination is not associated with the arenaceous pans. Indeed, on Uluru dipping strata rather than laminated rock are exposed in the sidewalls of some of the summit pans (fig. 9B). Several of the pans are unusually deep in terms of their diameter and their formation, like that of the previously mentioned its, may have been facilitated by the penetration of moisture along steeply dipping bedding planes in the country rock.

21It is concluded that though lamination is a potent factor that facilitates the formation of the flat floors characteristic of pans in granitic and similar bedrocks, such a fabric is not essential to their development. The water leveling process, evidently is sufficiently potent to generate - and extend - flat floored basins in both granite and arenaceous strata. Yet the process appears to have been relatively ineffective in pits, even where, as on the crest of Uluru, pits and pans occur in close proximity. Presumably the water and detritus charged with chemicals and biota accumulated in the longer term in pits is more effect than water level weathering.

Fig. 9 – Pans in arenaceous beds.
Fig. 9 – Vasques dans des formations arénacées.

Fig. 9 – Pans in arenaceous beds.  Fig. 9 – Vasques dans des formations arénacées.

A: Pans in flat-lying sandstone southern Drakensberg, RSA. B: Pans in arkose on summit of Uluru, central Australia Note steeply dipping grey-green strata expose in sidewall below weathered veneer, red in color and consisting of finely divided feldspar, each particle with a coating of hematite-goethite. C: Laminated sandstone on basal flared slope, southern Drakensberg. D: Laminated arkose exposed in piedmont of Uluru (C. Wahrhaftig).
A : Vasque sur un plan de grès au sud Drakensberg, RSA. B : Vasque dans les arkoses du sommet d'Uluru, centre de l'Australie. Remarquer les plans de stratification gris-vert plongeant abruptement dans le flanc de la vasque sous le placage altéré, de couleur rouge et constituées de feldspath finement divisé, chaque particule ayant un revêtement d'hématite-goethite. C : Grès stratifié sur la pente basale et évasée, sud Drakensberg. D : Arkose stratifiée et exposée dans le piémont d'Uluru (C. Wahrhaftig).

4. Fractures

22MacLaren (1912) drew attention to the many basins that have formed along partings. Granite is a crystalline rock that when fresh is of low porosity and permeability. It is, however, typically fissured and well jointed. Complex fracture patterns have given rise to basins of irregular plan form (fig. 10A). Some fractures are tight, do not permit water retention, and are not conducive to pool and basin development, but others are tensional, open, and favor the formation of pools and basin formation.

23MacLaren (1912) also referred to tanks as in water tank (Jutson, 1934) (fig. 10B) or cauldrons (Ormerod, 1859) which have formed at the intersection of several major fractures. Larger elongate forms developed along fracture zones or comparable linear zones of weakness known as bathtubs (fig. 10C). Some, like that illustrated by Boyé and Fritsch (1973) clearly was formed in the subsurface but whether all basins of similar morphology are of etch type or evolved after exposure, is not known.

Fig. 10 – Basin variations.
Fig. 10 – Variations au sein des cuvettes.

Fig. 10 – Basin variations.  Fig. 10 – Variations au sein des cuvettes.

A: Pan with irregular plan shape developed in Eocene quartzite, Mt Arden Creek Valley, southern Flinders Ranges. B: Sketch drawn from photograph in Jutson (1934), of tank or large pit in granite at Jumannia, in the Norseman district of the southeast of Western Australia. C: Bathtub developed on Lightburn Rocks, west of Gawler Ranges. No parting can be seen in the sidewall of the tub but one in line is visible on the slope beyond. The basin may be developed on a zone of strain susceptible to weathering and hence erosion. (Twidale and Vidal Romani, 2005).
A : Vasque à contours irréguliers développée dans un quartzite éocène, Mt Arden Creek Valley, au sud de Flinders Ranges. B : Croquis d'une cuvette d’altération ou d'une méga vasque en granite à Jumannia, dans le district de Norseman, au sud-est de l'Australie occidentale. Tiré d'une photographie de Jutson (1934). C : Vasque en ‘baignoire' ménagée sur les Lightburn Rocks, à l'ouest de Gawler Ranges. On ne voit aucune ligne de faiblesse structurale dans le flanc de la cuvette, mais une encoche linéaire est visible sur la pente en arrière-plan. La cuvette peut être développée sur une zone de déformation susceptible d'être altérée et donc érodée. (Twidale et Vidal Romani, 2005).

5. Water eyes

24Some shallow saucers developed in the humid tropics have been described as oricangas or water-eyes (Freise, 1936-38; Bremer, 1965) and illustrated by Bakker (1960) and Tschang, (1962). Presumably it was thought that shallow hollows filled with pools of water and standing on otherwise featureless rock surfaces resemble human eyes and they were called oricangas by inhabitants of the Oricanga River basin and olho d’aguas throughout Brazil.

25The name water-eye is however misleading. Externally the human eye consists of triangular sectors of the eyeball on each side of a circular pupil, and for some, the strings of depressions developed along major fractures, each consisting of a circular centre with alars or tapered triangular wings on either side and extending along a parting, are more eye-like (or oculiform), though this shape is lost where the depressions are occupied by sediment (fig. 11).

Fig. 11 – Fracture-controlled shallow clefts.
Fig. 11 – Fissures superficielles contrôlées par la fracturation.

Fig. 11 – Fracture-controlled shallow clefts.  Fig. 11 – Fissures superficielles contrôlées par la fracturation.

A: With ‘water eyes’ or oculiform basins on Disappointment Rock some 100 km west of Norseman, Yilgarn Craton Western Australia. B: Soil-filled water eyes on Peella Rock some 25 km ENE of Wudinna.
A : Chapelet de vasques coalescences appelées ‘water eyes’ à Disappointment Rock, à environ 100 km à l'ouest de Norseman, Yilgarn Craton Western Australia. B : Alignement de vasques en chapelets partiellement comblées par un sol sur Peella Rock, environ 25 km ENE de Wudinna.

6. Modifications by running water

6.1. Running water and renewal

26Rock basins developed on granitic massifs in midlatitude lands that receive regular and heavy rainfall frequently carry a pool of water that contains sediment and chemicals as well as biota. Some pools stand in basins that are clear of debris (fig. 12A) but as mentioned earlier most carry a partial cover of detritus derived from the weathering of the basin walls and floor or washed in from higher slopes and many pan floors are covered by mineral detritus at least for some of the time (fig. 6A). Moist detritus supports plants as do pool waters. Algal mats are commonplace, and like desiccated clays the mats release loose crystals and rock fragments from the pan floors (fig. 6B, 12B). The exploitation of weak minerals in the exposed floor has produced numerous small water filled depressions that herald the rotting and lowering of the pan floor (fig. 12C).

27The evacuation of the products of weathering responsible for the shaping of various rock basins has long been attributed to flowing water following heavy rains. Overflow not only removes soluble salts and fines in suspension, but also has replaced standing water rich in dissolved chemicals trapped in the basin pools with fresh, and hence more aggressive, water. Coarser fragments or less soluble minerals remain in the depressions where they are slowly reduced to a state in which eventually they can be removed (fig. 12D).

28In addition, clearance by humans to improve the water holding capacity of the basins, as for instance by Indigenous Australians, Afghan cameleers trading with interior settlements in southern Australia a century or so ago, and local pastoralists and farmers past and present, has also occurred (Bayly, 1999).

Fig. 12 – Pan fills.
Fig. 12 – Remblaiement dans les vasques.

Fig. 12 – Pan fills.  Fig. 12 – Remblaiement dans les vasques.

A: Clean pan on Haytor, Dartmoor, southwestern England. B: Algal mat in straight-sided pan. C: Pan on Hyden Rock, southwest of Western Australia, with coarse blocks awaiting rotting and disintegration.
A : Vasque nettoyée de ses débris sur Haytor, Dartmoor, sud-ouest de l'Angleterre. B : Tapis algal dans une vasque à parois droites. C : Vasque sur Hyden Rock, au sud-ouest de l'Australie occidentale, contenant des blocs grossiers en cours de désagrégation.

6.2. Cylindrical pits or hollows

29Most pits are shaped by standing water, but flowing water has evidently contributed directly to the modification of some basin forms. As noted by the farmers concerned, certain pits on the various Kwaterski Rocks, north of Minnipa, do not hold water. Removal of accumulated sediment in one revealed that the base of the 60 cm diameter hollow, at a depth of 1.2 m intercepted a sheet fracture along which incoming waters could escape. Spiral flow marks visible on the bedrock wall suggest that at times there have been considerable flows through the cylindrical hollow or depression (Twidale and Bourne, 1978).

30A similar hollow occurs at the margin of Myrtle Rock, northwest of Minnipa, where a cylindrical opening some 60 cm diameter is located close to the bedrock–soil junction and lower than the granitic platform or low rise from which it receives runoff. The hollow extends to a depth of about 230 cm, but only the uppermost 60 cm is defined by a cylindrical hollow with solid granite walls (fig. 13). Below that the aperture opens into a void that extends to a depth of a further 170 cm (230 cm from the surface) and laterally in all directions for several meters. The void is interpreted as a sheet fracture the opposed walls of which have been weathered and kaolinized (kaolinized granite in situ is exposed in an excavation some 50-70 m distant from the granite outcrop. Drainage along the fracture has allowed the evacuation of weathering products.

Fig. 13 – Diagrammatic section showing cylindrical hollow and associated underground chamber, at Myrtle Rock (for scale see text).
Fig. 13 – Coupe schématique montrant un trou cylindrique et la cavité souterraine associée, à Myrtle Rock (pour l'échelle, voir le texte).

Fig. 13 – Diagrammatic section showing cylindrical hollow and associated underground chamber, at Myrtle Rock (for scale see text).  Fig. 13 – Coupe schématique montrant un trou cylindrique et la cavité souterraine associée, à Myrtle Rock (pour l'échelle, voir le texte).

6.3. Armchair shaped hollows

31Most armchair-shaped hollows are initiated in laminated rock and stand in random isolation on bare rock slopes. They are asymmetrical in profile, with a high and steep upslope backwall but an open downslope aspect. Most are comparatively small (circa one-meter diameter) though much larger forms have been noted (fig. 14A). They have presumably been initiated by the exploitation of various moist sites in the country rock either at the weathering front (fig. 14B) or on the exposed flanks of the various bornhardts (fig. 1C). Subsequent development has been due partly to weathering by standing pools but, given their position of inclines, abrasion by running water armed with loose sand, and rendered turbulent by the roughness of the surface also is significant. Indeed, it appears that in time such hollows become linked by systems of gutters, as water flows over the open or lower side lips of the armchairs and gravitates toward other basins (fig. 15).

Fig. 14 – Armchair shaped hollows.
Fig. 14 – Vasques en fauteuil.

Fig. 14 – Armchair shaped hollows.  Fig. 14 – Vasques en fauteuil.

A: Large armchair–shaped hollow located on The Humps, near Hyden, Western Australia. B: Asymmetrical hollows initiated at the weathering front, Dumonte Rock, near Wudinna. Rivulets draining the exposed low dome or platform have flowed a short distance down the front creating isolated wet and hence weathered zones. X–X marks the former soil level.
A : Vasque en fauteuil située sur The Humps, près de Hyden, Australie-Occidentale. B : Creux asymétriques initiés sur le front d'altération, Dumonte Rock, près de Wudinna. Les rigoles drainant le dôme surbaissé ou la plate-forme se sont développées sur une courte distance, créant des zones humides isolées propices à l'altération. X-X marque l'ancien niveau de sol.

32Timms and Rankin (2016) have recently proposed a different causation for armchair hollows. They have produced plans of many rock basins, so detailed that it may be possible for future investigators (though unfortunately most likely not anyone alive today!) to identify rates of change. They rightly emphasize the role of haloclasty in the formation of rock basins (Bradley et al., 1978) though the reasoning behind this conclusion is flawed insofar as on northwestern Eyre Peninsula, the ocean is not the only source of salt in a salina-studded landscape. However, though they recognize that the weathering of granite is unequal, as is evidenced, for instance, by pitting, they reject the idea that random weaknesses in the surface exposures can be exploited eventually to form armchairs. Instead they attribute them to turbulent evorsive (or hydraulic) flows, that is, erosion by the force of water alone, accompanied by the downslope migration of pans. The authors do not explain either why basins have developed where they have, whether what is effectively identified as sheet flow can persist for any distance on what are in detail rough surfaces, or indeed why the implied lateral shift and relocation of the gnammas has occurred. Whether a single or a succession of pans is involved is not clear, and the deducible consequence of the hypothesis, namely, sets of basins aligned downslope and located in parallel in deep insets, are not matched by the field evidence, save where basin have developed along fractures. Thus, the basins featured in Figure 15 are offset in terms of downslope alignment.

Fig. 15 – String of armchair- shaped hollows linked by gutters on one of the Kwaterski Rocks, some 11 km north of Minnipa.
Fig. 15 – Succession de vasques en fauteuil reliées par des gouttières, sur l'un des rochers de Kwaterski, à environ 11 km au nord de Minnipa.

Fig. 15 – String of armchair- shaped hollows linked by gutters on one of the Kwaterski Rocks, some 11 km north of Minnipa.  Fig. 15 – Succession de vasques en fauteuil reliées par des gouttières, sur l'un des rochers de Kwaterski, à environ 11 km au nord de Minnipa.

6.4. Resonance pools

33Despite doubts concerning the recently suggested attribution of armchair hollows to runoff, asymmetrical basins shaped by running water are developed in groups or chains in preexisting narrow and shallow channels (fig. 16A). They develop downslope in cascading sequences in the direction of flow. They are the strings of beaded panholes of Pogue (2008) or Smith, (1941), who likened them to strings of sausages!

34Like the corrugations formed in unsealed (gravel) roads, such are still common in outback Australia, they are caused by resonance. An initial rut or depression generated by wheel erosion causes the redistribution of pressure and as a result ups and downs develop in the surface. Once such a transverse depression is formed caused by the rotating wheels of a motor vehicle, the oscillating wheel pressure causes similar forms to develop along the road in the direction of vehicle movement. This repetition is known as resonance.

35Similarly, in runnels formed by runoff such resonance causes flow oscillation and the formation of spaced depressions aligned the downstream channels (fig. 16B). Some pools are initiated behind resistant intrusive veins, others behind chance obstructions such as tree branches that initiate depressions, and hence potential pools. Whatever the initiating factor, however, once formed asymmetric hollows are repeatedly formed in sequence downslope and downstream - many armchair shaped basins are initiated and develop in channel floors. They originate partly as plunge pools, partly as rock basins. During dry spells in areas of episodic or intermittent (seasonal) rainfall regimes, such as are experienced in most of the areas considered in this essay, the basins are substantially shaped by weathering due to pools of standing water. During brief periods of possibly torrential rainfall and runoff there is erosion (abrasion) by turbulent water flows.

Fig. 16 – Resonance impacts.
Fig. 16 – Formes liées à la résonnance.

Fig. 16 – Resonance impacts.  Fig. 16 – Formes liées à la résonnance.

A: Resonance basins on Polda Rock. B: Spaced resonance depressions, Yarwondutta Rock.
A : Cannelure de vasques coalescentes sur Polda Rock. B : Dépressions de résonance espacées, Rock Yarwondutta.

36That pool weathering is the dominant process is suggested by a number of lines of evidence and argument. For example, like other parts of southern South Australia occupied by the Crocker Dunefield (Twidale et al., in press), northern Eyre Peninsula was a desert during the later Pleistocene, circa 300 k–25 k years ago (Hilgers et al., 2011). During that extended period runnels would have flowed infrequently, but pools of standing water, some fed by seepages, would have persisted after flows had ceased.

37The conservation of the bars separating pools also argues that abrasion by flowing water is minor and that the main contribution of flowing water may be to transport the products of pool weathering, that is erosion, the lowering of the basin floor, but not as result entirely or even largely of abrasion. Some deeply incised basins are developed in low gradient runnels serving only comparatively small catchments, by contrast with shallow basins found developed in runnels high on the slope.

7. Conclusions

38The 1963 morphogenetic classification of rock basins into pits pans and armchair shaped hollow differentiated on the basis of rock fabric and surface slope remains useful but additional factors and forms are now recognized. In particular, fractures are lines of weakness that have allowed the formation of eye-shaped basins. Tanks have been formed on major fractures and particularly fracture intersections. Vegetated basin fills have contributed markedly to the formation of large tanks and pits. Whereas basins were correctly attributed largely to alteration by standing water charged with chemicals and biota, it is now clear that running water has played a significant part in basin developments by replacing saturated with fresh water, by modifying basins, by generating cylindrical forms from pits, and initiating resonance hollows in channels draining upland. Bearing in mind these factors it may be appropriate to redefine gnammas as rock basins that were shaped initially and primarily by pool water and associated minerals and organic detritus, but in many instances modified by running water.

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Annexe

Version française abrégée

La distinction en trois types de cuvettes (vasques à encorbellement, vasque à fond plat, vasque en fauteuil) est utile, mais nécessite un examen minutieux. Elles sont toutes formées par l'eau stagnante et les sédiments provenant de l'altération de la roche encaissante (généralement granitique ou gréseuse) et chargés de résidus chimiques et organiques. Ces formes sont le plus souvent initiées à partir de dépressions causées par une altération inégale en fonction de la texture sur le front d'altération et évoluent à l’air libre après évacuation des altérites. Elles diffèrent également en fonction de l'agencement structural et de l'inclinaison de la surface.

Les vasques à encorbellement (pits) sont hémisphériques et se développent sur un plan ou sur des surfaces légèrement inclinées à partir d'une roche massive. Les vasques à fond plat (pans) sont en général peu profondes. Dans le granite, leur formation est facilitée par la texture foliée. Les vasques en fauteuil (armchair-shaped hollow) se développent sur des pentes plus raides. Cependant les vasques développées dans le grès (quel que soit le pendage) montrent que l'altération guidée par le niveau de l'eau est le processus de base. Certaines fractures retiennent l'eau et favorisent la formation de bassins. Ils sont également exploités pour former des cuvettes comme les vasques-baignoires et des chapelets de vasques coalescentes (water-eyes).

L’eau de ruissellement contribue à l'évolution des vasques de plusieurs façons, quand par exemple les eaux riches en sels dissous remplissant les creux sont remplacées par de l'eau douce plus agressive ou encore quand les sédiments qui s'y accumulent sont évacués. Là où les cavités sont guidées par les fissures de la roche, l'eau courante peut les sculpter en forme de creux cylindriques. Sur les pentes plus raides, des séries de vasques en fauteuil qui s’organisent dans des gouttières de ruissellement sont en partie façonnées en marmites par des écoulements tourbillonnaires, en particulier quand le climat favorise un écoulement intermittent ou épisodique, capable de remplir les cavités d'eau stagnante.

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Table des illustrations

Titre Fig. 1 – Rocks basins. Fig. 1 – Vasques.
Légende A: Pit on Pildappa Rock, near Minnipa, northwestern Eyre Peninsula, South Australia. Note ferruginous staining on rim of basin and blocky remnants of former sheet structure. B: Pan on Yarwondutta Rock, near Minnipa, north western Eyre Peninsula. Note flat floor, overhanging sidewalls, sediment on floor, and overflow channel on near side. C: Armchair shaped hollows on northern slope of Pildappa Rock.A : Vasque (Pit) sur Pildappa Rock, près de Minnipa, au nord-ouest de la péninsule d'Eyre, Australie-Méridional., Notez la coloration ferrugineuse sur le rebord de la cavité et les restes de blocs de l'ancienne structure desquamée. B : Vasque à encorbellement (pan) sur le rocher Yarwondutta, près de Minnipa, au nord-ouest de la péninsule d'Eyre. Noter le plancher plat, les parois latérales en surplomb, les sédiments sur le fond et le chenal de débordement. C : Les vasques en fauteuil avec exutoire sur le versant nord du rocher Pildappa.
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Titre Fig. 2 – Location maps. Fig. 2 – Carte de localisation des sites.
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Titre Fig. 3 – Irregular surface of a granite platform, north of Podinna Tank and some 25 km, north of Minnipa, northwestern Eyre Peninsula. Fig. 3 – Surface d'une plateforme granitique irrégulière, au nord de Podinna Tank et à quelques 25 km au nord de Minnipa, au nord-ouest de la péninsule d'Eyre.
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Titre Fig. 4 – Oblique aerial shot of Pildappa Rock seen from the northwest. Fig. 4 – Photographie aérienne oblique du rocher Pildappa, vu du nord-ouest.
Légende Location of pits A and B indicated. The half million-gallon (almost 2 m liter) tank at top right is the storage for water drained from the Rock and directed to flow under gravity in a series of drains. It was one of several State Government water conservation schemes based on local inselbergs and constructed early last century. Emplacement des vasques A et B indiqué par des cercles noirs. Le réservoir d'un demi-million de gallons (près de 2 000 m³) en haut à droite sert à stocker l'eau évacuée du rocher et écoulée par gravité dans une série de drains. Il s'agissait de l'un des programmes de conservation de l'eau mis en place par les gouvernements des États sur la base d'inselbergs locaux et construits au début du siècle dernier.
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Titre Fig. 5 – Pits. Fig. 5 – Vasques.
Légende A: Pit at margin of Pygery Rocks, near Wudinna, northwestern Eyre Peninsula. B: Upper limit of pool and fill marked by ring of pitted granite. C: Soil filled and vegetated pit located on fracture on beveled crest of Turtle Rock, near Wudinna. D: Hollows on crestal bevel of Uluru; Scale provided by rucksack (C. Wahrhaftig). A : Vasque à exutoire sur les marges de Pygery Rocks, près de Wudinna, au nord-ouest de la péninsule d'Eyre. B : La limite supérieure du remplissage de la vasque (qui correspond au niveau de l’exutoire) est soulignée par les marques circulaires dans le granite. C : Grande vasque remplie par un sol et de la végétation. Elle est située à l'emplacement d'une fracture sur la crête de Turtle Rock, près de Wudinna. D : Très grandes vasques étagées sur la crête biseautée d'Uluru ; l'échelle est donnée par le sac à dos (C. Wahrhaftig).
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Titre Fig. 6 – Pools. Fig. 6 – Mouilles.
Légende A: Pool with water weeds, Little Wudinna Hill, north of Wudinna. B: Pan with desiccated sediment and algal remains Yarwondutta Rock. A : Vasque remplie d’eau colonisée par un herbier aquatique, Little Wudinna Hill, au nord de Wudinna. B : Vasque asséchée avec des sédiments déshydratés et des restes d'algues, Rock Yarwondutta.
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Titre Fig. 7 – Laminated granite exposed in sidewall of pan Yarwondutta Rock.Fig. 7 – Exploitation des plans de foliation sur le bord d’une vasque dans des granites à texture orientée, Yarwondutta Rock.
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Titre Fig. 8 – Granite plates. Fig. 8 – Plaques de granite.
Légende A: Remnants of once higher plate of fresh granite in floor of pan, Yarwondutta Rock. B: Plates of granite released during Minnipa Hill earthquake of January 1999 (see Twidale and Bourne, 2000). A : Les résidus d'une plaque de granite frais sur le plancher de la vasque, Yarwondutta Rock. B : Plaques de granite déplacées dans l’exutoire d’une vasque lors du séisme de Minnipa Hill en janvier 1999 (voir Twidale et Bourne, 2000).
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Titre Fig. 9 – Pans in arenaceous beds. Fig. 9 – Vasques dans des formations arénacées.
Légende A: Pans in flat-lying sandstone southern Drakensberg, RSA. B: Pans in arkose on summit of Uluru, central Australia Note steeply dipping grey-green strata expose in sidewall below weathered veneer, red in color and consisting of finely divided feldspar, each particle with a coating of hematite-goethite. C: Laminated sandstone on basal flared slope, southern Drakensberg. D: Laminated arkose exposed in piedmont of Uluru (C. Wahrhaftig). A : Vasque sur un plan de grès au sud Drakensberg, RSA. B : Vasque dans les arkoses du sommet d'Uluru, centre de l'Australie. Remarquer les plans de stratification gris-vert plongeant abruptement dans le flanc de la vasque sous le placage altéré, de couleur rouge et constituées de feldspath finement divisé, chaque particule ayant un revêtement d'hématite-goethite. C : Grès stratifié sur la pente basale et évasée, sud Drakensberg. D : Arkose stratifiée et exposée dans le piémont d'Uluru (C. Wahrhaftig).
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Titre Fig. 10 – Basin variations. Fig. 10 – Variations au sein des cuvettes.
Légende A: Pan with irregular plan shape developed in Eocene quartzite, Mt Arden Creek Valley, southern Flinders Ranges. B: Sketch drawn from photograph in Jutson (1934), of tank or large pit in granite at Jumannia, in the Norseman district of the southeast of Western Australia. C: Bathtub developed on Lightburn Rocks, west of Gawler Ranges. No parting can be seen in the sidewall of the tub but one in line is visible on the slope beyond. The basin may be developed on a zone of strain susceptible to weathering and hence erosion. (Twidale and Vidal Romani, 2005). A : Vasque à contours irréguliers développée dans un quartzite éocène, Mt Arden Creek Valley, au sud de Flinders Ranges. B : Croquis d'une cuvette d’altération ou d'une méga vasque en granite à Jumannia, dans le district de Norseman, au sud-est de l'Australie occidentale. Tiré d'une photographie de Jutson (1934). C : Vasque en ‘baignoire' ménagée sur les Lightburn Rocks, à l'ouest de Gawler Ranges. On ne voit aucune ligne de faiblesse structurale dans le flanc de la cuvette, mais une encoche linéaire est visible sur la pente en arrière-plan. La cuvette peut être développée sur une zone de déformation susceptible d'être altérée et donc érodée. (Twidale et Vidal Romani, 2005).
URL http://journals.openedition.org/geomorphologie/docannexe/image/11880/img-10.png
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Titre Fig. 11 – Fracture-controlled shallow clefts. Fig. 11 – Fissures superficielles contrôlées par la fracturation.
Légende A: With ‘water eyes’ or oculiform basins on Disappointment Rock some 100 km west of Norseman, Yilgarn Craton Western Australia. B: Soil-filled water eyes on Peella Rock some 25 km ENE of Wudinna. A : Chapelet de vasques coalescences appelées ‘water eyes’ à Disappointment Rock, à environ 100 km à l'ouest de Norseman, Yilgarn Craton Western Australia. B : Alignement de vasques en chapelets partiellement comblées par un sol sur Peella Rock, environ 25 km ENE de Wudinna.
URL http://journals.openedition.org/geomorphologie/docannexe/image/11880/img-11.png
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Titre Fig. 12 – Pan fills. Fig. 12 – Remblaiement dans les vasques.
Légende A: Clean pan on Haytor, Dartmoor, southwestern England. B: Algal mat in straight-sided pan. C: Pan on Hyden Rock, southwest of Western Australia, with coarse blocks awaiting rotting and disintegration. A : Vasque nettoyée de ses débris sur Haytor, Dartmoor, sud-ouest de l'Angleterre. B : Tapis algal dans une vasque à parois droites. C : Vasque sur Hyden Rock, au sud-ouest de l'Australie occidentale, contenant des blocs grossiers en cours de désagrégation.
URL http://journals.openedition.org/geomorphologie/docannexe/image/11880/img-12.png
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Titre Fig. 13 – Diagrammatic section showing cylindrical hollow and associated underground chamber, at Myrtle Rock (for scale see text). Fig. 13 – Coupe schématique montrant un trou cylindrique et la cavité souterraine associée, à Myrtle Rock (pour l'échelle, voir le texte).
URL http://journals.openedition.org/geomorphologie/docannexe/image/11880/img-13.png
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Titre Fig. 14 – Armchair shaped hollows. Fig. 14 – Vasques en fauteuil.
Légende A: Large armchair–shaped hollow located on The Humps, near Hyden, Western Australia. B: Asymmetrical hollows initiated at the weathering front, Dumonte Rock, near Wudinna. Rivulets draining the exposed low dome or platform have flowed a short distance down the front creating isolated wet and hence weathered zones. X–X marks the former soil level. A : Vasque en fauteuil située sur The Humps, près de Hyden, Australie-Occidentale. B : Creux asymétriques initiés sur le front d'altération, Dumonte Rock, près de Wudinna. Les rigoles drainant le dôme surbaissé ou la plate-forme se sont développées sur une courte distance, créant des zones humides isolées propices à l'altération. X-X marque l'ancien niveau de sol.
URL http://journals.openedition.org/geomorphologie/docannexe/image/11880/img-14.png
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Titre Fig. 15 – String of armchair- shaped hollows linked by gutters on one of the Kwaterski Rocks, some 11 km north of Minnipa. Fig. 15 – Succession de vasques en fauteuil reliées par des gouttières, sur l'un des rochers de Kwaterski, à environ 11 km au nord de Minnipa.
URL http://journals.openedition.org/geomorphologie/docannexe/image/11880/img-15.png
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Titre Fig. 16 – Resonance impacts. Fig. 16 – Formes liées à la résonnance.
Légende A: Resonance basins on Polda Rock. B: Spaced resonance depressions, Yarwondutta Rock.A : Cannelure de vasques coalescentes sur Polda Rock. B : Dépressions de résonance espacées, Rock Yarwondutta.
URL http://journals.openedition.org/geomorphologie/docannexe/image/11880/img-16.png
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Charles Rowland Twidale et Jennifer Anne Bourne, « Rock basins (gnammas) revisited  », Géomorphologie : relief, processus, environnement, vol. 24 – n° 2 | 2018, 139-149.

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Charles Rowland Twidale et Jennifer Anne Bourne, « Rock basins (gnammas) revisited  », Géomorphologie : relief, processus, environnement [En ligne], vol. 24 – n° 2 | 2018, mis en ligne le 08 janvier 2018, consulté le 21 septembre 2018. URL : http://journals.openedition.org/geomorphologie/11880 ; DOI : 10.4000/geomorphologie.11880

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Auteurs

Charles Rowland Twidale

Geology and Geophysics, School of Physical and Earth Sciences, University of Adelaide – Adelaide, South Australia (rowl.twidale@adelaide.edu.au). Tel:+61 (0)8 8313 5392.

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Jennifer Anne Bourne

Geology and Geophysics, School of Physical and Earth Sciences, University of Adelaide – Adelaide, South Australia. Deceased 20 November 2015.

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