Regions > Western Cape

Kalk Bay Mountain

Peter Swart 11 December 1994

Introduction

The sandstone/quartzite mountains above Kalk Bay show a number of geological features more commonly associated with limestone terrain. This article gives a general description of the area, and gives a more detailed description of some of the karst-like features.

Location

The area described in this article is covered by the 1:50000 Geological Survey map; 3418 AB & AD. [Ref 7] It is a small section of mountain stretching from the Fish Hoek valley in the south, to the Ou Kaap se Weg road in the north and west. The eastern border of the area is bounded by Boyes Drive. The area forms the southern part of the Silvermine Nature Reserve.

General Description

The rock in the area consists of a layer of quartzitic sandstone on top of a thin bed of reddish sandstone and mudstone. [Ref 6] The mountains reach a maximum altitude in the north of the area at Upper Steenberg Ridge at an altitude of 537m. A plateau bordered by ridges running north-west/south-east, extends over part of the northern section. The main drainage is via the Silvermine River. The southern section consists of a series of north-west/south-east oriented ridges and valleys.

The sandstone occurs massively and thinly bedded units which may alternate with each other [Ref 6]. These are extensively jointed. In some cases the joints have been enlarged to form troughs which can be over 10m deep.

All of the major valleys carry small streams, most of which run dry towards the height of summer. Doline like depressions occur in these valleys.

There are a number of caves in the area. These range from a few metres in length to Boomslang, at over 600m. The deepest known cave is Drip Water Pot at 25m.

Karst Features found in the area

Karren, Clints and Grikes

Large areas resembling karren occur on or near the top of all of the ridges in the area. The best examples of this can be seen on Upper Steenberg Peak [Figure TBD] and Kleintuinkop. [Figure TBD pic rocktown] In these areas, the rock consists of large clint like blocks separated by grike like troughs. The troughs vary from 0.8m to 2.5m wide, and can be up to 4m deep. In certain cases, these trough structures become much deeper than those which occur in the immediate vicinity, and sometimes form the entrance to caves. Sunbeam Cavern is an example of one of the deeper troughs. It is 18m deep and the top is covered by a rock collapse. This collapse forms part of the roof of the cave. [Ref 5]. Another good example of a giant grike is the entrance to Robin Hood's Cavern. The trough is approximately 25m long, and 22m deep at its deepest. This forms one of the entrances to Ronan's Well, a major cave system in the area. [Ref 1]

Sinkholes, dolines

Sinkholes occur in three of the bigger valleys. [See fig TBD] A number of these are very small, 2m diameter x 1m deep, but are clearly defined depressions. The larger depressions are more than 20m long and 10m deep, and contain piles of boulders, some of which appear to have been part of the collapse, and some have fallen from the surrounding cliffs. Most of the depressions are quite stable, but a least one is still very active. This collapse, at the top of Echo valley, first appeared in the mid 1970s. It has grown wider and deeper each winter, and is currently approximately 14m x 11m in size, and 3m deep at its deepest. Sand eroded from the floor and walls of the depression disappears down a hole in the bottom of the sink. [See Survey of Sunbeam Cavern 5]. [Doline TBD in figure TBD]

Another large sink occurs above the Amphitheatre. The depression is TBDm long by TBDm wide, and is the highest collapse of this size on the mountain. Amongst the boulders at the southern end of the depression is the entrance to Drip Water Pot, a 25m deep cave [Ref 2]

The two indigenous forests in the area, in Spes Bona Valley and in Echo Valley, have boulder strewn, irregular floors. Although there are indications that there are depressions in these forest floors, the identification of these is made very difficult by the dense vegetation, and the debris which has fallen from the cliff walls.

Insurgences and Resurgences

There are many small streams throughout the area, most of which stop running at the height of the dry season. These streams either drain off to the northwest, and end up in the Silvermine River, thence into False Bay, or run off to the southeast, and directly into False Bay.

There appear to be two broad types of resurgence, the cliff resurgence and the valley resurgence. The cliff resurgences occur where streams emerge from the cliff walls, and flow down and feed the bigger streams in, and under, the valleys floors. The majority of these occur on the southwest facing cliffs and occur at widely varying altitudes. Caves entrances on the northeastern slopes of the valleys are seldom resurgences. In caves such as Bettie's cave, NW Kleintuinkop, the water runs into the cave, and soaks into the sand in a terminal chamber.

The valley resurgences occur where the already consolidated streams emerge from the valley floors. According to [Ref 7], these occur at the following altitudes:

410m Spes Bona Valley 400m Pecks Valley 320m Kleintuinkloof 320m Bailie's Valley 310m Echo Valley

Theses resurgences are all quite small, and are not the entrances to any currently known caves.

Caves

There are about 70 known caves in the area described. Although some of these caves are little more than 10m long, some of the them are over 500m long. Examination of the surveys of these caves shows that they appear to be formed along well defined joints, and show evidence of being formed by dissolution. [Ref 3]. In a number of cases, (Drip Water Pot/Tjoklet's Grotto, Ronan's Well, and Boomslang), water is carried either through or part way through a ridge. This shows that the direction of flow of underground water can be independent of the surface terrain.

The cave passages have a number of different characteristic cross sections. These vary from low, wide passages to narrow vertical cracks. The low, wide passages appear to correspond to bedding planes within the rock, and the vertical cracks appears to be widened vertical joints. A number of the cave passages end in domed terminal chambers. The joint along which the passage formed can usually be seen in the roof and floor of such chambers.

Although no spectacular spelothems occur in the caves, they are not entirely devoid of formations. Small 5mm to 10mm long, stalactite like nodules occur on the ceilings of many caves. These appear to consist of a mudstone like substance. No attempt to determine the composition of these nodules.

Other features often found in the caves are pillars. Some are free standing on the floor, and some are attached to both floor and ceiling. Partially formed pillars which are not separated from the cave walls, are also found.

Rock Features

Although the large bridges and arches sometimes found in limestone do not occur in the study area, many small arches do occur. These vary from less than a metre high to over 2m high. [see fig TBD] The most notable of these can be seen at the northern end of Echo Valley, and on top of Kalk Bay Mountain. Other solution features can also be found. The small flutes which often occur in limestone can be found on the surface rock. A good example of this occurs above the entrance to Climax Cave on Kalk Bay Mountain. [fig TBD]

In apparently solid rock, small tubes which penetrate into the stone can be found on most parts of the mountain. [See fig TBD] These can be many centimetres deep and often are divided laterally by thin rock ribs. During wet weather, water and silt can be seen pouring from some of these tubes.

Conclusion

From the above description, it can been seen that a number of features more commonly associated with karst landscape can be found in the quarztitic sandstone mountains above Kalk Bay. If these features are formed by the dissolution of limestone in karst areas, and the physical appearance of the features in limestone and quartzite are similar, it is not unreasonable to suggest that they may have been formed by similar processes in quartzite.

Whether this is called Karst or Pseudo-karst is a question to keep the academics busy. As it is only the rock that is not limestone, but all of the features associated with karst are present, I suggest that this landscape should be referred to as sandstone karst.


References

[1] Hine, S. & Hitckcock, A.N (TBD) SASA Bull

[2] Hitckcock, A.N. & Swart, P.G. (Unpublished) Drip Water Pot

[3] Kavalieris, I. (1977) The Formation of Ronan's Well Cave, SASA Bull.

[4] Martini J. (1984), The Rate of Quartz Dissolution and Weathering. The Bulletin of the South African Spelaeological Association.

[5] Swart, P.G. (Unpublished) Sunbeam Cavern

[6] Theron, J.N. 1984. The Geology of Cape Town and Environs, Department of Mineral and Energy Affairs, Geological Survey

[7] Theron, J.N. 1984. MAP: 3418 AB & AD KAAPSE SKIEREILAND 1:50000 Geological Series 1984

Houwhoek Adullam Cave

Kalk Bay Mountain