Vlêrmuis Grotte, Groot Winterhoek

 

Peter Swart

5 December, 1998

 

Location

 

There are a number of small sandstone caves in the Groot Winterhoek Wildness Area. All of these caves fall under the management of the Cape Nature Conservation Board. Permission to visit these caves must be obtained from the CNCB.

 

Caves around Die Hel pool

(pdf) Survey - Die Hel complex

Cave A, Vlêrmuis Grot

(pdf) Survey - Cave B

Cave A, the most conspicuous of the three caves, consists of a large passage, of approximately rectangular cross section, reaching 147m into the mountain. Approximately 80m from the entrance is a chamber, the roof of which is 15m high and the lower 8m is filled with bat guano and boulders. A stream issues from a crack at the very end of the cave, flows along the wall and disappears under the guano. It reappears on the eastern side, and flows out of the cave entrance, into the Groot Kliphuis river. There are a few small side passages, but these are little more than narrow cracks, when compared to the main passage.

 

Amphipods and white worms were found in the stream, and many insects and spiders were collected and sent to UCT for identification. A small mixed colony of fruit bats and Cape Horseshoe bats inhabit the main chamber, and the entrance passage of the cave. The first 40m of cave floor is covered with driftwood, which lies up to half a metre deep in places.

 

Cave B

(pdf) Survey - Cave A

Cave B is the southern most cave on the eastern side of the pool. The northern section consists of a large Y shaped passage, with an entrance at the tip of each fork of the Y. There is a large pillar at the junction of the passages, and the remnants of another two pillars on the roof. The stem of the Y is a narrow passage, 6m high by 0.5m wide and 35m long. The ceiling of the cave is flat and level.

 

The southern part of the cave has a lower and upper section. The entrance is in the lower section, and the entrance passage terminates in a flat wall, which has a stream emerging from beneath it, at floor level. The upper section of the cave is one large chamber, 20m long by 4m wide, with piles of sand and rubble on the floor. A stream emerges from the southern side, and disappears into a the boulder choke in the floor. This chamber was full of Miniopterus schreibersii bats during our November visit.

 

Minerals

 

At the end of the long thin passage in cave B, strata in the passage walls are quite conspicuous. There are bands of a soft clay/kaolin like material, some of which are pure white, while others are mustard coloured. Close to ceiling are the remains of a band of conglomerate, which is cemented to the wall.

 

Flowstone-like formations occur at the entrance to the large, upper chamber. These vary in colour from dark brown, to red, to a dark yellow in colour. They appear to consist of hardened clay.

 

Bats

 

During the survey, most of the bats that were captured were males. As the cave is a breading cave, and the young are usually born during summer, it appears that the males migrate to this cave before the females. More research is required to determine when the bats arrive, and why the males arrive before the females.

 

Cave C

(pdf) Survey - Cave C

Cave C is the northern most cave on the eastern side of the pool. The cave can be split into three parts: The eastern chamber, midsection, and the western network section. The eastern chamber has a high level entrance on a cliff side balcony, and is linked to the network section via a narrow passage and a small chamber, one side of which has collapsed. Most of the entrances of the network section of the cave open out directly onto the pool.

 

The balcony entrance to the eastern side of the cave was formed by a collapse of part of the cliff. The collapse must be fairly old, as the amorphous silica formations, examples of which are found throughout the cave, have formed on top of some of the collapse debris. This entrance leads down a steep, narrow section of passage, to a large chamber. (24m long by 4m-5m wide) Like most of the passages in this cave, it is much higher than it is wide (6m high x 1m wide). The floor of the passage consists of course grains of deep red silica material. The floor of the chamber, to which the entrance passage leads, slopes up towards the east. Bats roost in the roof of this chamber, and much guano lies piled on the floor. Deep red flowstone-like formations decorate small sections of the wall of the chamber and small stalactites (approximately 1cm long x 4mm diameter) were found. No minerals were collected, so the composition of these formations is not known.

 

The mid-section of the cave consists of another vertical passage, which leads to a small chamber. This chamber opens onto the same collapse, which forms the balcony entrance, and also leads down to the network section. A stream flows from the eastern chamber, through the midsection, and towards the network section. The floor of most of this midsection of the cave is made up of fine, red dust.

 

As the plan of the cave shows, the network section consists of a number of parallel passages, all running approximately northwest. These are interlinked by smaller cross passages. Some of the cracks, in which the parallel passages have developed, have passage at more than one level. These passages appear to have been truncated by the development of the river gorge, as they all end in entrances overlooking the pool. Much drift wood and river debris has collected in these entrances, some of it more than 4m above the current water level. From the outside, the driftwood caught in the cave entrances looked like a number of giant birds nests, so this series of entrances became known as Birds Nest Windows.

 

Biology

 

Many invertebrates were collected from the cave, although none have yet been identified.

 

A group of well over a hundred bats was found in the eastern chamber, and a number of Horseshoe bats, some of which were carrying babies, were found in the network section. A ghost frog was also found in the twilight zone of the network section of the cave.

 

Notes:

 

Species collected:

  • Invertebrates - not yet known

 

Bats:

  • Rhinolophus capensis (Cape Horseshoe bat)
  • Rhinolophus clivosus (Geoffory's Horseshoe bat)
  • Miniopterus schreibersii
  • Miniopterus ??
  • Rousettus aegyptiacus leachii (Cape fruit bat??) (not confirmed)

 

Frogs:

  • A ghost frog was observed in the twilight zone of Cave C.