Gansbaai Area

A.Hitchcock, P.Swart, August 2003

Geology

The Heidehof caves are formed in a series of low (225m max) hills known as Die Koppies. These hills are formed from a cream coloured, crumbly limestone of the Bredasdorp group. (1) The limestone probably originated as wind-blown calcareous sand dunes on the coastal plain. These dunes later consolidated into loosely consolidated limestone.

The Byeneskrans Cave is an archaeological site in the same area.

There are also caves at sea level along the coast. These caves have formed by sea action against the low coastal limestone cliffs.

Speleogensis

Most of the caves are generally horizontal, with little vertical development. The floors are covered in fine light-coloured dust and sand. There are isolated rocks on the floors, and slabs of rock that appear to have fallen from the ceiling. The horizontal nature of the caves, and the lack of distinctly cut water channels, indicate that they were formed by dissolution of the parent limestone below the water table. The slabs probably fell from the cave ceilings when the water levels dropped, and the weakened ceilings were no longer able to support their own weight.

Caves

Heidehof Caves 1 & 2

The first discovery was a disappointing 15m long cave situated in the hill a couple of hundred meters east of the windmill. The second cave is situated on the lower slopes of a hill approximately half way between the windmill and the farm house. This cave proved more promising and challenging and occupied Greg Waller for a few hours. It consists of a low passage and is rather unstable at the entrance. Concerted digging yielded about 15m of tight passage that continues as a low shelf.

Martha’s Cave

Martha’s Cave is situated in a small alcove in the hill behind the farm house. The low and narrow entrance is on the left-hand side of alcove (facing the hill), and the sandy floor is strewn with rocks. There appears to be another entrance on the right-side, as light enters the first chamber from area. This second entrance was not investigated due to the thick surface vegetation. This first chamber is approximately 8-10 across, 6m deep and less than a metre high throughout.

There are two passages leading off the first chamber. The first is approximately opposite the main entrance and the second is to the right of the first. The first passage is about 4m long, but becomes low and narrow. The second passage is about 12m long and very broken. This passage also becomes too low and narrow for further progress.

These caves were not surveyed.

Other Caves

For Bat Cave, Dassie Cave and Martha's Cave, see the associated cave folders.

Biota of Heidehof Caves

The caves at Heidehof are very shallow, and closely linked to the outside environment. The animals that are found in this type of cave are usually not specifically adapted to cave life, but rather use the caves for part of their life cycle.

Mammals

Although no porcupines were resident during our explorations, the number of quills and large bones in the caves indicate that porcupines often frequent the caves. Whilst exploring Dassie Cave, Stephan came face to face with a small mouse. This was approximately 20m from the current entrance to the cave.

Bat Cave is so named for the resident population of Horse Shoe bats. Although no measurements of the bats were taken, they appeared to be quite small for Rhinolophus, and are possibly Rhinlophus capensis, the endemic Cape Horseshoe bat. Roosting in amongst the Rhinolophus were a few Miniopterus. sp.

The bats were all in deep torpor when we entered the cave, and woke up very slowly as we moved about. Some of the bats fell to the floor of the cave while waking, and took about 10-20 seconds before flying off from the ground. Disturbances of this nature are very disruptive for the bats and potentially life threatening. Visits to this cave should therefore be kept to an absolute minimum, especially during winter.

Invertebrates

The caves are rich in organic material due to the presence of the mammals, which make an ideal hunting ground for invertebrates. We found an orange coloured, parasitic winged fly of the family Streblidae. Despite our efforts, we could not catch the fly for further identification. Many crevices in the ceiling of Bat Cave was crammed full of another blood sucking parasite, and relative of the human Bed Bug. These insects belong to the family Cimicidae(2a). Most of these appeared to have fed well, and had dark red, blood filled abdomens.

We also found a number of small dark hairy beetles These were later identified as Moldy Beetles(2b). We also found large, dry looking ticks on the floor of the cave.

Apart from the bats, the most visible forms of life in the cave are the Woolly Worms. These are the caterpillars of the moth Apisa canescens(2c), and are common in caves along the southern coast of the Western Cape, although appear to be absent from the Cape Peninsula. The caterpillars are quite inactive until one shines light on them and then they move off quickly. They spin complicated, messy nests from the ceiling in which they pupate. A few of the webs were also simple and conical in shape and contained the remains of pupae.

References

1. Gresse, P.G., Theron, D. The Geology of the Worcester Areas, and accompanying sheet 3319 Worcester 1:250000 (1997) Geological Survey of South Africa

2. Picker, M., Griffiths, C., Weaving, A, Field Guide To Insects of South Africa, (Struik 2002)
   a. Bed Bugs (Family Cimicidae) p116 no 2
   b. Mouldy Beetles (Eurychora sp?) in bat dung p 230 no 4
   c. Woolly Worms, Apisa canescens p352 no 5