As we saw in the Gondolin region, a sinkhole is formed when the roof collapses on an underground solution cavity (or cave) in the dolomites. Sometimes these are very large, and may extend deep enough to fill with water, as at Boesmansgat south of Kuruman. This sinkhole does not hold any known fossil deposits, but it is an excellent (and impressive!) example of the geographic and geologic features of the palaeocaves for which we are searching. It is thought that the water filling Boesmansgat is connected underground to another large water-filled sinkhole about 60km to the north – the Kuruman Eye – which supplies all the water for the local population.
At least one diver has died trying to reach the bottom of the Boesmansgat sinkhole, which is 274 meters below the surface (some 337 meters when corrected for altitude) leading to the claim that this is the deepest and largest natural sinkhole in the world.
Our interest in Boesmansgat was to learn more about the occurrence of sinkholes on the farms in the area. With local farmers as our guides, we were able to visit several smaller sinkoles to inspect them for fossil deposits and ancient cave formations. We did find some small speleothem (calcite cave formations) occurrences, and one sinkhole led to a large underground chamber.
Without proper equipment and safety gear, we could not fully explore the underground caverns on this trip, but in the main chamber at a depth of 10-15m we did find some rock breccia, speleothem formations, and recent micro-mammal bone — as well as one deceased tortoise that fell in to this ”animal deathtrap” and was lodged between the rocks! We could not see how deep the chambers extend underground, but there are at least two deeper tunnels that we will explore when we return on a later trip.
We have now learned that several more farms in this region also have sinkholes, so we will pursue those contacts and plan to return for a more extensive survey – in addition to lime mines, we are now also looking for sinkholes!