Sinkholes: 7 June 2012

We’ve surveyed several caves in the hills near Gondolin. Most of these caves are visible from the surface as sinkholes, which are created when the roof of an underground chamber collapses. The walls of sinkholes can contain any number of geological formations including speleothem (flowstone and dripstone), breccia, sedimentary infill, and the rocks that comprise the regional geology, which in this area are dolomites.

Because caves are formed by erosional processes of underground water movement, vegetation is usually abundant at cave openings. Sometimes it’s so heavily vegetative that it’s difficult to see the sinkhole:

Andy and Kristi collect data at edge of sinkhole

But if you crawl in a bit (mandatory disclaimer: don’t do this at home) you can see the opening and the surrounding rock:

We also found a breccia dump outside this cave. Breccia is a conglomerate rock consisting of cave infill that forms when material–bones, rocks, and vegetation–falls into the cave. In South African fossil sites, the presence of fossils (hominin and other) in breccias is normally attributed to the action of carnivores (eg, leopards) that drag carcasses into trees to devour them without scavengers hanging around. Alternately, some animals might just fall into the sinkhole and subsequently die. The presence of this breccia dump outside the cave indicates that it was a historical lime mine. In the breccia we did find several small fossils, including this beaut:

My thumb for scale.

All in all, a successful day!

-KL Lewton


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