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THE LATEST PALAWAN DISCOVERY: THE “HONEYCOMB PAVEMENT” OF THE NEW BRANCHES OF THE PPUR Jo De Waele, Paolo Forti With the exploration of the new branches of the PPUR, we have added yet more concretions to the already exceptionally high number that we know were formed due to the particular climate of Palawan. This time, the structure is constituted by small bowls (Fig. 1, centre) grouped together so as to resemble very closely the cells of a beehive – a “honeycomb pavement”, in other words. The credit is due exclusively to Vittorio Crobu and his passion for cave photography, which has made it possible for us not only to know of their existence but also to study them in detail. Indeed, everything that we now know about these strange concretions comes from the shots that he was kind enough to take whilst exploring the new upstream branches of Daylight. On the basis of our previous knowledge of the climate of Palawan, which is characterised by short but heavy rains, and above all on what we have been able to discern from the images, it is clear that there has been
a relatively long genetic process here, encompassing at least three distinct evolutionary stages. Initially, the (sub-horizontal) area was covered by a thin layer of loam and mud that was completely soaked during the wet season, whereas in the dry season evaporation caused its total desiccation, with the formation of a series of cracks resulting from the consequent decrease in the volume of clay (Figs. 2.1-2.2). Given that, during the subsequent wet period, the water failed to seal up entirely the fractures formed in the clay, the alternation of the wet and dry periods gradually deepened and widened those fractures (Figs. 2.1-2.2), which thus conserved ever-greater volumes of water. The evaporation of this water trapped in the mesh of the mud cracks led to the development of an internal concretion. Over time, then, CaCO3 filled the lattice of the cracks right up to the outer edge of the mud and, thanks to capillary rising, the concretion rose slightly higher than the mud (Fig. 2.3A). Had this process continued unchan-
Fig. 1 – In the centre, the honeycomb pavement surrounded by the details that have enabled us to trace its development; 1) bowl partially destroyed by the flow of water along a drainage channel; 2) fragment of concretion on the recent mud base; 3) drifting pebble trapped in a bowl; 4) almost entirely destroyed structure, from which the original clayey substrate has also been eroded; 5) fragments of tubes within a highly eroded bowl; 6) recent mud sedimented within a corroded bowl; 7) close-up of the dividing walls with clear detachments of the structure; 8) fragment of stalactite and piece of non-drifting rock within a bowl; 9) recent, still-plastic mud; 10) basal portion of a totally dismantled bowl showing the mud solidified by the CaCO3 deposited within it; 11) close-up of a bowl, showing internally the concretion films formed after every flood; 12) calcite films that have developed over an almost entirely destroyed structure.
