On the way back to Koh Phi Phi Don from Koh Phi Phi Ley, having cruised by one of the caves from which the locals harvest swifts' nests for birds nest soup, we got some up-close views of the base of the limestone cliffs. The cave action is clearly undercutting the rock over the course of the years. The vegetation clinging to the cliff faces reaches into the fissures and expands them, weakening the rock mass further. Eventually, gravity takes over and a chunk of the cliff will cleave off into the ocean. This is what forms the near vertical cliff faces and exposed rock of the limestone outcrops in this area. I think the face to the left of center in this image is a prime candidate to fall away. There's a big fissure that you can trace from the base of the overhand up at about a sixty degree angle into the yellow area where it then turns vertical. I can imagine there'd be a sharp crack followed by a thunderous roar as the rock face gives way and tumbles into the ocean below. I think this is what happened to expose the cave in yesterday's photo since there a stalactites exposed on the cliff wall. I think these were once in a cave within the rock but as the rock face was undercut and then the rock above fell away, the cave was exposed.
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