The cave-like openings and pock-marks in the sandstone and conglomerate at Papago Park, for example, are called tifoni.  Weathering of the rock, and chemical action from acidic rainfall, breaks down the mineral "cement" that holds the particles of the rock together.
   Over geologic time, those particles then wash out from the formation in subsequent rainfalls, or are blown out by wind, leaving the strange appearance of the rock.


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