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It's in a sink, water will filter through the protection, and still enter the cave, but not a massive amount of silt and debris. Also to prevent animals from falling in. It was covered, au natural,...

Just my .02 on digs--
Get to know your local BLM guy
Get to know your local forest service guy
Better yet, get to know your local fish and game guy ( more likely to see him/her in the field)
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