The Estuary Karst Arch Formation

 

Hidden along a remote and muddy stretch of a large harbour, this impressive karst arch is a rare find accessible only by a long trek across the mudflats.

Over thousands of years, the relentless action of the tides and rainwater ate away at a soft point in the limestone cliff, eventually carving a hole completely through to the other side. While the lower section is constantly being hollowed out by the salt water, the upper bridge remains intact because it sits above the high-tide line, where a thick mat of plant roots helps bind the rock together to preserve its natural shape.