Ruakuri Natural Bridge Walk

This area is formed of Oligocene limestone that accumulated as shell banks on the shallow sea floor of a submerged Zealandia about 30 million years ago. After burial beneath another 500-1000 m of mud and sand the shells began to dissolve and new calcite crystals grew between the remains to create a hard recrystallised limestone. In the last 15-18 million years the whole area has been pushed up out of the sea and eroded down. Slightly acidic groundwater passing through humus and soil beneath native forest has slowly dissolved passages along cracks in the limestone to form a wide variety of caves and tunnels.

Many of the rocks have both horizontal flaggy surfaces and vertical runnels down their sides. The horizontal flagginess is the original seafloor surfaces that have been exaggerated during the recrystallisation of the shell banks. What created the runnels down the rock faces?
When the small caves and tunnels you walk through were formed at what level do you think the river water was at? What has happened since?
How do you think the natural tunnel was formed?
Where does the water come from that flows out of the bottom end of Ruakuri Cave?