Mangapohue Natural Bridge

The Waitomo area is renowned for its limestone caves and karst landforms. This is because there are extensive sheets of limestone eroding at and near the surface in this area. The limestone was formed during the Oligocene period when Zealandia was mostly submerged beneath the sea and there was very little mud or sand eroding off the limited low-lying land areas. Thus little sand or mud accumulated on the floor of the shallow seas that overlay much of New Zealand at that time. Instead shell banks slowly accumulated, composed of the calcareous shells of a number of kinds of marine organisms, especially molluscs (sea shells) but also moss animals (Bryozoa), echinoderms, calcareous red algae and foraminifera (shelled amoeba). Over 5-10 million years, many metres thickness of shell bed accumulated and when these were subsequently buried by 500 m-1000 m or more of sand and mud the shells partly dissolved and recrystallised thereby cementing themselves together into a dense hard limestone rock.
In the last 15-20 million years the Waitomo region has been pushed up out of the sea and has been eroding. This erosion has stripped off most of the younger softer sandstones and mudstones and exposed the limestone layers.
Limestone is soluble in slightly acidic ground water. Over time cave passages may be dissolved along fractures through the limestone. This has happened here with the Mangapohue Stream flowing through the cave. Much of the roof of the cave has collapsed and been carried away by the stream creating a narrow steep-sided gorge. The natural bridge is the only remaining part of the cave with some roof still in place.

When walking through the natural bridge look up at the roof. Can you see some speleothems hanging down? How do you think they formed? Why are they angled and not vertical? Hint: Read the sign.
In the farmland on the far side of the loop track see if you can find the rocks containing Oligocene (30 million year old) giant oysters? The seas at that time were warmer than around New Zealand today and these oysters grew much larger than modern ones. How big is the largest? How many people would one oyster have fed for a meal? Unfortunately humans were not around 30 million years ago - they had yet to evolve.