Harwood's Hole

The carbonate rocks common throughout the Golden Bay region present a unique weathering feature at this locality – Harwoods Hole, New Zealand’s deepest vertical shaft (176 metres).

Karst landscapes form by the dissolution (dissolving into water) of soluble rocks. Limestone and marble, both prolific across Golden Bay, are susceptible to dissolution from rain and ground waters. These natural waters react with the slightly alkaline carbonate-bearing rocks to produce a weak carbonic acid. Karst landscapes develop as these acidic waters exploit weakness (usually fractures or joints) in soluble rock, chemically dissolving the rock and eroding it. The chemical susceptibility of such rocks develops into mechanical susceptibility, as the flow of water is focused into the depressions created by dissolution. As well as producing fluted channels and other karst landforms above the ground, localised weaknesses in the rock can amplify dissolution to the point that sinkholes – depressions in the ground caused by dissolution – develop.

Harwood’s Hole itself formed from thousands of years of dissolution and erosion and, for a time, would have exhibited a spectacular waterfall from the lip of the sinkhole right to the floor. Continued erosion allowed water to flow from the bottom of the hole out through the Starlight Cave system, to re-join with the nearby Gorge Creek. These days, the Hole and the associated cave system remain largely dry, except for flood periods, and together possess a total combined depth of 357 metres.

The river which created and once flowed through the Harwood system has changed its course, and now descends into the earth approximately 3.5 kilometres north of Harwood’s Hole, into the Homestead Creek sinkhole. The Homestead Creek system flows underneath the Harwood system and, in time, will itself likely be abandoned.

Observe the karst landscape as you walk toward Harwood’s Hole; can you imagine the slow dissolution process of the acidic water working away at the marble rock? See if you can find joints or fractures in the rock which might be subject to water accumulation and subsequent dissolution in the future.

Gazing out over Harwoods Hole, try to picture the impressive sight of a waterfall plunging 176 metres into the depths of the sinkhole must have created. Have a go at imagining the underground network of caves which connected Harwood’s Hole with Gorge Creek, and the caves beneath those which connect the currently active Homestead Creek sinkhole with Gorge Creek.