Over millennia, the elements have carved this and many other gorges radiating away from the volcano’s caldera to reveal the basalt columns that you see before you. When lava cools, it contracts. Hot things generally take up more space than cool things do. So when hot things cool, they get smaller and often crack or fracture. As molten lava cools to form basalt, this process forms the vertical cracking and spectacular columns on display here.
This example is somewhat unique to this area as the jointing process has stopped some distance below the surface and vanishes into an irregular basalt formation known as “Upper Colonnade” at ground level. This phenomenon is caused by an abnormally rapid cooling period of the surface lava. It is unclear what caused this more rapid cooling; the action of rain and water, possibly from the creek water itself as it eroded the gorge in it’s early stages are likely causes.
Diagram showing columnar jointing process and the different layers that can form:
Getting to this Earthcache involves a 5km round trip mainly off track bushwalk through Denham Scenic Reserve at Beechmont. We recommend taking the route as detailed in our cache “Twin Falls” GC69NZQ as you will pass GZ on the way. Alternatively, you can take the main track down to Lip Falls and rock-hop downstream past several waterfalls to Twin Falls, then head up to GZ from there. This is not an easy walk, take usual bushwalking precautions and tell someone where you are going.
To claim this Earthcache you must go to GZ and answer the following questions:
- Why do you think the columns end below the surface?
- Estimate how far under the surface the columnar jointing ends.
- Estimate the height and width of the columns at their highest point.
You can log your visit immediately and message your answers to the CO later. Pictures of you at GZ are welcome but optional.