About 230 million years ago, when cycads and conifers were king and dinosaurs roamed the land, a slippage in the Earth’s crust created the Demon Fault. On geological maps, the Demon Fault runs directly north-south about 200 km from the headwaters of Demon Creek to Ebor where it becomes hidden under basalt flows of smaller faults.
The Demon Fault Line coincides with the north-south valley of the Guy Fawkes River and occurs along the junction of two major geological blocks of Palaeozoic sediment. Geological maps of the area show the Demon Fault as a continuous line although it has numerous splays, deviations and splits. The nature of deformation features adjacent to the faulting indicates that the fault was shallow and was created in a brittle environment.
The Demon Fault is of geoheritage significance and was first recognised as a major strike-slip fault in 1969. The Demon Fault is a prominent feature because it is evident from a series of valleys from near the Queensland Border to Dorrigo. Here in Gibraltar Range National Park the landforms are a legacy of ancient volcanic activity, followed by faulting and uplift.
The Demon Fault is a large transverse-type fault meaning that movement on the fault was horizontal rather than vertical. It displays very obvious topographical features. It has formed valleys where the rock of the fault has been broken down into what is called gouge or rock-flour. Gouge is very weak material. It is easily eroded and rivers often preferentially follow the route of the fault carving out the gouge into deep valleys. In this case, Boundary Creek has cut its way into the land by following this major north-south crack in the earth. Several waterfalls, including the nearby Boundary Falls, occur along this fault line.
The Demon Fault marks sharp boundaries between geologies. The ancient igneous formation east of the Demon Fault is in stark contrast with the even older underlying meta-sedimentary rocks that dominate the western half of the park.
The rocks and soils - and the plants and animals that live on them - are very different on either side of Boundary Creek. Soils vary in the area according to parent material and landform. On one side are soils formed from old metasediment rocks which support a dry forest. Soils on the other side of the creek support a wet forest with bigger trees, more shrubs and more insect-eating birds. These soils were formed from younger volcanic and granitic rocks that ‘slid down the fault’ from many kilometres away.
One day you may impress someone with such a great throwaway line about this seemingly innocuous part of the road by pointing and saying, “That’s the Demon Fault!”
You may log this Earthcache straight away but then please email your answers to the questions to the CO. We will contact you ONLY if your answers need extra work. Logs with no answers sent will be deleted.
The answers can be found by observing the landscape and reading the information boards.
Q1. On this side of the fault (when standing at GZ) what kind of rocks are you standing on?
Q2. How far down the fault did the younger volcanic and granitic rocks slide?
Q3. Which direction did the rocks slide towards?
Q4. Name a bird that lives in the dry forest on the other side of the fault?
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Remember to take only photos and leave no trace of your visit.
We hope you enjoy your visit to this ancient geological formation and the nearby Boundary Creek falls.