Nature, culture and history
Perched high above the head of the Pioneer Valley, Eungella National Park is the central part of the extensive plateau of the Clarke Range, extending from well north of Dalrymple Heights farming area, through Eungella town, to south of the Crediton locality. The eastern edge of the plateau is a spectacular escarpment.
The plateau is largely carved on hard granitic rocks resistant to erosion. Most were intruded into the Earth’s crust as molten bodies during an episode of crustal heating and mountain building in the late Carboniferous to early Permian periods (300–275 million years ago). Look for evidence of the granitic rocks on the slopes of the escarpment and in the riverbeds such as in the Broken River south of the national park picnic area (Granite Bend walking circuit).
Notice that the granitic rocks are not evident on the plateau surface, here deep red soils over pale clays predominate the landscape. The plateau is believed to be an ancient, higher-level land surface, deeply weathered from the granite sometime between 35 and 15 million years ago and is now eroding around its edges. The red soils have developed from iron and aluminium oxides accumulating in the soil surface profile, with other elements leached out from below. The soil here is somewhat more fertile than other granite soils. This would have encouraged the growth of rainforest in the high rainfall belt along the plateau’s eastern edge, and with settlement the clearing of the plateau for farming.
Basalt lavas on the plateau in the Crediton State Forest area were erupted from volcanoes to the south near Homevale National Park about 34 to 32 million years ago.
Eroding Finch Hatton Gorge
The popular Finch Hatton Gorge is well below the plateau surface and reveals rainforest growing on typical granite terrain. Most of the boulders have careered down from the steep slopes above, but some of the original granite formations are still in place. The straight north-south trend of the gorge results from strong fracturing in the granite in this direction, with the stream finding it easier to pluck out blocks along these lines of weakness. In addition, a major dyke of black basalt has been intruded into the granite in the same direction, and the stream has found it easier to erode this softer rock in preference to the surrounding granite. The creek has thus been trapped for much of its length in a narrow slot eroded into the basalt dyke.
Geology content courtesy of Warwick Willmott.