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View from the top of Cathedral Rock
The base of the ascent to Cathedral Rock
An aplite dyke dipping at a low angle beneath a large tor
Close up of above

Cathedral Rock National Park

Latitude -30.439739, Longitude 152.25074

Located in the New England Region of NSW, nearest large town Armidale

Source: Bob and Nancy's Geological Tour Site http://brovey.yolasite.com/resources/Cathedral_Rock.pdf

A detailed geological guide to Cathedral Rock National Park is available at the above website.

Link to Detailed Map

The best way to experience the geology of this park is to walk the loop track around Cathedral Rock and then take the detour to the top of the rocks. Be cautious of the unfenced drops as you ascend: the result of a fall from these heights would be fatal. The forest loop walk around the tors is a very pleasant shady stroll crossing open woodland and swampy areas. The wildlife is prolific particularly the variety and colour of native birds. Please be aware of snakes especially in the summer and warm weather.

Geology

The Cathedral Rock region mainly comprises Carboniferous and Permian sedimentary rocks which have been intruded and metamorphosed by a number of Permian to Triassic granites. These have been subsequently intruded by Eocene and Miocene basaltic magmas which locally reached the surface to extrude as lava and ash. Accelerated erosion accompanying uplift of the Great Dividing Range (of which Cathedral Rock is a part) has incised deeply into these rocks, forming the Great Escarpment and associated gorge-dominated terrain.

Cathedral Rock National Park is situated on one of these large Triassic granite masses that also incorporates some basaltic rocks derived from different volcanic vents of various ages including the extinct Ebor Volcano.

Harsh weathering in the Park has exposed large granite tors (see below explanation) most notably at Cathedral Rock itself, where a series of granite boulders are perched, one on top of another, to a height of about 200 metres, extending approximately one kilometre. The granite outcropping at Cathedral Rock is "I-type granite", formed from the melting of igneous rocks or sedimentary or metamorphic rocks which were rich in igneous material. Known as the "Round Mountain Granite", this mass was deposited about 250 million years ago.

Tors

Tors are rounded, block-like outcrops of granite. They form in hard, massive granite which has cracked along vertical and horizon fractures. The fractures are known as "joints". Vertical joints form as molten granite cools and contracts within the earth's crust. Some granites have abundant, closely-spaced (metres apart) joints, whereas others have very widely-spaced (hundreds of metres apart) joints. Horizontal joints form as rocks covering the granite are eroded away, reducing the mass of overlaying rock. This is the process of "unloading". Unloading results in the expansion of rocks nearing the earth's surface, which results in horizontal cracking. Then as groundwater moves into the joints it begins to react with the fresh minerals in the granite, causing the minerals to break down into other varieties, whilst the granite itself weakens and begins to exfoliate from the underlying fresh rock. Rinds of weathered granite wrapped about fresh granite may resemble the structure of an onion, giving rise to the term "onion skin weathering".

Joints in the granite can be observed in individual outcrops, and their effects upon the landscape are also evident in satellite images. Joints provide pathways for weathering and erosion of the granite, which in turn leads to the formation of gullies, and then creeks and rivers. It is common for jointed granites to produce a landscape with parallel watercourses abutting many angular bends, such as is the case in the Cathedral Rock National Park.

Aplite

Also in evidence at Cathedral Rock are dykes of aplite cutting through the granite. Aplite is formed from the final, residual magma left over when the bulk of granite has crystallised. That final magma is composed of low temperature minerals, such as quartz and orthoclase, which subsequently crystallise to form aplite. These aplite dykes force their way through the crystallising granite using energy generated by the contraction of the newly formed rock.

Getting There

Cathedral Rock National Park is located in the New England Region of New South Wales. Travelling via Waterfall Way it is approximately 65 kilometres drive east of Armidale and 59 km drive west of Dorrigo. Look for the sign to Cathedral Rock National Park and turn off from Waterfall Way onto Round Mountain Road. Proceed 8 kilometres to Barokee Rest Area to access the loop walk. From the north you can also access the loop walk and Native Dog Creek Camping Area from the Ebor-Guyra Road.


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