The Nimbin Rocks are volcanic extrusions of rhyolite left over from the Mount Warning Tweed Volcano that erupted around 20 million years ago in what is now northern New South Wales, Australia.
As part of an eroded dyke of the volcano, the Rocks are situated just outside the present day caldera wall about 20 km from Mount Warning and three kilometers from Nimbin village. The three most prominent were named by early white settlers as the Thimble, Cathedral and Needle. They are an extremely significant cultural site to the local Bundjalung tribe of indigenous Australians who believe the rocks were home to the Nmbngee, or Clever Men. They were also initiation grounds for young boys and the dreaming story can be read at the Nimbin Museum.
The Nimbin Rocks are comprised mainly of the quartz rich volcanic rock called rhyolite overlying a section of agglomerates (reworked volcanic rock) and volcanic glass known as perlite. Below the perlite lies basalts of the Kyogle Basalt. And here may lie the clue. The rocks appear to be layered because they are deposited on top of each other. First the Kyogle Basalt, then the perlite and agglomerates and then the rhyolite lavas (with some bands of perlite within it). The rhyolitic lavas are referred to as the Georgica Rhyolite Member according to Duggan and Mason (1974), or historically and more recently as Nimbin Rhyolite according to McElroy (1962) and Cotter (1998) and others.
If the Nimbin Rocks were related to a dyke they would have formed through pushing through the surrounding rocks such as those of the Kyogle Basalt or the Clarence Moreton Basin sediments, metamorphosing them and displaying different diagnostic textures than those I know about. However, it is still quite possible that the rocks may have been vents since the nearest identified vents seem to be about 8km away to the north east in the Nightcap Ranges and rhyolite lava flows tend to not move great distances, indeed rarely greater than 5km. However, the vents located further into the Nightcap Ranges are characterised by thick erosion resistant units of rhyolite which we don't see so much near Nimbin other than the Nimbin Rocks themselves. But conversely, the shape of the rock monoliths does imply a dyke.
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What type of rock overlays the substrate rock?
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Why are the rocks layered?
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Are the Nimbin Rocks part of the Tweed Caldera?
4. Standing at the co-ordinates, what color are the rocks? Are they the same color?
Source - Wikipedia