They are of a 'tumulus' form of weathered granite outcrop.[2] They are made of a pink, massive, coarsely equigranular rock consisting mostly of quartz and orthoclase. Their appearance may be due to a combination of erosion by underground rainwater and then by subsequent weathering after they were exposed. Most of the pillars emerge without a break from the underlying granite. Their structural base may be of orthogonal or vertically-aligned sheet jointing.[3][4]
They obtained their name because a traveller in a coach saw the formation in the distance. He asked how a farmer could produce so much hay. As the farm was on a property owned by a man called Murphy, the rocks became known as Murphy's Haystacks.[5][6]
^Main, B York (September 1997). "Granite outcrops: A collective ecosystem"(PDF). Journal of the Royal Society of Western Australia. 80 (3). Department of Zoology, University of Western Australia: 115. Retrieved 4 April 2017.