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.
The haystacks are formed from ancient granite inselbergs dating back 1,500 million years. [The name inselberg comes from German insel, meaning island and berg, mountain.]
The pillars and boulders in their present day form only go back as far as 100,000 years and have been formed by the uneven weathering of crystalline rock as densely fractured compartments break down through weathering more quickly than massive unfractured compartments. The distinction between the two forms is that pillars merge unknown with the underlying bedrock, but boulders are clearly detached.
The haystacks are of a pink granite named Hiltaba granite after the homestead of the same name in the Gawler Ranges. It is extensive over the north-western Eyre Peninsula. The mottled colours on the surface of the haystacks are caused by growth of lichen, a tough plant organism which thrives on exposed granite.
The granite hills of the district, including the haystacks, were buried by calcareous dune sand about 30,000 years ago. Subsequent erosion of the surrounding land surface has gradually revealed the forms we see today.