Murphy’s Haystacks
These large features are found on the Eyre Peninsula, hanging out over the ocean in southern Australia. The name originates from them being on the property of a family named murphy and a group of stagecoach riders in the 1800s seeing them at a distance and asking why the property had such big piles of hay. Up close, they are very much not made from hay; instead, they are granite.
Murphy’s Haystacks are big blobs or “tumuli” eroded from a block of a unit known as the Hiltaba Granite. This granite was one of the events in the Precambrian that was part of Australia growing outwards from the ancient cratonic core found in the country today. Continents grow by adding rocks to them at subduction zones and by forming new island arcs; those island arcs often produce granites like these. These granites are just over 1.5 billion years old.
The shapes of the Haystacks are thought to be controlled by joints – fractures that form in the rocks as they approach the surface. Water that flows into these joints will open the fractures up, shaping them into blob-shapes. These rocks are not true boulders as the fractures are thought to be only vertical, not horizontal, meaning that the bottom of the Haystacks are still hooked to the granitic body below.
-JBB
Image credit: https://flic.kr/p/5hqHdt
References: https://bit.ly/2E6qyYE https://bit.ly/2GTfpNw