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The rocks of Aragunnu

March 11, 2014

Barry Tucker                    11 March, 2014

Mimosa Rocks National Park is well known for the rocky outcrops and headlands that separate its many beaches. The northern beaches of the park are remarkable for the rounded rocks, stones and pebbles that cover them.

It’s hard to find sand on the northern group of three beaches; it might exist beneath the rocks. In the area between the rocks and the vegetation at the back of the beaches, you can see how the sand is being made. It’s a coarse mixture of rock particles, sea shells and other debris that is slowly being pulverised by the grinding of the rocks, themselves being further shaped and rounded by weathering and the pounding of high tides, coastal storms and a persistent wind. Any fine sand produced is carried out by the receding tide and falls into the gaps between the rocks.

Opening shot

The castle-like Mimosa Rocks are the main feature at the northern end of the national park. The area is named for the paddle steamer Mimosa, which ran into these rocks and sank in 1863. The Indigenous name of Aragunnu applies to this area and a sandy beach just south of here.

The fishing and sea life that clung to the submerged rocks attracted Indigenous people for centuries, perhaps even many centuries. Three streams run through the park. For Indigenous people, for anyone really, the area where a stream or river meets the sea is usually a special place. Aragunnu was a meeting place and a training ground for the utilisation of resources.

Man made mound

The grass covered mound beyond the rocks is a midden, formed of discarded sea shells and bones. It’s more than 30 metres long and maybe three to seven metres high in places. According to the signage, middens may also be places of birthing and burial. At the request of the Indigenous caretakers, a boardwalk has been built along the far side of the midden to protect the area and its relics. A stream, usually dry, runs under the boardwalk and over the rocks at the northern end of this beach.

I want to focus on the beach rocks and speculate about where they may have come from. On my first of several visits I figured the rocks, because of their rounded appearance, must have tumbled down from the hill behind the midden and the stream. A closer look at the landscape during my last visit indicates this scenario is unlikely and the rocks must have been formed right where they are.

Rocks 2

Shelves of rock, undulating hills, ridges, mountain ranges and volcanoes are thrust upwards to appear above the surface by the unimaginably powerful action of the collisions and movements of tectonic plates (the large masses into which the earth is divided) and by volcanoes themselves. In the picture above, the dark shelf of rock has been lifted at one end by several degrees. An eroding cliff face to the right (not visible here) is about 13 metres high and marks the southern end of this beach. The island which has formed just offshore is slowly breaking up due to weathering of its natural fault lines.

Fault lines are usually vertical, causing rock faces to break up into column shapes. Later horizontal fault or stress lines often appear, causing the columns to break up into smaller cube shaped blocks that eventually fall to the ground. The wave action of high tides would roll the rocks about, rounding them over time. Further weathering and erosion comes from blasting by wind driven sand and debris. I believe this is how the rounded rocks on the beach were formed.

Notice the many different colours of the rocks; different types of rock too. It is not unusual to find different types and coloured layers of rock in any exposed cliff face. As the cliff face breaks up, it produces rocks of different types and colours. Some of these rocks contain quartz veins. The molten quartz is created well below the earth’s surface. It has to go somewhere and it exploits fault lines or weaknesses in the surrounding rock. The same phenomenal pressure eventually brings the rock shelves to the surface and they are replaced below by new molten rock.

Rocks 3

The cliff face above shows that it has been formed by regular layering, sometimes with material of a different type and colour. Each layer is about one centimetre thick. It is remarkably consistent and regular. When lumps fall off they contain sections or stripes of these different colours, eventually producing an intriguing rock resembling a child’s multi-coloured marble.

Rocks 4

Obviously, natural forces have produced the rounded rocks of the northern beaches of Mimosa Rocks National Park. But Man, ever playful and creative, sometimes amuses himself and attempts to entertain or leave a message for others by rearranging the rocks. These balancing figures remind me of the carved stone Moai monuments on Easter Island, whose purpose may have been religious but whose message appears to have been lost. Some of the Moai were eventually lowered face down because they no longer served their purpose or were replaced by other beliefs. The beach rocks of Aragunnu invite wonder, play and creativity – a natural thing to some people but an objectionable thing to others. The balanced rocks I saw on my fifth visit to Aragunnu today are of a different style to those I saw on my first visit. On my second visit I noticed they had been knocked down. Some of these new figures have more substantial bodies – the smaller portion of cliff faces or hills that have split into pillars or columns and fallen to the ground. Over time they will break up and add to the rocks that surround them.

Cube shapes

The vertical and horizontal fracture lines on the face of the outcrop above show how huge lumps of rock will eventually fall to the ground. Some will break up when they hit. Pieces will break off over time, be broken into smaller pieces and be rounded by weathering and the rolling action of high tides and storms.

As I walked along the rock covered beach, looking for signs of where the rocks may have come from, I started to form the idea that they were produced on the beach and did not come from another place. That is, they were not carried here by torrents of water (as you will find in a river bed) and they did not roll down the side of a mountain – there isn’t one close enough.

Caldera Mimosa rock pit

I found what I think is conclusive proof when I saw the walled feature in the picture above. These walls must be the remains of a cliff face that has collapsed to the front, the rear and all sides. The main clue is the accumulation of rocks in the centre of the feature; they are identical to the rocks on the beaches. They have been rounded by weathering and wave action – the broken front of the feature is open to the sea. The walled feature may also have an opening at the rear. This opens up the possibility that the rocks have tumbled down the hill, entered the broken wall at the rear and accumulated inside the feature. Somehow, I do not think this is the case. The feature is surrounded by the same rounded rocks you find elsewhere on these beaches. It’s more likely that the feature was once part of a ridge that ran along these rocky beaches and ended in a cliff face. Over time the cliff face has broken up, littering the beach with the rocks we see today.

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