Rosetta Head
Rosetta Head, known as Kongkengguwar by the Ramindjeri people, but more commonly known as ‘The Bluff’, is a headland located a few kilometres south-westerly from the township of Victor Harbor on the Fleurieu Peninsula. Regarded as the most prominent feature along the Encounter Bay Coastline, the granite headland stands at around 101m above sea-level, providing spectacular views along the coastline. Also, previously used as a lookout for whales during the early European settlement.
Geology:
Rosetta Head was formed and shaped through the processes of intrusion and metamorphism, followed by two phases of erosion. The headland itself was made through the intrusion of molten granite 5-10km deep in the Earth’s surface. Due to the extreme pressure and heat inside the Earth, this molten rock was forced upwards toward the surface, through the weak points in the Earth’s crust. This intruded on existing folded metamorphic rocks known as Kanmantoo Schists which can be seen in some pieces of granite. The molten rock eventually cooled and hardened on the surface, forming granite. This all occurred roughly 500 million years ago. Over millions of years, the layers of sediment which covered this granite were eventually eroded away. The bluff as we know today, is part of a very large body of underground granite (also known as a batholith), which is connected to Granite Island and extends as far as Kangaroo Island.
Part of the intrusion, Rosetta Head was covered by 1-2 kilometres of ice during the Permian glaciation, suggesting that it was partly formed by glacial action, this has subsequently eroded to form the distinctive dome shape of the headland which can be seen today. The ice also left large deposits of glacial sediments across the Inman and Hindmarsh Valleys, some of which can be seen along the beach. The second phase of erosion is still in action to this day due to weathering and hydraulic action. Waves from the surrounding ocean constantly collide with the landform, causing the water to, extremely slowly, erode away the sides of the headland. This erosion doesn’t occur very quickly, but over the span of many, many years changing the visible shape of the land.
Rosetta Head mainly consists of the very hard and durable stone, granite. Granite is an igneous rock which forms from molten magma deep in the earth’s surface. Granite is granular in texture due to the compound of quartz, feldspar and mica which are the most common minerals which make up granite. Feldspar being often white or light orange in colour, quartz being clear or a milky colour and mica being an opaque white to green or red to black colour. Other minerals can also be found in the compound, just not in such quantities as those listed above. The strength of the rock is due to the chemistry of the feldspar and quartz together, which creates this strong property of resistance to weathering. This means that granite takes significantly longer to erode/weather than other rocks in general.
Logging requirements:
To log this EarthCache as found, you will need to climb Rosetta Head and go to the listed coordinates via the distinct walking trails. At GZ, you will find a large granite rock which will need to be examined. Answer the below questions and message your answers through to the CO. You can log a ‘found it’ as soon as this is done, but please do not include your answers in your log. Logs without timely accompanying answers will be deleted.
Questions:
- Looking at this large granite rock up close, what minerals do you believe this sample mainly consists of?
- Have a look at the rock and run your hand along the surface. Describe the texture and shape of this specimen of granite.
- Do you believe that erosion has shaped this certain piece to look how it does today?
- (Optional) Take a photo of yourself or the view across the coastline and attach it to your log to show that you have indeed visited.