This cache is placed with the acknowledgement of the traditional owners of this land, the Jardwadjali people, and pays respect to elders past and present.
A Brief History of Time
The gates in this area are subject to seasonal closure and you will need a 4WD if coming from the North.
This area is of historical and geological significance and is somewhat of a local secret. Please be mindful of this when visiting this site.
Follow the path and carins up to the saddle, then stick to the left of the buttress until you reach the cave.
The Lesson
The western Black Ranges lay to the west of the Grampian Ranges and consists of dipping sandstone beds of the Grampians Group sediments.
Millions of years ago the region was not a mountain range, but was instead the ancient Eastern shoreline of Australia. Massive forces, related to continental drift, uplifted, faulted and folded the sediment accumulated along this shoreline, and turned sand into stone. This cache aims to explore this process, both in geological history and in current location.
The sediments accumulate
Approximately 430 million years ago the coastline of Victoria ran north-south through the Grampians region. To the west of the Grampians the ancient Australian coastline was quite mountainous. To the east, the region that is now the rest of Victoria was a deep ocean. Over quite a few million years several thousand metres of sand and mud were laid down along this shoreline as a layer cake of flat lying sheets that eventually become converted into the rocks we now called the Grampians Group.
Below: Map of Grampians Group with representation of different sedimentary rocks and order they were deposited. Column width refers to the grain size of the sediment – mud, find sand, coarse sand, gravel.
At the time, the climate was probably quite wet, and land plants were small, so erosion of the mountains was high. Large rivers drained away from these mountains, leading to the ocean. These rivers carried sediment- sand and gravel eroded from the mountains-and some of this was deposited along extensive river flats as the Grampians Group, burying the older, folded bedrock beneath.
Not all the Grampians group was deposited by rivers however, where the river diverted creating large lagoons which accumulated thin fine red mud, which became the Silverband formation. In some places the lagoons were separated from the oceans by wind-blown sand dunes, which created the Black Range region. This region still displays the distinctive large scale high angle cross beds which are visible in the location of this earth cache.
Turning sand into stone
Once a few kilometres of Grampians Group sediment had accumulated the sheer weight of the overlying material caused an increased in pressure and in temperature. This squeezed much of the water out of the sediments and altered the mineral grains into an interlocking network that turned the sediment into stone.
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Shortly after the Grampians Group were deposited, plate motions and continental drifts caused huge compressive forces to exerted on the rocks. The ocean to the east began to close, and the sediments beneath the ocean were buckled into folds and uplifted and converted into a new part of the Australian continent, which now includes the goldfields of central Victoria and the Alps of eastern Victoria, making the Grampians Groups in the centre of a newly emerging continent!
These compressive forces squashed the Grampians Group into a thickened pile by folding and faulting, a process called deformation. In some places, the sandstone slid and stacked up over layers of mudstone, making it even thicker, folding and faulting it into its present configuration. These sandstone beds took many shapes and forms, from the shallow bed dips at Venus Baths, to the horse shoe shaped Mount Difficult ranges.
The Black Ranges
The western Black Range consists of dipping sandstone beds which are comprised of sandstone, mudstone and overlain by a sandstone known as ‘quartzone’, (composed primarily of quartz), which makes them extremely resistant to weathering. The beds here formed into cuesta landforms, meaning a ridge with a gentle slope on one side and steep slope on the other.
Similar soils to the Grampians Ranges occur here with the major soils including sandy soils with pans and acidic, grey texture contrast soils with sandy surfaces. The sandy soils with pans (Tenosols) comprise organic rich surface sands over a conspicuously bleached subsurface horizon. A further change occurs where a yellow organic and sesquioxide discontinuous pan (coffee rock) sits over mottled clay further down the profile. Grey texture contrast soils (Kurosols) have sandy surfaces with yellow and red mottled light grey clay loams to medium clay subsoils. Both soils have an acidic profile trend with the texture contrast soils strongly acidic.
Questions:
- In your own words, explain how sandstone beds occurred in the Grampians Group, and why is this unique in the Black Ranges?
- What evidence do you see of sandstone dipping beds here? What direction do they run?
- What do you believe has caused the formation of this cave?
- What colour pan and soils do you see present here?
- What evidence do you see around you of continued erosion?
References: Geological Survey of Victoria, 2002.