A society grows great when old men plant trees whose shade they know they shall never sit in.
Greatness is measured in many ways. Something to ponder while thinking about this Earth Cache.
Welcome to a place Geo mum often stops on her way to or from work. Having grown up on the river flats of the Murray River the Green Rolling Hills of South Gippsland are somewhat of a novelty, nearly always green, ever changing and always beautiful.
What’s the geology behind them?
The oldest outcropping rocks in the South Gippsland area are of Cambrian age (approx. 580 - 500 million years old). During the Cambrian period, Victoria was covered by mainly deep ocean.
South Gippsland remained under deep ocean throughout the Ordovician, Silurian and early Devonian periods (approx. 380 m.y.a). At this time, Australia was still a part of Gondwana and did not lie in its present position. It is estimated that during the Ordovician period, Victoria was located within 20 degrees of the Equator. During the Silurian it moved southward and by the early Devonian period had moved close to its present position. Throughout these periods, thousands of metres of sediment, mainly sandstones, mudstones and shales were laid down within a deep oceanic trough, which extended from Tasmania into New South Wales.
From the middle Devonian period to the beginning of the Cretaceous period most of Victoria was no longer covered by ocean and several thousands of metres of sediments were eroded away forming a nearly level plain. This erosion exposed granites around Wilson's Promontory, which were formed in the middle Devonian period (385 - 400 m.y.a) as intruded magmatic material crystallised slowly at depth.
In the late Jurassic and early Cretaceous periods (160-96 m.y.a), Australia and Antarctica began to split apart. As they moved apart a large basin or 'rift valley' was formed. This basin was then slowly filled by up to 3 000 metres of sediments such as sandstones and mudstones. Accumulated swamp material would later be compacted to form the coal seams in the Wonthaggi area.
From the Middle Cretaceous period, Australia moved further away from Antarctica. The stresses caused by this separation resulted in significant folding, faulting and uplifting of the Early Cretaceous basin sediments. The uplift resulted in the development of the Otway and Strzelecki Ranges. The rocks forming these ranges have a high feldspar content and weather relatively easily. This together with the high rainfall has resulted in the development of rounded hills with generally deep soils.
During the upper Cretaceous and early Tertiary periods, extensive lava flows occurred within many of the valleys in South Gippsland. Extensive present day outcrops are located in uplifted areas of the South Gippsland Highlands, around the eastern margin of the ranges and around Leongatha and Dumbalk. In other areas they have been covered by subsequent Tertiary and Quaternary sediments ( Lang Lang River basin).
Victoria experienced a warm and wet climate throughout much of the Tertiary period and temperate rainforest was widespread over the state. During the Tertiary period (65 m.y.a) Australia continued to drift away from Antarctica (at rates of up to 6 cm per year). Three interconnected basins (Otway, Gippsland and Bass) formed along the southern edge of Victoria's continental plate and significant marine sediments were laid down in shallow seas. Some parts of these basins now lie onshore but most are to be found beneath Bass Strait. The sea level fluctuated significantly during Tertiary times.
Following the uplift of the South-Eastern Highlands in the Tertiary period (approximately 5 m.y.a), extensive river systems developed over dry land areas. These rivers developed deep channels, which were filled with coarse gravels. Some of these have been preserved as river terraces along present-day streams. As the uplands became more and more eroded, the rivers draining them became much reduced. These slower flowing streams then deposited only fine grained sediments such as silt. As a result the coarser gravels were covered by finer-grained sediments.
The most recent geological period is the Quaternary which extended from 1.6 m.y.a to recent times. During the Quaternary there were several ice ages which resulted in significant sea-level changes. The most recent ice age peaked at approximately 25 000 years ago and sea levels were some 150 m lower than present. Quaternary deposits include: Coastal deposits such as dunes, beach deposits, windblown sand sheets and barrier deposits. Swamp and lagoonal deposits. Stream alluvium (sands, silts and clays deposited by streams carrying eroded material from the highlands).
In modern times this particular location was considered to be apart of the mighty Blackwood forest. In the early 1800’s timber was the primary industry in the area, timber was felled and shipped to Melbourne through ports such a Queensferry, the majority of buildings in Melbourne built at this time were made from Blackwood forest timber. In 1860’s land here was opened up for selection. Dairy, Beef and Mutton prospered.
So now we know how the hills were formed and why they are currently bare, what does this mean for the hills now?
Naturally weathering continues, weathering actually proceeds faster than it should as the land has been cleared. This location has some great examples of past present and future problems and solutions both natural and man-made.
Cattle grazing (or any hard hoved animal) on such steep lands result in ruts that look like 1000’s of mini terraces, these ruts are referred to as catsteps or terracette’s. By definition a terracette is a type of landform, a ridge on a hillside formed when saturated soil particles expand, then contract as they dry, causing them to move slowly downhill. It may also be described as a small, irregular step-like formation on steep hillslopes, especially on those used for pasture which are formed by soil creep or erosion of surface soils exacerbated by the trampling of livestock such as sheep or cattle.
This changes the shape of the land and speeds up the process of erosion. This can also compact the land resulting in pasture health declining. Farmers will often plow these ruts in order to restore the land, they then either plant new pasture or lease their land to Pea growers (popular in South Gippsland)(Pea growers are required to return the land to pasture at the end of a lease), the problem with this is the top soil is often to shallow due to the erosion that has already occurred, this means the clay subsoil is brought to the surface, a lack of nutrients in the subsoil paired with South Gippsland’s high rainfall, results in pasture not being quick to take hold causing more erosion, the Pea growers often plow the soil very fine which also speeds up the erosion process and the cycle continues.
Landslips are common in the area, Tunnel erosion and Silting of streams is very common. Planting trees, Plowing and Rotational grazing are all tactics commonly used in this area to combat the aforementioned.
To log this Earthcache we require you to read the cache notes provided and to visually explore GZ, then, to the best of your ability message us with the following answers;
1. Suggest why a farmer digging at the top of a hill in this area would reach clay quicker than a farmer digging at the bottom of the same hill?
2. Looking North East from GZ can you see any signs of Erosion or a Solution to Erosion, name at least 1?
3. Looking South West from GZ can you see any signs of Erosion or a Solution to Erosion, name at least 1?
4. A photo of your Team, GPS near GZ with your log and answers. (optional)
You are welcome to log your answers straight away to keep your TB's and Stats in order but please message us with your answers within a couple of days. Cachers who do not fulfil the Earth Cache requirement will have their logs deleted.
Acknowledgments http://vro.agriculture.vic.gov.au, wikipedia
