To successfully complete this Earth Cache you will need to walk along Pinnacles loop walking track and make some observations and answer some questions. It is a 1.1km loop.
The Pinnacles is an erosional feature in soft sediment. A small stream has cut into the soft sediment, forming a deep gully. Numerous runnels cut into the exposed sides of the gully, forming ‘pinnacles’.
A spectacular feature of the gully walls is the contrast in colour in the sediment: rusty red above and pure white below. These colours are accentuated by the bright green of tea trees and the sapphire blue of the ocean backdrop – truly a feast for the eyes.
Although some red sediment has washed down over the white sediment, the sharp, level boundary between rusty and white colours is clearly seen. This looks like a geological boundary marked by a change in rock types. In fact, the sediments are the same above and below the colour change. They consist of fine sand and clay, with thin beds of small pebbles. Note, too, that the erosional features are the same above and below the colour change: you might expect a difference if there was a change of rock type at the colour boundary.
What has caused this sharp colour change? Most likely, it preserves the level of a former water table, whose acid waters removed rust-coloured iron minerals from the sediment. In fact, you can see a similar effect in the sand under your feet at the lookouts. The sand should be rust-coloured - but it is grayish-white. Plant acids carried into the sand by rainwater have bleached out the rust colour. The slight grey colour is from plant organic matter.
Now we need to be a little more technical to present a good earth science lesson so in short the geological formation exposed at the Pinnacles is the Long Point Formation. It is of Tertiary age.
This amazing sandstone formation is made up of aggraded deposits. Aggradation is the term used in geology for the increase in land elevation due to the deposition of sediment. Aggradation occurs in areas in which the supply of sediment is greater than the amount of material that the river system is able to transport away. This clearly reflected a time of very high erosion due to the high rainfall conditions, high sea level, and the creation of the escarpment during the formation of the Tasman Sea.
Deposition of the Long Beach Formation is thought to have occurred during the Oligocene to Early Miocene (23-30 Mya). This is in accord with the 31 million year date for the Coila Basalt, a volcanic flow that is sandwiched between two similar sandstone formations in the Tuross Head part of the region. Aggradation of the south coast mid-Tertiary sandstones is thought to be attributed to an increase in sea level rise during the Oligocene to Early Miocene rise resulting in the development of coastal barriers of transported sediment at higher levels than today. The outcrops seen today are a remnant of the original extent of the sandstones. The diagram below matches up the contours of current outcrops and remnants that could conservatively indicate the extent of the aggraded sediments, which, in the forming basins, consolidated into sandstone.
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Studies have identified four phases of weathering during the Tertiary recognized in these deposits.
The first - Kaolinization - formation of soft white clay under high rainfall is well preserved in the deeply weathered rhyolite (harder volcanic rock)
The second Silicification - Whilst the connection is speculative, it is interesting to note that the local deposition and silicification coincided with a major worldwide increase in silica accumulation (following extensive global volcanism), known as the Silica Burp.
Silicification was followed by two weathering phases of ferruginization. Ferruginization is a complex chemical reaction in a weathering soil profile where iron silicates are altered to iron oxide (haematite) and quartz (silica). The process is usually indicative of a hot arid climate but it also can occur in climates that alternated between warm and humid and dry conditions. The latter is the likely mechanism for the formation of the south coast red Tertiary sandstones.
To log this Earth Cache we require you to consideree the information presented to you in this Earth Cache, walk the Pinnacles loop track and look at the area, perhaps you will need to do some research of your own, then message us with the following answers to the best of your ability;
1. From your chosen vantage point how deep do you estimate the ancient water table to be?
2. The Ferruginized layer is probably one of the most remarkable parts of the pinnicales, from your chosen vantage point how thick do you estimate it to be ?
3. A photo of your team or GPS with the pinnacles in the background, upload as many photos as you like we would love to see the area again and again. (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 24 hours. Cachers who do not fulfil the Earth Cache requirement will have their logs deleted.
Many Thanks to Anne a local Geologist who kindly offered her own personal notes to be used on this EC.
Source: Anne Felton, who’s work also appears on sapphirecoast.com.au.
