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Hidden : 12/09/2016
2 out of 5
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Geocache Description:

The posted coordinates will have you standing with dry feet at any time of the day, however to visit our reference points you will be best to do so at low tide. You can Click here to check tide times at Tooradin.

The Earth Science behind Western Port Bay is very complex, we decided to simplify it a little for this Earth Science lesson as our aim is to encourage interest in Earth Sciences not frighten people with complex scientific jargon.

Western Port covers 270 square kilometres and is an unusual geological feature, formed when the lower reaches of a river system were inundated by a rising sea level during the Holocene period. This ‘Western Port sunkland’ now forms an extensive tidal bay. Deep channels cut the tidal flats, with several catchments draining into the Bay. All these components make for a complex water circulation system within Western Port Bay. The bay as a whole, and features within and around it, are of considerable geological and geomorphological significance. This large embayment is unusual in the variety of habitats that are present, from relatively exposed rocky reefs to mangroves and sheltered mudflats inclusive of extensive seagrass beds. Each of these habitats has a unique mix of fauna and flora, and some sites in Western Port support a high species diversity. Elephant Fish and opisthobranch molluscs being the most notable.

Over the past 200 years, Western Port Bay has undergone significant changes as a result of land use e.g. vegetation clearing within the catchment, draining of the Koo Wee Rup swamp and agriculture, industrial and residential area growth. Of most significance was the draining of the Koo Wee Rup swamp in the 1890s for conversion in to agricultural land. Originally there was no natural drainage systems from the Koo Wee Rup swamp, and in effect no direct water or sediment movement into Western Port. As a result of the draining, excess water is channelled through surface drains and in to the Bay. Consequently, these channels increase the sediment sources for Western Port Bay.

The tertiary aged Western Port groundwater basin is structurally controlled sedimentary basin. The main features controlling the basin structure are the Tyabb Fault, which lies on the eastern side of the Mornington Peninsula, and the Heath Hill Fault further to the east. The western side of the Western Port groundwater basin coincides with the Clyde Monocline-Tyabb Fault system, and to the north the basin wedges out against uplifted basement rocks. The major lowland area in the Western Port catchment, the Koo Wee Rup swamp was formed during the Paleocene and lies between the Tyabb fault in the west and the Heath Hill fault in the east. The basin is composed of a sequence of Tertiary and Quaternary sediments overlying a Devonian granite and Silurian to Devonian mudstone and sandstone bedrock. The Tertiary aged sediments consist of the Childers Formation overlain by the Older Volcanics, which are consecutively overlain by the Western Port Group, composed of three main units: the Yallock, Sherwood and Baxter Formations, comprised of sandstones and sands. The Western Port Group consists of fine to coarse sands and gravels with minor limestone and carbonaceous clay. The entire sequence is in excess of 250m thick in some areas. The Quaternary sediments consist of clay, shoe-string sands and dune sands with up to 2m of peat occurs in the former swamp areas around Koo Wee Rup. The Western Port Sunkland was formerly a major river drainage system, however it was inundated together with Port Phillip by the rising sea in the Holocene period; the Western Port sunkland now forms an extensive tidalbay; Western Port Bay.

Western Port Bay gains international interest as two principal tidal drainage systems occur. In terms of volume, the major system is to the west and north of French Island, and the second lesser arm is to the south and east of French Island. The two systems have their ebb tidal headwaters to the north-east of French Island, between Palmer Point and north of the Lang Lang River. There is a broad area of intertidal mudflat and sandflat exposed here at low tide, the surface being incised by an intricate network of ebb and flood tidal channels. The area is known as the tidal divide or tidal watershed, but the flow of water is complex and it is difficult to determine a single line that separates the two tidal systems. The configuration of the north-westerly draining system differs markedly from that draining to the south. The materials exposed in the deeply incised channels include freshwater peats and organic muds that are indicative of a former extension of the Koo Wee Rup Swamp during lower sea levels.

It is one of the most intensively investigated tidal watershed systems on the Australian coast. The sea floor sediments here are of considerable interest for the data they hold concerning the development of the on-shore swamplands, and for the history they record of late Quaternary sea level changes in northern Westernport Bay.

To successfully log this Earth Cache we require you to consider the information given, check out the reference points and perhaps if you need to do some research of your own, then message us with the following answers to the best of your ability;

1. The posted coordinates should give you a good vantage; describe what geological features you can see? (at least 3)

2. WP2 Check out the mud flats and mangroves, describe the mud, is it thick and sticky, soft and fluffy or hard and dense? Why is it like this?

3.WP3 Check out the ‘Beach’ is it a clean beach with lots of soft clean sand or muddy and mixed up? Why do you think it is like this?

4. A photo of your team or GPS nearby. (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.

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Source: Victorian Resources Online, Melbourne Water.

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