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Let's do the Time Walk again! EarthCache

Hidden : 12/19/2014
1 out of 5
1 out of 5

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Geocache Description:

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It's just a jump to the left, then a step to the past.

The Ulladulla Time Walk is a 255m concrete path being developed in Brodie Park, Ulladulla. The path will lead you past boulders and rocks in landscaped gardens representing 510 million years of the Shoalhaven's geological history. The rocks are placed in chronological order along the path, spaced according to their age. The plan was for 510 million years to be represented by 510 child-sized footsteps so each half metre brings you 1 million years towards the present.

Parking is available at the Ulladulla Rotary park close by, where there are toilets, BBQ's, a water fountain and a great kiddies playground.

The project is not yet complete. All the rocks are in place, the gardens are landscaped and most of the signs are installed. As funds become available, the volunteers who run this project will install the additional signs.

In order to log this cache you will need to visit the walk and answer the following question:

As you walk along the path, you will see signs by each set of rocks. At the sign headed "Siltstone" from Ulladulla Harbour (dated 270MYA) there are two large boulders and three smaller ones. Please describe the appearance (colour and texture) of the large boulder further from the path. In what way is the large boulder nearer the path different?

Please send me the answers by e-mail, please do not use the new message centre. Do not include your answers in your log. You do not need to wait for my reply before logging. If there is a problem with your answer I will contact you for clarification. More than one cacher may log the cache with one e-mail, but all names must be included in the e-mail.

The first sign describes the walk. Attached is a pocket to hold brochures explaining all the features of the walk. In case the brochures are missing, the following is copied from the brochure and is quoted with the permission of the Gondwana Coast Fossil Walk volunteers.

1. ~510Ma. These 4 boulders all formed in water more than 4km deep to the east of the ancient super-continent, Gondwana. Basalt lava belched out of sea mount volcanoes but since lava doesn't mix with water, it formed discrete pillows of Pillow Basalt. The ancient basalt pillows are no longer recognizable, changed by metamorphism into Greenstone. The black shiny rounded rock is a modern-day pillow basalt from the sea floor east of PNG. It shows what the greenstone would have looked like sitting on the seafloor 510Ma ago. Sediment in the form of organic-rich mud very slowly accumulated in this deep sea environment forming Black Shale (metamorphism later changed it into Black Slate). Silica, mostly from tiny particles suspended in ocean currents and from the accumulated shells of microfossils (=Radiolaria), was deposited on the sea floor to form the banded rock, Chert (now changed to Meta-chert). These original igneous and sedimentary rocks were changed by heat and pressure into the metamorphic rocks we see here as a result of eastern Gondwana’s collision with an oceanic plate around 440Ma ago.

2. ~450Ma. Through 60 million years, little has changed. This region was still on the sea floor in >4km deep water east of Gondwana and the same types of rocks were still forming there. This boulder is another Black Slate. Its age (~450Ma) has been determined by palaeontologists who have correlated species of graptolite fossils with those from other global localities where the age is known from isotopically dated zircon crystals found in volcanic ash layers.

3. ~395Ma. Two of these boulders are Moruya Granite which was used on the Sydney Harbour Bridge portals. As the oceanic plate continued to converge on east Gondwana buried crust melted to form granite magma which intruded from Tuross Heads to Nelligen. This granite is exposed in road cuttings and as tors around Moruya and Nelligen. Activity on the margin of eastern Gondwana would have been on a similar scale to the mega quakes, volcanoes, and granite intrusions caused by the South American plate’s convergence with the Pacific plate. This collision is continuing to build the Andes Mountains and a similar mountain range would have formed here. The third boulder, a weathered brown banded rock, occurs adjacent to the margins of the Moruya Granite intrusion. lt once formed layers of black mud (shale) lying on the sea floor but was changed into a metamorphic rock called Banded Hornfels. Hornfels formed when the shale, heated by a huge volume of granite magma, formed new minerals and re-crystallised into a much harder rock. This rock, often used on road surfaces and as an aggregate in concrete, is commonly known as Blue Metal.

4. ~380Ma. This Granite boulder forms part of another major granite intrusion which formed around 380Ma ago and extends from Mt Durras to north of Granite Falls. Unlike the Moruya Granite, the Shoalhaven Granite is rarely seen at the surface. It was covered by horizontal layers of younger sedimentary rocks and is only exposed where the younger layers have been removed by erosion. This granite formed during the last of the tectonic cycles that impacted in this region. Through each of these cycles Gondwana grew eastwards with new continental crust being added in this region which had been deep sea floor. Following this final tectonic cycle, the Shoalhaven became a stable part of the supercontinent. Gondwana’s active plate boundary continued to move farther east, progressively adding more land that became New Zealand.

5. ~280Ma. By this time, eastern Gondwana had begun to sag, from Durras to Newcastle, and the sea covered the coastal lands. Sediments were deposited over an extensive area now known as the Sydney Basin. These Sandstones form part of the Snapper Point Formation seen locally at Bannister Head. Bivalve molluscs with their large thick robust shells and the burrows of worm preserved as “trace fossils” are preserved in these sandstones. These animals lived on the sea floor in sand deposited by strong currents in at least 20m of sea water.

6. ~270Ma. Deposition of sediment by ocean currents continued on the shallow sea floor of the Sydney Basin. These finer sediments (silt and mud) rich in organic carbon were deposited by gentler currents. It is the carbon which gives the Wandrawandian Siltstone on both sides of Ulladulla Harbour its characteristic dark grey colour. At times the currents were stronger, transporting and depositing layers of sand which became sandstone. Life was prolific on the sea floor and fossils presen/ed in the siltstone include sea lilies(crinoids); sea fans (bryozoansj; lamp shells (brachiopods); bivalve mollusc shells; sea snails (gastropod molluscs); horn corals (solitary rugose corals); “trace fossils" of worms; and carbonised logs of wood; all common in the rock platforms around Ulladulla.

7. ~245Ma. At this time, sediments viz. the Hawkesbury Sandstone were still being deposited in the Sydney Basin, from Marulan to Newcastle, but here in the Shoalhaven sedimentation had ceased around 267Ma ago. In all, a thickness of about 800m of sediments, rich in marine fossils, had been deposited in the Sydney Basin here. The rock in these 2 boulders formed when molten magma cooled to produce the 10km diameter Milton Monzonite intrusions. This magma is believed to have risen from a mantle “hot spot" that initially produced the Jervis Bay intrusion about 247Ma ago. As Gondwana drifted north at around 2cm per year, the Monzonite was intruded when Milton sat over the hot spot about 245Ma ago.

8. ~241Ma. The two dark coloured boulders form part of the Termeil Gabbro intrusion which is centred on Bawley Point. As Gondwana continued its move further north, Bawley sat directly over the “hot spot" and magma rose into the crust to form a circular shaped intrusion. The best known mantle "hot spot", now under Hawaii, has persisted for around 80Ma, and continues to produce many spectacular volcanic landforms. In contrast, magma from the south coast hot spot did not reach the surface to form lava flows or volcanic structures. The hot spot produced 3 intrusions all of similar size and the magma only reached up as far as 1 or 2km beneath the surface. The third boulder was collected from directly above the intrusion at Kioloa. lt was part of a layer of sandstone of the Snapper Point Formation but heat from the gabbro magma baked (metamorphosed) the sandstone and it became a very hard Quartzite.

9. ~100Ma to 84Ma. These are geometric shaped blocks of Wandrawandian Siltstone the result of the pattern of cracks visible in the harbour rock platforms. The rocks fractured to form this tessellated pavement during the period when a major rift developed in eastern Gondwana. The rift resulted in the NZ subcontinent, including a trail of islands stretching to New Caledonia in the north, broke away to open up the Tasman Sea.

10. ~31Ma to 30Ma. By this time, Australia had broken free of the last remaining piece of Gondwana (= Antarctica) around 45Ma ago. The Australian plate was now able to move north at a much faster rate (7cm per year) and frictional drag produced deep fractures (mostly E-W) in the stretched and thinning crust of eastern Australia. This Basalt rock formed when molten magma ascended one of these vertical fractures to form a basalt dyke. Similar dykes and other volcanic structures are found intruding older rock strata in eastern Australia from Burnie in Tasmania to far north Queensland with ages ranging from around 50Ma to recent.

11. ~15Ma. As a result of Australia/Antarctica split, the landscape continued to erode and the climate in Australia became more and more arid. Around 15Ma ago the climate changed from being essentially dry to warmer, wet conditions. Extreme leaching of sandy soils produced Silcrete in coastal regions of south-eastern Australia. Silcrete forms a very hard near surface layer usually less than 3m thick. It typically comprises 99.9% silica (SiO2). For more than 50 years, from the early 1900s, Silcrete was mined at several sites to the north of Ulladulla as a source of pure silica, mostly used in brick making.

12. ~5Ma. This boulder of Shelly Limestone (“Coquina”) was recovered from a horizontal sedimentary layer on the edge of the continental shelf about 30km east of Ulladulla. The layer now lies in more than 300m water depth, yet the fossils in it are of species similar to modern day animals that live on or near shore in very shallow water. This confirms that major sea level fluctuations have occurred here in the past 5 million years and that the sea level is now more than 300m higher than it was at that time. Erosion and the significant fluctuations in sea levels, particularly those documented for the last 200,000 years, have determined the shape of our present coastline and the nature of the hinterland.

13. ~3-4 Ma. During another wet period, extreme leaching produced Ferricrete, a very hard reddish or brown sedimentary rock with high iron content. It forms caps on top of the silcrete horizon, especially on headlands composed of Sydney Basin sedimentary rocks. The cement that binds Ferricrete is hydrated iron oxide, produced because iron has low solubility. The iron oxide is only precipitated after other more soluble metals, such as sodium, potassium, magnesium and calcium have been dissolved and flushed out of the soil and rock by percolating groundwater.

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