In this earthcache you will learn about and how to recognise anticlines and synclines.
Examine the tall exposed rock face directly east of you at the listed coordinates and answer the following earthscience tasks. Provide your answers to the cache owner in a timely manner, i.e. less than a fortnight.
-What angle are the rock folds presented?
-Which direction is the anticline?
-Which direction is the syncline?
Rocks behave plastically and become folded in response to stress. Folds result from the deformation of rocks. Deeper in the crust, where the rocks are more ductile, folding happens more readily. The most basic types of rock strata folds are anticlines and synclines.
Anticlines and synclines. An anticline is a fold that is arched upward to form a ridge. A syncline is a fold that arches downward to form a trough. Anticlines and synclines are usually made up of many rock units that are folded in the same pattern.
Time Line
- 4500 million years ago (mya) the earth forms.
- 3500 to 2 500 mya the first record of bacteria.
- 2500 mya oldest known rocks of Kakadu form as granite intrusions in the earth’s crust.
- 2000 mya erosion of the crust exposes the granite. Faulting forms a wide shallow depression, a geosyncline.
- 1870 to 2 000 mya oxygenated atmosphere evolves. High areas erode and are deposited s sediments in the geosyncline. Faults sag and layers of sediment 10 km thick collect in the basin.
- 1860 mya the weight of sediments destabilises the earth’s lower crust and mantle, leading to mountain building. Pressure and heat fold and metamorphose sediments to gneiss and schist rocks. New granites intrude. Faults open up rift valleys in southern Kakadu and volcanoes fill them with lava. There is very rapid erosion by high-energy rivers.
- 1650 mya micro algae evolve. Flash flooding of large braided rivers spread a one km thick layer of sand from an unknown land mass to north west. This was followed by a long erosion period in an arid climate produces a flat, desert-like landscape with scattered low ridges and hills. Rocks are deeply leached.
- 1000 mya sands consolidate to form the Kombolgie Sandstone Formation which now forms the Arnhem Land Plateau. The sandstone is often ripple-marked and cross-bedded and there is basalt lava near the top.
- 500 mya, the time of the first macro organisms. A long geologically stable period apart from crustal warping that forms faults and joints in sandstone, now weathered to gorges and waterfalls.
- 500 - 140 mya is the time of the first land plants and age of the dinosaurs. Mesozoic seas spread across the area, eroding older sandstone into sea cliffs (escarpments) and islands (outliers). Sandstone and siltstone containing fossils are deposited over lowlands.
- 100 mya Mesozoic seas receded and most of their sediments are eroded away. The major components of today’s landscape are apparent.
- 50 mya ancient faults in southern Kakadu move once more to form local depressions. Swampy sediments with fossil tree palms are laid down and preserved.
- 10 to one mya continued erosion of highlands and deposition of sand on lowlands. Rocks are deeply leached and laterite forms.
- 60000 years ago the first humans arrive in Australia
- 10000 years ago, in the Estuarine Period, the sea invades lowlands. Beach ridges show the sea advancing 20 to 30 cm a year.
- 7000 years ago the sea level drops and freshwater swamps form as the coastline advances.
- 20 years ago in 2004 earthcaches are created.