How the Geography of Schouten Island was Formed:

Schouten Island took over 375 million years to form into the island it is now.

To fully understand the geology of Schouten Island, we must attempt to understand exactly how the island was formed and what geomorphic processes created its fascinating geology.

During the Paleozoic Era (period of change from about 542 million years ago, to about 251 million years ago), the bedrock of Eastern Tasmania was deposited on the strata called the Mathinna Supergroup. 

400 million years ago (mya), off the edge of a continental shelf, layers of mud and sand accumulated. This sometimes occured due to underwater landslides, and also happened slowly in still water. During this time, Eastern Tasmania was very close to the equator.

380 mya: Collision of two tectonic plates compressed the layers of rocks, folding and faulting them. The high mountain range that was created began to drift slowly downwards.

370 mya: Folding and faulting caused weaker areas in the Earth's crust, through which huge masses of molten granite pushed upwards and melted the sedimentary rocks. When the magma cooled, solid granite remained, surrounded by baked sediments. Much of the sediments previously covering the granite was eroded to expose the bare granite at the surface.

290 mya: As Tasmania was moved closer to the Polar Region by continental drift, the East Coast sunk into the sea  and glaciers dropped large stones into small sediments which then buried the granite under several layers.

170 mya: Instead of being compressed, upwelling magma forced apart the crust along the East Coast of Tasmania. Large masses of dolerite were pushed between horizontal layers previously deposited. As the crust slowly rifted, the Tasman Sea began to open up. 

The faulting occured on Schouten Island during this this time, and caused the prominent division in the island's geology.

60 mya: As the Tasman Sea opened up further, some blocks of land were lifted up while others sunk. What is now the Freycinet Peninsula became a low range, while Great Oyster Bay opened up as earthquakes caused underlying rocks to sink downwards.

2 mya: The earth entered the Ice Age. During a series of glacial periods, Tasmania was covered with ice. However, there was not enough precipitation on the East Coast to build up large snow banks.

Today and Onwards: Weathering and erosion of the granite areas continues today, and this can be observed in the discolouration on rock surfaces where water emerges from joints.

Devonian: The devonian granite that makes up the the Eastern half of the island is the same as the red granite produced at Coles Bay used for architecture. Red granite usually consists of course graicontaining a large amount of orthoclase, which gives it the reddish appearance. Separate patches of finer grained granite are also present on Schouten Island, however they are infrequent and contain the same minerals and are the same age as the more common coarser type. 

Permian: Rocks that date back to the Permian age have been reported on Schouten Island, consisting of two small rocks that lie at either ends of the fault line, which separates the granite and dolerite areas. 

On the northern coast, there is a small outcrop of indurated siltstone, approximately 15ft long and 14ft wide. 

On the southern coast, the area where the fault line meets the sea is a small bay edged with Permian Cliffs, 35ft in height, over which flows a small creek.

These Permian cliffs, while they are limited in area, are very prominent. They are similar to the famous Tessellated Pavement, at Eagle Hawk Neck, located on the Tasman Peninsula.

Triassic: The two main divisions of the Triassic are very prominent on Schouten Island; the Knocklofty or Ross sandstones and shales which are large, prominent cliff sections and the Feldspathic Sandstone, which contains coal.

The main exposures of the triassic Ross Sandstone and shales are along the western coastline. They form steep cliffs that rise up to 250ft above the water.

The Feldspathic sandstones are not as prominent as the Ross Sandstone and only occupies narrow coastal areas over short distances.

Jurassic: The typical Jurassic dolerite occupies the majority of the Western half of the island, and rises to a height of about 900ft above sea level.

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