Origins of Tamar Island

Origins of Tamar Island

The Tamar Valley originated about 65 million years ago when a major series of faults formed a ‘graben’ type valley, as part of global plate tectonics. The Tamar River subsequently followed this valley to the sea. The graben structure placed the dolerite rock, as seen in Mt. Arthur and the Notley Hills, below the valley floor in the location of today’s Launceston.

30 million years ago:

The Tamar valley was blocked near the present location of the Batman Bridge. This was as a result of volcanic activity which was widespread in Tasmania at that time.

A large freshwater lake formed, stretching as far back as Cressy, which was quite narrow because of the nature of the fault valley it filled.

In the city area sediments of the eastern lake shore can be identified south of Waverley, while near the

Silverdome entrance traces of the western shore

sediments have been found.

For over ten million years the lake occupied the valley and muds, sands and gravels several hundred metres thick were deposited in it. These sediments still occur under the central city, East Launceston and Kings Meadows. To the west, Prospect, Trevallyn and Riverside above the highway rest on the solid dolerite rock as seen in Cataract Gorge. To the east, Rocherlea, Ravenswood and St. Leonards all sit on solid dolerite. In a few locations, such as Dunn’s Monumental Masons at Prospect, the lake sediments can be found resting on the dolerite.

Eventually the lake drained away, no doubt because the Tamar had eaten through the lavas, ashes and muds blocking its valley. There then followed an extensive period of erosion, which removed much of the lake sediments in some areas. At the same time, the river system was depositing its own sediments on top of the lake deposits.

14 million years ago: The ice age began and for much of this period sea level was up to 100 metres below the present levees – there was no water in the Bass Strait area and the Tamar flowed westwards across a flat , sandy plain past some hills, now King Island, and into the sea near where Kangaroo Island (South Australia) is today.

The various rises and falls of sea level during the ice age produced an extremely complex series of river sediments around Launceston. Streams ate into their old deposits when sea level dropped, then depositing gravel, sand and mud in the same area when sea levels rose again. Just to complicate the picture, many of the river sediments are comprised of recycled lake sediments.

10 thousand years ago: Present sea levels were reached and for a brief period just before, the sea level rise had resulted in the dposition of vast quantities of mainly mud, such as what underlies Inveresk and Invermay. This makes the ground so unstable that the vibrations of passing heavy trucks can be experienced by pedestrians and stationary car occupants.

The flat part of Tamar I sland is comprised of foreshore estuarine deposits formed since the ice age. Around its edges the higher part of the island is comprised of gravels of the same age as the muds to the north.

Deeper inside our ‘hill’ is a compacted sediment comprised of dolerite boulders – definitely a lake sediment, over 20 million years old. Similar boulder beds have been found in excavations at the university and the Aurora site at Rocherlea. The very core of the ‘hill’ is an even older material which has not yet been fully identified.

Although we have only theories to explain why today’s Tamar Island exists, it would appear that after the sea level reached its present height there was a hill of resistant gravel exposed in the river valley. This hill would have survived erosion and seen other sediments deposited around it as time went by.

It is worth pointing out that even during the first part of the 20thCentruy, there were changes to the river system and the sediments being deposited at Tamar Island. This occurred because of the millions of tons of dredged mud deposited between the island and the western riverbank by the Port of Launceston Authority.

Douglas Ewington

Geologist

(after Launceston Urban Geological map 1996, SM. Forsyth project geologist).