By Peter Cox
The Tamar Valley is the product of 200 million years of geological evolution. The story begins with the splitting up of the old super-continent of Gondwanaland, which began about 200 million years ago.
About 145 million years ago, at the beginning of the Jurassic period, lava began to intrude into the earth’s crust and spread out below what is now the central, eastern and southern areas of Tasmania. It cooled slowly below the surface to form dolerite, a very hard rock. Over the next 80 to 90 million years the rocks above the dolerite were gradually worn away until the dolerite was left as a hard erosion-resistant cap, which tops many of Tasmania’s mountains.
At the beginning of the Tertiary Period, some 65 million years ago, earthquakes began to split up this dolerite capped plateau. The faulting created a trough from central Bass Strait through northern Tasmania into today’s Midlands. The eastern edge was the Eastern Tiers of Ben Lomond, Mt Barrow and Mt Arthur. The Western Tiers formed the western edge.
The central trough was split into two main valleys, separated by a central horst, or upland ridge. The rivers draining the mountains flowed into the central valley, creating two huge lakes, known by geologists as Lake Tamar and Lake Cressy. They stretched out into the Bass Basin.
Over the next 30 million years rivers eroding the highland areas flowed into these lakes and deposited their sediments, filling the two lakes with clay, sands, gravel and boulders to a depth of up to 800 metres. Eventually these lakes were breached and drained, to create the Midland Plain in the east and the Norfolk Plains in the west. Through these plains flowed today’s South Esk and Lake Rivers. The Tamar Valley itself was a continuation of the Midland Plain, and it was at the same height.
The faulting created a continuous high ridge along the western edge of the valley. To the east the ridges rose in a series of steps towards Mt. Arthur and Mt. Barrow. Over millions of years the river has succeeded in eroding away the lake sediments, down to the level of the river today. These sediments were carried out into the Bass Basin and deposited there.
The original course of the South Esk ran to the east of present-day Evandale. It flowed through the valley now drained by Rose’s Rivulet and joined the North Esk River at Corra Linn before flowing through the Bass Basin. The Lake River flowed north, joining the Meander River to flow through the Bass Basin between present day Devonport and Port Sorell.
Periodically between 40 and 20 million years ago volcanic eruptions created lava flows in many parts of the valley. They cooled to form basalt rock. These flows also occurred over much of Northern Tasmania. One flow, some 50 metres thick, originated from Cocked Hat Hill, near present day Breadalbane. It blocked the channel of the South Esk River near Evandale and diverted it to the west to flow into the Macquarie/Lake River near Longford. They then joined the Meander River, which had been diverted eastwards by further lava flows. The combined waters of these rivers overflowed the dolerite barrier, and they carved out the Cataract Gorge, forcing their way into the Tamar Valley at present day Launceston.
Further lava flows covered the lake sediments at Windermere and East Arm, preventing their further erosion. They formed the hilly areas in the middle valley – Gaunt’s Hill, Murphy’s Hill and Brady’s Lookout. Basalt flows also blocked the river, creating a series of lakes and diverting the course of the river. This happened at Whirlpool Reach, between George Town and Clarence point, and possibly between Legana and Dilston. The river cut its way through the blockages, but the lakes remained.
Stretched and thinned by the geological movements and filled by sediments brought down by the rivers, the crust in the Bass Basin sagged, and later was encroached by the sea. As a result of this sagging, the lake sediments are at a lower level downstream from Whirlpool Reach. The dolerite hills each side of the valley were also lowered, disappearing beneath the sediments as they approach today’s coastline. The river flows between sedimentary plains at Rowella, Bell Bay and Beauty Point, plains, which are also protected from erosion by small areas of basalt beneath the surface.
About 3 million years ago the sea invaded the Tamar Valley for the first time, filling up the old lake basins and forming the estuary we know today. Over the last 2 million years there have been dramatic changes of sea level, caused by a series of ice ages. During times of low sea level, the river has continued to erode the floor of the valley. This has created a narrow channel, up to 50 metres deep that forms today’s shipping channel and makes the Tamar navigable for small ships as far inland as Rosevears.
[This article was written for the Society’s newsletter following Peter’s presentation in Nov 2020. During the week prior to his sudden passing in early October, Peter was researching further into the topic in preparation for a presentation to Tamar Valley U3A the next week. You can also watch Peter explanation among the videos produced for the TEER (Tamar Estuary & Esk Rivers) Program in 2021-22. https://www.teer.org.au/videos/coordinated-management ]