Neck and Neck – An Island of Isthmuses

isthmus [ˈɪsməs]: n pl -muses, -mi [-maɪ]

1. (Earth Sciences / Physical Geography) a narrow strip of land connecting two relatively large land areas

 

An island is defined by its coastline. A coastline can be seen as an island’s signature and in the case of Tasmania there is none more unique. The south-east quadrant of the state stretching from Freycinet Peninsula on the east coast, to Bruny Island in the south, is home to five beautiful examples of the rare coastal landform known as the isthmus. Listing them in a clockwise direction we have:

  1. the world famous Wineglass Bay on Freycinet Peninsula
  2. Maria Island which is divided in two by an isthmus halfway along its length
  3. Eaglehawk Neck on the Tasman Peninsula, renowned for its convict era “dog-line”
  4. South Arm on the Derwent River narrowly connected to the “mainland” by a narrow isthmus
  5. and finally there is the isthmus which separates North and South Bruny Island.

There is no comparable stretch of coastline in the world which has so many well developed examples of the isthmus. What is so special about the Tasmanian coastal environment that has led to their formation? There are three essential ingredients: waves, sand and a rising sea-level. Remarkably, four of the five isthmuses have a consistent north-south orientation. What are the forces at work that have made this pattern? To form an isthmus you need equal strength waves coming from opposing directions to allow the sand from the sea-bed to be pushed together to form a central ridge. Waves are generated by wind, so let’s look at the prevailing wind regime around the south-east coast of Tasmania.

The strongest winds in Tasmania are of course the famous Roaring Forties which blast their way, in from the west. They are felt at their maximum during the winter months, with the west coast of the state bearing the brunt of their force. The south-east coast of the island is relatively sheltered from the westerlies by the protective mountains that dominate the south-west wilderness. But as any Hobartian knows, the chilly westerlies of winter can still make their presence felt in the south-east corner of the state. When it comes to waves, the westerlies in the south-east quadrant have limited ability to make waves. This is due to the relatively short distance that they travel over water (the “fetch”) before they hit the peninsulas and islands which wrap around our south east coast.

The balancing force to winter’s westerly winds and waves in south-east Tasmania are the summer sea breezes. They are reliable as clockwork in coming ashore early each summer’s afternoon. As they form five to ten kilometres off-shore they have sufficient fetch to allow small to medium waves to be generated. Sea breezes typically make their land-fall at right angles to the shoreline – so around the south-east coastline they are typically blowing in from an easterly direction. This fortuitous combination of two opposing wind and wave directions has resulted in one of the ideal conditions for isthmus formation to be met.

But where has the sand that makes the isthmus come from? The sugary, white sand that makes up the bulk of the sediments that go into forming the isthmuses is derived from two sources. They are the Devonian era granites which make up the prominent back-bone of the Freycinet Peninsula and the Triassic era sandstones that were so beautifully used as a building stone for Tasmanian Georgian period buildings. The grains of sand were eroded from the bedrock by wind, rain, ice, rivers and waves over many hundreds of millions of years. They have probably been “recycled” through younger generations of sediments in that time until their most recent (but one) resting place – on the shallow sea-floor, close to their current home – where they provide the sand to make an isthmus.

It’s one of nature’s blessings that grains of sand are the perfect size to allow beaches to form. Fine particles of silt and mud needs only gentle currents to move them and they can easily travel long distances. They typically settle in quiet backwaters and form mudflats. Pebble and cobble-sized stones of course require a very energetic wave environment to move them. This is more typical of the storm-exposed West Coast. Sand on the other hand is in the “Goldilocks Zone”, being just the right grain-size to be carried along by waves and currents – which are of just the right strength.

Another critical factor that allows an isthmus to form is an environment where the sea-level has risen. With the ending of the last ice age 10 to 12,000 years ago sea levels around Tasmania rose by as much as 50 metres. This was the period when the land-bridge between Tasmania and the mainland was severed, and river valleys and low lying coastal areas were inundated by the rising sea. When the sea level began to stabilise, the combination of lots of sand on a relatively shallow sea floor and the mutually opposed wave directions allowed isthmus formation to begin. The final ingredients of course are the two land masses, located well above sea level, that get linked by the isthmus.

Our Tasmanian isthmuses have probably been forming over the last 8,000 years. Unlike other coastal landforms such as the sand spit, the isthmus is a relatively long–lived feature. One reason for this is that the forces that make them are relatively balanced. Waves break on their shorelines as “beach breaks” that are parallel and close to the beach. In contrast, when waves come in at an angle to a beach such as with a “point break”, the sediments that make up the beach get transported along the coast as “long-shore drift”. This sort of setting typically sees new beaches constantly forming and old ones being eroded.

Of course, there is always an exception to the rule. Of the five main isthmuses in south-east Tasmania, all but one, run in a north-south direction. The exception is the South Arm isthmus which runs in an east-north-east direction. Using opposing wave directions as a guide its formation probably more relates to the south-south-east swells that come up Storm Bay. The South Arm feature is more closely related to the Seven Mile Beach Spit which was probably built by similar southerlies coming up Frederick Henry Bay.

Footnotes:

  1. In some parts of the world isthmuses are called tombolos. Some famous tombolos are Mont St Michel in France and St Ninian’s in the Shetland Islands of Scotland.

  2. The preference for using the term tombolo or isthmus may depend on your comfort level with the tricky pronunciation of the latter. The problem can be solved if you adopt my dictionary’s suggested pronunciation. It says the “th” in the spelling is silent.