The Henty Sand Dunes of the West Coast

 

Located just north of Strahan, Tasmania, the Henty Dune field is the largest dune field in Tasmania. The sand dunes reach heights of about 30 meters and extend the western coast for 15 km. Although we do not know the absolute age of this formation, we know it formed during the Quaternary period (the last 1.6 million years).

The dune field is important for the potential for understanding variations in the intensity of westerly winds during the Holocene (the last 10,000 years).  This embayment is similar to a natural amphitheater in that it creates a concentration of sediments. In the area, mountains were glaciated and created glacial rivers. The ice margin supplied much of the sand for the Henty Sand Dunes. The Henty Dunes are an important geologic formation because it not only serves as an archeological record but also as a natural archive for the geologic history of the area.

​The main factor in the creation of the sand dunes is the wind current known as the Roaring Forties. This is a largely uninterrupted wind current at the latitude 40° S. The yearly Tasmanian climate has cold fronts moving west to east with low pressure systems passing to the south of the island. The cold fronts, which usually have steep pressure gradients, allow for strong northerly winds. The Roaring Forties are able to generate to generate great speeds because they are largely uninterrupted and because of the relative change in rotational speed of the land at the Earth’s surface.

Therefore as air moves from the equator to the South Pole, it does so over land that is rotating more slowly. Since the air rotates more slowly on land, the air appears to speed up, making these winds seem furious in their speed and constancy. The northerly winds are the dominant wind direction.

The strong winds gave rise to the Tasmania’s largest transgressive dune field, the Henty Dunes.

The dunes illustrates the winds dominant wind direction of 330°. The sand dunes are most evident areas of sparse vegetation and less evident in areas of denser vegetation. There is typically less vegetation near sand dunes because of the spray or sea salt and due to the high porosity and permeability of sand. The Henty River contains numerous small river and creeks that flow into an exposed, small blocked river mouth.

​The river alluvium, sand dunes and gravels represent the Quaternary system in the area. The alluvium can be seen on the margins of the Henty River and on marshy flats. The sand dunes occur extensively between Hemine and the Henty River. They have their greatest development at the northern end of the Ocean Beach, where they extend inland for about a quarter mile. The gravels are most visible near the coastline near the mouth of Montagu Creek. It is in this area, that one can see a section of thick inteterbedded fine sands, grit and coarse gravels. The section is likely deposits from Montagu Creek later eroded by the sea.

​The vegetation serves an important role by locking in relic dune forms. Dune vegetation is specially adapted to withstand the windy, salty environment. The type of dune vegetation plays a large role in developing the shape and profile of the dunes. The protection from foredunes allows more complex vegetation to develop in the hind dunes.

Relict dunes and sandsheets are widespread at the margin of the Bassian Plain that once provided a land bridge between Tasmania and the mainland. They are also located in western Tasmania and in areas of inland southern Tasmania. This area now contains wet eucalypt forest and rainforest and receive mean annual rainfall > 1500 mm. In the south they have been dated > 87.5–19 ka. They illustrate a long period of semi-arid climate in an area extending well to the west and south of the present semiarid zone.

Coastal dunes, such as the Henty Dunes, play an important role in our environment. They help protect against the shoreline against erosion and protect the backshore from flooding. The sand bank provides for natural replenishment of the beach. There are many animals, such as shorebirds, that depend on sand dunes as a nesting site.  Sand dunes are crucial in understanding the impact of changes in sea level because they erode quickly when the sea is rising and are able to provide sea water levels during different times.

The Henty dunes are not only a site of geologic interest but is also a site of archeological interest. At many of the dunes, shell middens and other Aboriginal artifacts have been found making these sites important for understanding Aboriginal culture and heritage.

Acknowledgement: I would like to thank Eugene Domack for leading this field course and for his support with my research. I would also like to thank Hamilton College for assisting in this program and also the University of Tasmania for allowing access to their library

Work Referenced available on request to Editor