CLIFF-TOP DUNES

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Ever since my undergraduate days, I have been fascinated by pits in sand deposits around the eastern suburbs of Sydney. I grew up in Rose Bay. Trevor Langford-Smith had me digging a pit in the backyard of our home in Roberts Street. This was part of a soils project in Year 3 Geography at Sydney University. What joy! I dug down 5 feet (no metric then) and uncovered a well-defined A2 bleached horizon overlying a brown humic-rich B horizon; my mother was not impressed with this hole so I could not invite Trevor home to inspect. However, it was with some urgency that I rushed to tell him what I had found.

It was soon apparent on my walks and runs around Dover Heights that this soil type was quite common. Excavations were high on the agenda to visit. This was back in 1959-60. In 1962 Trevor asked me to assist in drafting a paper on NSW coastal morphology to go into a volume on the Geology of NSW, the so-called pink book, published in the Journal of the Geological Society of Australia (v. 16, 1969). Note the year. It took seven years to produce this volume and during this time we circulated copies of our paper. In 1967, Joe Jennings in his paper on dune sands on top of coastal cliffs referred to our then circulating paper. We had written: “A feature of the NSW coast which has not been satisfactorily accounted for is the sand dunes which mantle the cliff tops in many localities. These dunes have been fixed for a long time interval, for they are well podzolised and support a vegetation cover which includes scrub forest” (Langford-Smith and Thom, 1969, p.579).

Of course this reference included locations well beyond the urbanised footprint of Dover Heights. By late 1962 we had discovered more locations along the NSW coast. Jennings looked at several sites in his paper published in Australian Geographical Studies (1967, v. 5, p. 40-49). He examined several possible explanations for these well-sorted sand deposits located in places above rocky cliffs accepting that they were most likely aeolian in origin. Since then there have been several studies of these or similar deposits with the added advantage over early work that some of the deposits have been dated.

I now live in close proximity to my old home and remain entranced when I see a building excavation with an exposed sand soil profile. Over the years they have been observed at numerous sites along the cliffs of North Bondi, Dover Heights and Vaucluse. Gale and others have recently described similar soils from the nearby Botany Basin (Australian Geographer, 2018, v. 49, p.291-316). While in other areas I had previously struggled to date aeolian sands using underlying organic deposits, the arrival of TL and then OSL dating techniques offered new ways to get an age of deposition. I turned to first David Price and then Tom Oliver of Wollongong University for help. The results have been very satisfying.

In 2016 it was time to get serious and start looking for a range of sites to get samples for dating. It was a dream come true to date these grains of sand. But there was a problem. I needed permission to get onto a building site safely and dig into the sand exposure. I found three acceptable sites and sent them to the Gong. Tom Oliver, now located at ADFA, became very interested in the study. This year we published a paper in the Australian Journal of Earth Sciences, with the title “Evidence for a change in wind regime during the Last Glacial Maximum from the Sydney region”. While we recognised the limitations of sampling, there was no doubt that our results yielded a story that had to be told. It is now up to others to do more work to test the hypothesis we developed in this paper.

Bruce Thom

Words by Prof Bruce Thom. Please respect the author's thoughts and reference appropriately: (c) ACS, 2018, posted 17 December 2018, for correspondence about this blog post please email austcoastsoc@gmail.com

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