Tuesday, 28 October 2014

Pretty Beach Rocks

On the NSW South Coast, just south of Kioloa, there is a little place tucked away in the northern reaches of the Murramarang National Park called Pretty Beach. And it is. One of the features of the place that I love is the sculpturing of the rocks along the northern headland. The sandstone there seems particularly prone to honeycomb weathering, and here it takes on some intriguing forms that are really cool. So you can interpret this blog post’s title as Pretty Beach rocks, pretty beach rocks, or Pretty Beach rocks!

I’ve camped in and explored the area several times in the past, especially when my son was about 8 years old, and have some very fond memories. But perhaps another large part of the reason I love the area is that it is within the Sydney Basin, albeit towards the southern edge, so has the typical geology and floral ecology of the sandstone landscapes that I grew up with as a kid in northern Sydney.

James in his speed boat at Pretty Beach, October 2004.
James at Pretty Beach, October 2004.

The Sydney Basin stretches from South Durras in the south to Newcastle in the north, reaches west to Lithgow, Mudgee and north-west of Muswellbrook, and east to the edge of the continental shelf some 30 odd kilometres offshore. The total land area of the Basin is about 44,000 square kilometres (plus a further 5,000 square kilometres offshore). The bit I’m referring to in this blog post occupies about 300 square metres, just 0.0000006% or 6 billionths of the Basin.

The Sydney Basin is defined geologically by the sedimentary sequences of Permian and Triassic sandstones, coal seams and shales that were laid down some 290-200 million years ago. The earlier Permian Sandstones derive mainly from marine sediments, the later deposits from freshwater alluvial fan and fluvial deposits. Uplifting of the area during the middle Triassic about 230 million years ago raised the whole basin, the eroded outer edges of which now manifest as impressive landforms such as the massive sandstone cliffs and escarpments, hidden valleys, and plateaux of places like the Blue Mountains, Capertee Valley, Morton National Park etc.

At Pretty Beach, the sandstone is from the early Permian and is derived from marine sediments, so fossil shells are often seen in the rocks. I recall going on an excursion to Kioloa back in 1979 during my first year of university, on which we made a brief visit to some fossil beds at Merry Beach, just around the headland from Pretty Beach. 

The strata in the sandstone are evident in the cliff faces. 

Both the rock platforms and the massive fallen boulders are subject to honeycomb weathering.

Rock platform at Pretty Beach showing honeycomb weathering of the soft sandstone.

Fossilised scallop shells exposed in a slab of Permian sandstone fallen from higher up the cliff.

The next set of photos just gives an impression of the diversity of forms the honeycomb weathering can take. 

And weirdly, for some reason pitting is sometimes concentrated along cracks in the sandstone.

And the play of light made depressions look like raised nodules.

And probably best of all are the miniature 'volcanic' landscapes - I've no idea how or why these develop quite like this.

Eventually the sandstone is eroded to small grains of sand, producing the stunning golden beaches for which the Sydney region is famous. 

And being a birdo, I can't possibly leave this post without including at least one bird!

Eastern Reef Egret (or Pacific Reef Heron) flying past O'Hara Island at Pretty Beach, NSW.

It's been a few years now since I last visited Pretty Beach - time to go again, I think. 

For anyone interested, there is some good information, particularly about the geological and geomorphological aspects of the Sydney Basin, but also a little about the ecology, at these sites:


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