Bombay Display Board: Geology
The text, images and references of the 'Geology' column as featured on the Bombay display board at Bombay reserve.
The oldest bedrock in the region forms the spine of the western Tallaganda Range and Mt Palerang. These sedimentary sandstones, mudstones and conglomerates were once laid down in a shallow sea system sometime during the Ordovician period (485.4 to 443.8 million years ago) before being folded and uplifted over time.
During the Silurian period came the ‘Long Flat South Volcanics’ (443.8-419.2 MYA). These ‘tuff’ deposits underpin much of the Bombay landscape and were formed by an explosive volcanic eruption. Pyroclastic flows deposited huge volumes of hot ash material, quickly cooling to form the fine-grained rock exposed along the river corridor. These quartz-rich rocks weather down to form acidic pale orange-white clay soils - known locally as ‘Bombay spew’ - which are unfortunately susceptible to waterlogging, erosion, salinity and bogging vehicles.
Rocks of the Long Flat South Volcanics exposed at Bombay Reserve. Credit: M Gill
To the east, the bedrock is dominated by ‘Braidwood Granodiorite’. This rock formed from magma cooling slowly, deep underground during the Devonian period (419.2-358.9 MYA), underpinning features like Mt Gillamatong. More than 90% of land over the Braidwood Granodiorites was cleared historically for cattle and sheep grazing, where rocky outcrops and tors are common.
Outlook over Bombay with granodiorite outcrops, Shoalhaven River valley and Tallaganda range in the background.
Credit: E Brinkley
Over time, the Shoalhaven River has cut a deep channel through the landscape. Along the river and its tributaries, you can find the much ‘younger’ sediments which have eroded and washed down the catchment over the last 40 million years. Dominated by the Illogan Park and Larbert soil landscapes, these alluvial deposits were extensively dredged for gold back in the day, with some sand and gravel extraction for building industries occurring at a minor scale downstream near Warri.
Simplified map of the geological landscape around Bombay discussed above. Credit: E Brinkley, eSPADE NSW DPIE 2025
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Geology References:
- Special thanks by Dr Leah Moore, Associate Professor Land and Water Science, Fenner School of Environment and Society, FSES, ANU & Centre for Applied Water Science, University of Canberra for the to workshops and handouts about the Bombay region she provided.
- State of New South Wales through Department of Planning and Environment 2023, eSPADE version 2.2, accessed 20 November 2023 <https://www.environment.nsw.gov.au/eSpade2WebApp#>
This Bombay Landcare Information Board project was proudly co-funded by Bendigo Community Bank Braidwood, WaterNSW
Bombay Landcare Group and Upper Shoalhaven Landcare Council. With Special thanks to Sheep Station Creek Landcare's Christine Payne and Michael Gill for illustrations and design. Artwork © 2025 C Payne & M Gill. All rights reserved.