Back by popular demand, Leah took the group to visit several geological sites of interest across member’s properties. Starting in the foothills of the Tallaganda Range, Leah described the processes which created the underlying sedimentary rocks. The sandstones, mudstones and conglomerates which form the spine of the Range were once laid down in a shallow sea system sometime during the Ordovician (485.4 to 443.8 Mya) before being uplifted and faulted over time.

Moving down from Tallaganada and east toward the Shoalhaven River, the group got to see the Long Flat South Volcanics. Leah indicated these formed sometime during the Silurian (443.8-419.2MYA) from an explosive volcanic eruption, depositing huge volumes of pyroclastic ash material. These fine grained rocks are quartz-rich and weather down to form white-pale orange clay soils - known locally as ‘Bombay spew’ - susceptible to waterlogging, erosion, salinity and bogging vehicles.

Next, the group visited an outcrop of the Braidwood Granodiorite which dominates the landscape east and west of Braidwood. This rock formed from magma cooling slowly deep underground, creating features like Mt Gillamatong and the rock used to build St Bedes Church.The tour wrapped up along the Shoalhaven River, where you can find the much ‘younger’ sediments which have eroded and washed down the catchment over the last 40 million years or so.

The field trip was designed to compliment a geological mapping session held last year, where Leah showed Bombay Landcare how to use the free online tool ESPADE to access information about local soils, geology and hydrological landscapes (www.environment.nsw.gov.au/eSpade2Webapp/). 

The tour proved to be a great day for ground truthing the online maps, seeing practical examples and rock samples to help explain what's going on geologically underneath the Bombay landscape. The field trip concluded with a BBQ and Bombay Landcare put on a great spread to thank Leah for her time and expertise. We hope she’ll visit again soon.