The underlying geology of south eastern Australia is sedimentary rock, laid down as thin sheets under a once vast ocean which covered much of the continent, some 500 million years ago. Sedimentary landscapes are gently undulating and usually have a high portion of their native vegetation still present. Topsoils are shallow, fertility is naturally low and risks like soil erosion and dryland salinity must be managed in a proactive manner. Sedimentary soils surround Lexton and Evansford, as well as east of Burnbank and around the Horseshoe Lagoon area.
Through this sedimentary rock have been a number of granite incursions, notably Mounts’ Beckworth, Bolton and Misery, as well as the ridge immediately west of the Waubra Township. Granite is volcanic rock which never reached the earth’s surface and solidified as solid rock some 350 million years ago. Granite is very hard, hence the dominant position these hills now have across the area. Visible large boulders or slabs of exposed rock are present and usually native vegetation dominates. Topsoil and fertility are limited and erosion risks must be actively managed.
Across this landscape some 1.5 million years ago, volcanic activity was intense. Molten rock pushed through the sedimentary layers and discharged across the landscape through a number of volcanic vents. These vents are the domed hills like Mount Mitchell, Gordon, Big, Quoin, St Marys and Foran Hills, plus many others, which dot the landscape of the area.
This volcanic rock is the basis of the rich, arable soils which cover two thirds of the district. Underground water is also a key feature of the district, as beneath the volcanic rock are the old rivers that once ran across the sedimentary soils. Rivers still run here through the sediments of these ancient watercourses and have been the subject of much interest for the water and for gold which is held within them. Best examples of this gold mining activity are in the Burnbank area, and as recently as 2012 further investigations have taken place.