Beneath the surface

Soils

The Beyond Bolac catchment area contains high fertility soils that are highly suited to agriculture. The soil characteristics vary across the region, and they reflect the influences of parent material, topography and weathering processes.

Farmers are very aware that the characteristics of soils can be highly variable over a short distance, even when mapping assigns them to the one soil class. The image below is of basalt soil samples from sites only several metres apart. The samples are from approximately 0.5 m depth.

Underground water

The beyond Bolac catchment is located in a section of Victoria that has subsurface water. This can be many metres below the surface. The water an also be found to lie in separate layers, with each layer having a different salinity, and separated from each other by intervening layers of impervious rock. 

Salt

What happens to the water that soaks into the ground? Why do some areas have salty water, while others close by are fresh? These are important determinants of the health of streams and lakes. Researchers from the Department of Environmental Geoscience at La Trobe University, Melbourne have been working in Western Victoria for many years to find answers to these questions. There are many relevant research papers that have been published, that can be accessed from Associate Professor John A. Webb’s ResearchGate webpage here.

Geology

Older basalts

Newer basalts

Great Dividing Range

Volcanoes

Volcanoes

The Western Victorian landscape is dotted with volcanoes. They occur in distinct patches that are covered by basalt soil. Most Australian rocks are very ancient, and some of the oldest rocks in Victoria are located near to Wickliffe. The more recent basalt can be thought of as analogous to icing spread over a fruitcake.

In general the age of the volcanoes are younger further to the west. This is because the Australian continential plate has been gradually moving eastwards over a ‘hotspot’ that has magma is closer to the surface.

The H11-H12 catchments have two ages of lava flows. The older flows have rocks that have weathered to into a basalt soil, often layered as a thin topsoil over a heavier impermiable clay. There are some areas of deep soil lower in the catchment. More recent lava flows have not weathered as much, and can often be identified by raised areas of rocky barriers. Scoria cones are common, and were formed where magma rose through a waterlogged layer of rock or soil. The rapidly exapanding steam produced an aerated lava, similar in texture to an aero chocolate bar.

More information is available from Julie Boyce of Monash University, with this ten minute presentation on the different types of volcanoes in Western Victoria.