Structural controls on the orientation of erosion gullies in mid-western New South Wales, Australia

The role of geology in determining the drainage network of gullying has been evaluated by quantitative methods of analysis. At gully erosion sites in New South Wales, field evidence suggests that soil cracks are sites of preferential flow and erosion at both the surface and subsurface. Soil fabric and rock structure and weathering studies were undertaken to establish the inheritance of soil cracks from the underlying parent material. Gully orientation was correlated with the strike of joints and bedding planes in a range of rock materials, including sedimentary, intrusive and extrusive igneous, and thermal and regional metamorphic rocks. These units vary not only in terms of mineralogy but also in degree of weathering. However, all are steeply dipping structures (usually exceeding 60°). Correlations exist between gully orientation and joints in the underlying material, but the strength of the relationship varies according to the parent material. In the Ordovician hornfels, where no correlation between joints and gully orientation occurred, the evidence showed that gullying was controlled by faulting along the hornfels–granite boundary. These relationships support the view that throughflow processes are dominant in the development of gullies, since convergence of runoff would not be expected to align with subsurface soil discontinuities.