Walk down into Little Oberon Bay and you'll immediately be struck by how rocky the hillside and shoreline is.

In this photo, Mount Oberon lies to the left and Little Oberon behind you. The narrow beach has prominent low cliffs and a major drainage channel that takes rainfall off the western slopes of Mt Oberon. The beach deposits are well sorted quartz sands, however, as you'll see, the deposits in the cliffs behind the beach are very different!

There are a range of materials present within the cliff faces and shoreline. Here's an exposure of poorly sorted angular silt, sand and gravel (sand- to pebble-size particles) composed of quartz and granite lithic sediment. Notice the centimetre scale bar on the exposure. The sediments are also weakly stratified (bedded).

When describing regolith materials we use the Wentworth size classification scheme, which attributes grain size ranges to particular names:

The examples below illustrate the range of colluvium at Little Oberon Bay from the smallest to largest grain size materials.

Here's another example of poorly sorted sediments. In this case, very poorly sorted (notice the boulders in the cliff face!).

The sediments are debris flow deposits, which are classified as colluvial regolith. Colluvium may be deposited by gravity (landslides), as debris flows in high- to moderate-relief landforms including high-relief colluvial fans along rangefronts and mountainsides and as moderate-relief sheetwash fans in areas with more subdued relief.

The important thing to remember with colluvium is, even though it might be deposited by gravity or water (so how is it different to alluvium you ask?) it's always near to its source and is generally poorly sorted with angular fragments (although there are always exceptions to generalizations in geology!).

In this example the sediments have a larger grain size and are extremely poorly sorted. You'll notice that some boulders have appeared in the cross-section, however, the deposits are characterized by matrix-supported clasts (large clasts suspended in smaller particles).

Debris flows occur in areas of high relief on mountain or hill slopes and have been described by the USGS as being "soupy". They feature a dense mixture of poorly sorted materials and can move downslope slowly as slumps, or more vigorously as mudflows, depending on the amount of water they contain. Densities of debris flows vary depending on the types and grain sizes of materials being carried and amount of contained water, but can be as high as 2200 kgm^-3, measured in volcanoclastic debris flows at La Fossa volcano on the island of Vulcano, in Italy (Ferrucia et al. 2005). Considering the typical density of granite (2700-2800 kgm^-3), debris flows are almost as dense as the materials they contain. Large blocks of rock are rafted within debris flows (almost "floated") and are deposited in a chaotic assembly once the flows comes to rest.

Debris flows represent a considerable geohazard in areas where there is high relief, lots of rainfall or snow accumulation or areas that are tectonically active (like volcanoes and mountain ranges). They are capable of scouring out channels and, because of their density and erosive capabilities, can undermine and physically lift and remove infrastructure like road and rail bridges, weirs and dams.

In this example, we're looking at a clast-supported debris flow deposit on the beach. This deposit has very little fine material, consisting mostly of granite boulders and cobbles, and would have moved quite slowly down hill to its resting place.

Finally, this is what the debris flow deposits look like once the waves have got into them and washed away the finer material. Notice that these boulders are subrounded to rounded in shape. They have been tossed around during storms and have had their sharp edges knocked off.

Learn more about Little Oberon Bay and colluvial landscapes by reading:

L.H. Blikra and W. Nemec 1998. Postglacial colluvium in western Norway: depositional processes, facies and palaeoclimatic record. Sedimentology 45, 909-959.
W.D. Birch ed. 2003. Geology of Victoria. Geological Society of Australia special publication No. 23. 842 p.
M. Ferruccia, S. Pertusatib, R. Sulpizioa, G. Zanchettaa , M.T. Pareschid and R. Santacroce 2005. Volcaniclastic debris flows at La Fossa Volcano (Vulcano Island, southern Italy): Insights for erosion behaviour of loose pyroclastic material on steep slopes. Journal of Volcanology and Geothermal Research Volume 145 (3-4), pp. 173-191.
D. Meagher and M. Kohout 2001. A field guide to Wilsons Promontory. Oxford University Press, South Melbourne, 352 p.
G. Taylor & R.A. Eggleton 2000. Regolith Geology and Geomorphology
S.M. Hill 1999. Mesozoic regolith and palaeolandscape features in southeastern Australia: significance for interpretations of denudation and highland evolution. Australian Journal of Earth Sciences 46, pp. 217-232.
E.S. Hills 1975. Physiography of Victoria. Whitcombe and Tombs Pty. Ltd, 372 p.
E.C.F. Bird 1984. Coasts - an introduction to coastal geomorphology. Australian National University Press, Canberra, 320 p.

Visit the Geoscience Australia Natural Hazards Group to learn more about geohazards in Australia and southeast Asia, including landslides and debris flows.

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