[Proc. Roy. Soc. Victoria, 33 (N.S.), 1921.] 



Art. VIII. — An Example of Gravitational Drift of Rock 

 Debris in Parallel Lines in Sub-Arid Western Australia. 



By J. T. JUTSON. 

 (With Plate V., Figure 3.) 

 [Read 12th August, 1920.] 



Gravitational Drift of Rock Debris in General. 



One of the most striking features of portions of the interior 

 -of sub-arid Western Australia is the widespread surface cover- 

 ing of fragments of the hardest rocks. These fragments may rest 

 on bedrock, or, as in most cases, on the surface soils ; and the 

 action of rain and wind removes any fine material that lodges 

 on or between such rock fragments, so that the stony character 

 of the ground is maintained over wide areas. These rocky frag- 

 ments 3 re usually not more than three or four inches in size, and 

 are frequently less than this. They form in places the "pebble 

 armour of the desert," as described for other areas, by Hobbs 1 

 and by Mawson 2 . Such areas are also termed " stone fields." 



This rock debris must often have travelled considerable dis- 

 tances, but the rocks that do so are only the hardest, such as 

 quartz, dense ironstones and jaspers. Fragments of most other 

 rocks, even those usually regarded as hard and weather-resisting, 

 such as some fine-grained basic ones, are practically never found 

 far from the parent rocks, showing that their disintegration must 

 be comparatively rapid. This fact is of value in geological 

 mapping. 



As to the mode of travelling of the hard fragments, the direct 

 transporting action of water seems, except along the beds of some 

 watercourses, to be out of the question. The travelling rock 

 debris, however, is not restricted to these watercourse beds, 

 but is found in all positions, and, amongst others, on gentle 

 slopes where nothing but gentle rills of water, without any de- 



1. Hobbs, W. H, — " The Erosional and Degradational Processes of 

 Deserts, with Especial Reference to the Origin of Desert Depressions." 

 Annals Assoc. Am. Geog., Vol. VII., p. 48. 



2. Mawson, D. — " Geological Investigations in the Broken Hill Area." 

 Mem. Roy. Soc, South Aust., Vol. II., 1812, p. 230. 



