Gravitational Drift of Rock Debris. 11$ 



On this eastern side of the hillock, the white quartz debris- 

 (which is in fragments of all sizes up to six or more inches- 

 in length) is arranged in parallel lines, which are in turn parallel 

 to the strike of the rocks, with spaces between the various lines,, 

 free or largely free, from the quartz debris. These interspaces^ 

 therefore, are the colour of the underlying shales, which here 

 are almost black. There are thus more or less alternate dark- 

 coloured and white bands parallel to one another and to the 

 strike of the rocks, represented by the shales and by the quartz 

 debris respectively. The width of the bands varies from about 

 three to six inches. The dark-coloured areas are not entirely 

 free from quartz, nor are the quartz bands quite* continuous, but- 

 distinct parallel bands do exist as described above. 



As regards the mode of formation of these parallel bands,, 

 the direct action of rain must be excluded. No fall would be 

 sufficient to remove the larger fragments of quartz. Moreover 

 the inclination of the ground, and consequently the flow of 

 water when rain falls, are across and not with the parallel 

 bands, and hence the rain could not form the furrows described 

 below. It may also be noted that immediately after rain, the. 

 banding is much less distinct, in consequence of fine sand being 

 washed out from the quartz bands on to the shale bands. The 

 only possible explanation of the banding that occurs to the writer 

 is that the wind is mainly responsible. Fine sand is available 

 from the quartz decay, and from adjacent areas. The shales 

 are eroded differentially by ordinary disintegration and weather- 

 ing along their line of strike in their exposed parts, that is,, 

 where the quartz rubble is scarcest. The wind sweeps this dis- 

 integrated portion away as well as probably rasping the surface 

 with the fine sand available. This tends to create a furrow- 

 running at right angles to the slope of the ground. In addition,, 

 the quartz rubble becomes undermined by removal of the under- 

 lying shales by the same action, with the result that the quartz: 

 fragments topple forward — no doubt gradually — and fall into- 

 the furrow. The " ridge " lately occupied by the quartz debris- 

 then similarly becomes furrowed, and the band of quartz debris- 

 behind, then occupies such furrow. Thus there is a slow gravi- 

 tative drift of the quartz debris in parallel lines, and at the 

 same time the shale surface is gradually being lowered as a 

 whole. The furrows formed in this way are somewhat akin to> 

 the " yardangs " of arid North America and elsewhere. 



