390 Prof. W. G. Woolnough — Later ite in W. Australia. 



The thesis I wisli to establish is that the laterite was produced 

 under peneplain, not plateau, conditions, that is, when the land surface 

 stood at a very slight elevation above sea-level. Youthful streams 

 cut down their valleys to base-level before they begin to widen them 

 at all sensibly. Hence the development of maturity of river erosion, 

 that is, the evolution of a peneplain, can be completed only at a slight 

 altitude above base-level, which in this case was undoubtedly 

 sea-level. 



With a very gentle gradient mechanical transportation of sediment 

 would be insignificant, and even in solution the lateral movement 

 of material would be extremely slow. Chemical weathering, however, 

 would be strongly favoured, and, given alternations of wet and dry 

 seasons, the conditions for laterite formation, postulated by Simpson, 

 would be ideally fulfilled. 



If this view is correct it follows that the importance of laterite, 

 from an historic point of view, is greatly enhanced. It owes its 

 present position on the summit of the plateau to the uplift of the 

 peneplain, the criteria for such an elevation along the Darling 

 Escarpment, with down faulting of the coastal strip towards the 

 Indian Ocean being complete in every particular. Hence it follows 

 that the laterite capping serves as a strati gi'aphic horizon of no mean 

 value. If we find areas of laterite markedly elevated above, or 

 depressed below the general laterite level, we have a prima facie 

 reason to suspect earth movement as a cause. In applying this 

 principle, however, it is important to remember that the peneplain 

 was never a perfect plane, but was always an undulating surface. 

 Under these circumstances certain initial differences of level of the 

 laterite capping must be postulated. In the Darling Plateau area it 

 seems probable that these differences of level were of the order of 

 200 feet.i 



Another possible source of error is the fact, mentioned by Simpson, 

 that considerable areas of redistributed laterite occur. When due 

 allowance has been made for these possible sources of error, a sufficient 

 number of outstanding cases has come under notice to indicate that 

 the importance of the general principle has not been overrated, and 

 that extraordinary differences of laterite level in adjacent areas indicate 

 hlock faulting. In most instances other criteria of faulting may be 

 discovered which convert probability into certainty. As examples 

 may be cited the occurrence along the foot of the Darling Range 

 Scarp, in the immediate neighbourhood of Perth, of isolated 

 remnants of a laterite-covered shelf or step. At Greenmount and 

 Ridge Hill (10 miles E.), where two railway lines enter the scarp, 

 this shoulder of laterite is prominent, while it can be detected at 

 least three points immediately to the south of those mentioned. 

 At Armadale (15 miles S.E.) and Waroona (60 miles S.) similar 

 areas of low-level laterite occur. Erom these occurrences I believe 

 we may suspect that the faulting of the Darling Range Scarp is of 

 the nature of a step fault and not simphr a single fault (see Eig. 2). 



1 The difference in altitude of Chidlow's Well (30 miles E.N.E.) and 

 Wooroloo (37 miles E.N.E.) on the eastern railway, both on the laterite 

 "level", amounts to 256 feet. 



