too THE BURDEKIN VALLEY. 



The conclusion arrived at in regard to the Burdekin 

 foasin, that what would ordinarily be considered a youthful 

 and an ancient type of topography are really in all prob- 

 abiUty of the same denudational age, naturally leads one 

 to review the reasoning on which physiographers have 

 based their classification. For a summary of modern views 

 on the processes of stre-am erosion, oiu' can hardly do 

 better than refer to Mr. E. C Andrews' '* Erosion and its 

 Significance," read before the Royal Soc. of X.S.W. in 1911. 

 The principles have mainly been evolved by American 

 writers and are fascinatingly told in such works as Ohamber- 

 lin and Salisbury's Geology or W. X. Davis" Physiography. 



A perusal of these writings shews that it is assumed 

 that in a new or rejuvenated land surface, the action of 

 denudation is in effect almost purely mechanical. It is 

 not until the streams are '' graded," and thus less active, 

 that the relative importance of weathering aetibn is thought 

 to be sufficiently great to be worth considering. 



This assumed predominance of mechanical action 

 is certainly applicable in rocks which are physically soft, 

 but chemically resistant to the atmospheric agencies, or 

 to exceptional conditions in which stream action is very 

 great compared with the decomposition resulting from local 

 rainfall. Thus a large stream passing through an arid 

 region, or a stream possessing such a fall that its corrosive 

 power is very great compared with the work done on its 

 valley sides by the rainfall would certainly approximate 

 in its conditions to the hypothetical cases. 



An examination of the usual American illustrations 

 show, in the majority of cases at any rate, that the conditions 

 are such as are mentioned above. For instance, the Grand 

 Canon of the Colorado is formed by a powerful stream 

 in a region, of light rainfall, and is -excavated in sedimentary 

 rocks which probably are in their nature, mechanically 

 soft but chemically resistant. 



In some text books, iu'order to illustrate the principles 

 of erosion, an island of regularly dome-shaped surface is 

 supposed to have risen suddenly above the sea and to be 

 subjected to the action of the rainfall. Tn considering 

 its subsequent history only mechaaiical erosion is considered. 



