826 



AN INTERESTING FORM OF SUB-SURFACE 

 DRAINAGE. 



By M. Aurousseau, B.Sc. 



(With Plate xlv.) 



During the autumn and winter of 1915 my attention was at- 

 tracted by certain features of minor drainage, at two widely 

 separated points on the Darling Range, W.A., namely, in the 

 Greenmount District, east of Perth, and at Roelands, 102 miles 

 further south. There has been no opportunity of examining 

 other parts of the range at the same time of the year, but it is 

 highly probable that the type of drainage to be described is of 

 wide occurrence. I have been informed that it is familiar to re- 

 sidents of the Donnybrook District. 



In the regions examined, the soil covered slopes of the range, 

 and the piedmont deposits, exhibited, in places, a dangerous 

 series of small holes, of varying sizes, up to one foot in dia- 

 meter and three feet in depth. They were frequently spaced 

 irregularly along definite lines, which suggested a certain con- 

 nection between holes on the same line. Their true relation to 

 one another was shown on the steeper soil-clad slopes. Here 

 the lines consisted partly of a series of holes, and partly of 

 varying lengths of trench-like depressions which terminated in a 

 tunnel at either end. The walls of the depressions were gener- 

 ally steep, even at the ends. They had obviously been formed 

 by the breaking away of the surface soil, which was extremely 

 liable to collapse if trodden on between the holes. 



It was evident that the holes and trenches indicated lines of 

 sub-surface drainage, which were found in all stages of develop- 

 ment, from strings of widely separated holes, to long, deep 

 rifts, separated by narrow bridges of soil, through or under 

 which the water would run, as under a culvert. All were dry 

 when examined (PI. xlv., Fig. 2) . At Roelands, a small gully 

 with steep sides and a cirque-like termination appeared to repre- 

 sent an extreme stage of development. 



The sub-surface channels had no rational relation to contour 

 such as that of ordinary surface drainage. The collapsed roof 

 soil had in many cases been completely removed from the chan- 



