"Dimpling" of Granite Hills. 109> 



may be approximately uniform in height, but frequently, opposite 

 walls taper to a low lip over which surplus water may pass. The 

 " dimples " may be scattered irregularly over the surface of the 

 hill, or they may be collected along somewhat definite lines, such, 

 as a drainage furrow incised by erosion in the side of the hill, 

 in which latter instance they form a series of hollowed-out steps;. 

 or both classes may occur. Rain water remains in these shallow 

 holes usually but a short time. 



The granite hills known to the writer on which the characters 

 described may be observed, are the 19 Mile Rocks situated about 

 17 miles east-north-east of Goongarrie railway station 1 , and at 

 the Donkey Rocks, about 18 miles farther east-north-east; also- 

 to a less extent at the 22 Mile and 25 Mile Rocks, to the east 

 of the 19 Mile Rocks. 



With regard to the mode of formation of these " dimples," 

 they are probably formed in much the same way as the more- 

 normal rock or " gnamma " holes, of which the " dimples " merely 

 form a variety. The mode of formation of " gnamma " holes 

 has been discussed by Maclaren, by Talbot and by Woodward.. 

 Maclaren 2 favours solution as the essential process, the narrow 

 openings of some holes being due to a hard surface crust resisting 

 erosion more than the rock below. Talbot 3 believes that they 

 may have originated either by the decay of a rock with more 

 felspar than the adjacent rock, or by a shallow crack in the 

 granite. Solution by water charged with carbonic acid would 

 enlarge the incipient hole, and animals and aborigines would still 

 further enlarge them by scratching the sides, and removing the 

 weathered rock to obtain the last drop of w r ater. Woodward's 

 ideas 4 are practically the same as Talbot's, except that he does 

 not refer to the possibility of a crack or joint facilitating the 

 commencement of a hole, although one of his figures shows the 

 effect of a joint in the making of a hole. Woodward suggests 

 that a beginning may be made by the more rapid weathering 

 out by water of segregations in the form of pegmatitic bunches 

 than of the containing rock. Animals then scratch out the re- 

 maining grit. 



1. This railway station is on the Kalgoorlie-Leonora line, and is about 

 miles north of Kalgoorlie. 



2. Maclaren, J. M. — Geol. Mag., 1912, pp. 301-304. 



3. Talbot, H. W. B. — Bull. 45, Geol. Surv., W.A. (1912), pp. 38, 39.. 



4. Woodward, H P.— Bull. 57, Geol. Surv. W.A. (1914), pp. 33-35. 



