PROCEEDINGS OF SECTION C. 471 



20.— NOTES ON CRUSH PHENOMENA IN THE CAMBRIAN 

 ROCKS NEAR BLACKWOOD, SOUTH AUSTRALIA. 



By H. BASEDOW and J. D. ILIFFE, B.Sc 

 ^WiTH Plate.] 



The following notes on crush phenomena in and surrounding the 

 Cambrian conglomerate of the Sturt Valley are a brief resume of a 

 paper read by us before the Royal Society of South AustraUa in 1905,* 

 references to which may be found in the Transactions and Proceedings 

 of that society. 



On the eastern side of the conglomerate certain siliceous and 

 felspathic quartzites and clayslates occur in alternating bands of hard 

 and soft rocks, varying in thickness from a fraction of an inch to a foot 

 and more. 



The fine-grained beds contain pseudo ripple marks and pseudo 

 sun cracks, the latter being produced by fracture along lines crossing 

 each other obliquely, the result of cross-folding on a small scale in 

 beds of brittle rock. Examples may be noted in the first railway cutting 

 east of the Metropolitan Brick Works. 



Contortions in the beds adjacent to the conglomerate are very 

 pronounced, and, in the case of overfolds in the harder rocks, have 

 resulted in fracture ; thus the first step in the production of false 

 pebbles becomes evident. 



Near to the eastern junction of the conglomerate the argillaceous 

 rocks have been minutely puckered. The cataclastic phenomena of 

 obliteration of bedding planes and laminse is marked, acute angular 

 wrinkles developing into thrusts along the middle limbs of the folds, 

 giving rise to an unstratified mass closely resembling the matrix of 

 the conglomerate. Illustrations of this occur about 100yds. west of 

 the 10th milepost on the Hills railway line, and, neglecting a few 

 crushed and fractured quartzitio bands, continue right up to the con- 

 glomerate, barely another 100yds. to the west. 



In the same cuttings several of the quartzitic bands have been 

 pinched out, lenticular masses resulting, one overlapping another. 

 In the clayslates of Tapley's Hill, to the west and south of the con- 

 glomerate, bands of Umestone occur, which thin out similarly, and are 

 in parts brecciated. The separated fragments in parts show signs of 

 rounding. The cleavage of these limestones does not agree with that 

 of the enclosing slates. The phenomena is best seen in sections 27, 

 28, and 70, hundred of Noarlunga, The hard bands of quartzites in 

 the railway cuttings to the east of the conglomerate have further 

 broken up along planes oblique to the bedding, each fractured 

 portion having slipped shghtly along the plane of fracture, forming 

 small step faults. This movement, combined with that which resulted 

 in small cross folds, has produced false pebbles. Abundant examples 

 occur in the railway cuttings. In several instances one and the same 



* Vol. XXIX., pages .334 and 335. Vol. xxx., pages 233 and 343. 



