192 



ME. T. W. E. DAVID ON EVIDENCE OE GLACIAL ACTION 



be correlated with the Bacchus-Marsh glacial beds, the only fossil 

 found in the latter being Oangamopteris, a plant of doubtful geolo- 

 gical horizon. 



In 1884 Mr. C. S. Wilkinson examined the Upper Marine Coal- 

 measures (No. 4 in the table) in the neighbourhood of Branxton, 

 near Maitland, and observed in the road-cutting west of the railway 

 station a very coarse conglomerate containing large subangular 

 boulders of clay-slate and other rocks foreign to the district, which 

 he considered to be erratics ; no ice-scratches, however, were at that 

 time noticed. 



It was reserved for Mr. R. D. Oldham, A.R.S.M., Deputy 

 Superintendent of the Geological Survey of India, on a visit to the 

 Colony in 1885, to make the important discovery of the presence of 

 boulders and pebbles, unmistakably striated and polished by ice, in 

 a railway-cutting at Branxton, close to the spot where the erratics 

 were first found by Mr. Wilkinson. Mr. Oldham has described these 

 beds in the ' Records of the Geological Survey of India,' vol. xix. 

 part i. 1886, page 44. The marine fauna associated with this 

 formation shows it to be homotaxial with the Bowen- river and 

 Wollongong beds. 



Carboniferous Glacial Beds at Grass-tree. 



Since this discovery by Mr. Oldham, the author, when commen- 

 cing a survey of the Northern Coal-field of New South Wales in 

 1886, found another extensive glacial deposit, probably of the same 

 age, at Grass-tree, near Musclebrook, 28 miles north-westerly from 

 Branxton. A fine section of the Carboniferous glacial beds is exposed 

 here in the railway-cutting, showing them to extend for at least a 

 mile horizontally, and to have a thickness of not less than 30 feet. 



The deposit here consists of reddish-brown to greenish-brown 

 shales, and is crowded with round and subangular fragments of rock, 

 from pebbles no larger than marbles up to blocks ^ ton in weight. 



The matrix in which the boulders are imbedded is a fine calcareous 

 sandy shale, reddish to rusty brown near the surface, and passing at 

 a depth of 15 feet first into a pale greenish brown, then into a 

 leaden grey. At intervals of from 3 to 20 yards spherical concre- 

 tions from | to 2 feet in diameter, as round as cannon-balls, occur 

 in the shale. When broken open these are found to be composed 

 of a nucleus of carbonate of lime surrounded by a shell of shale 

 partly cemented by calcareous matter. 



From the surface downwards, for about 15 feet, there is an 

 appearance in the shale suggestive of bedding, but due probably to- 

 the formation of ferruginous bands in the shale through the perco- 

 lation of surface-water carrying iron in solution. Lenticular patches 

 of gravel may be observed in places filling contemporaneously eroded 

 hollows ; but stratification, if it exists, is not strongly marked. 



Most of the boulders in the beds are more or less rounded, 

 angular fragments being rare. Their shape is very irregular, but 

 generally one end is more pointed than the other, and as a rule 

 their outlines are less convex than those of water-worn pebbles. 



