5 2 Excursion to Sydenham , Bulla , and Diggers Rest V" "'\'^^^^^^' 



old flood-plains laid down at an earlier stage in its history when 

 it was a more mature stream. But in this case it seems that 

 the river has cut down through a thin capping of basalt on to 

 the Tertiary sands and clays. Through these the stream has 

 corraded its channel into the underlying Pakeozoic strata, and 

 not into a previously deposited alluvium. The variation in 

 slope of the valley sides at different heights is noticeable. The 

 highest parts of the valley walls are very precipitous where 

 they consist of basalt, but the lower parts have a gentle slope 

 where they consist of the softer Tertiary sediments. At the 

 north of this meander a tributary valley running along the 

 junction between granitic rock and metamorphosed Pala;ozoic 

 strata mouthed at a height of about 80 feet above the level 

 of Deep Creek, giving a good example of a hanging valley. 

 The cause of the presence of this valley, as at Werribee Gorge, 

 where a fine example in a syncline is to be seen, is that the 

 tributary rarely has very much water flowing down it, and so 

 cannot corrade its valley at anything like the rate the main 

 stream does. The main stream continues to flow at lower and 

 still lower levels, leaving the tributary to debouch over a ledge 

 as a waterfall at a more and more badly adjusted level. This 

 goes on until the main stream becomes mature, and almost 

 ceases further corrading its valley. The tributary continues 

 gradually corrading its valley until it enters at main stream 

 level, the river system then becoming mature. A hanging valley 

 is always an indication of immature physiographic conditions 

 and of bad adjustment of the various parts of the drainage 

 system. This hanging valley practically marked the boundary 

 between basalt on the upper part, with Palaeozoic strata on the 

 lower part of the west side, and granite on the east. The 

 Palaeozoic strata consisted mainly of hornfels— a rock the product 

 of intense metamorphism caused by the intrusion of the 

 granite— a plutonic igneous rock. Into these strata had been 

 intruded many small acid igneous tongues and veins from the 

 main plutonic mass. In one case a compact fine-grained 

 granitic apophysis, about 20 feet wide, was examined. This 

 had been squeezed in between the strata along the bedding 

 planes. In the creek-section the amount of dip of the strata 

 is 65°, with a direction E. 20° S., the strike being N. 20° E. 

 The strike of the strata continued, with no deflection whatever, 

 right up to the granite. This seems to point to the conclusion 

 that the plutonic rock, as it came up, quietly ingested and 

 altered the sediments that overlay it, instead of pushing them 

 up into an arch or dome. Daly calls this " magmatic stoping." 

 These highly metamorphosed strata consist mainly of hornfels, 

 a very fine-grained rock consisting of cordierite, secondar\' 

 brown mica, secondary white mica (sericite), secondary quartz. 



