206 PROCEEDINGS OF SECTION C. 



The geological formation of the country explains its peculiar 

 physical features. The old Permo- Carboniferous valleys have been 

 choked with thick accumulations of glacial waste— a stiff, tenacious, 

 blue clay being characteristic of the lower portions, and a gritty, 

 more or less argillaceous sandstone, with scattered pebbles and 

 conglomerates the upper. 



The upper set of beds form ridges, 200 feet and 300 feet above 

 the valley bottoms, and as they are soft and easily disintegrated 

 they yield much free sand, which forms a poor soil and subsoil. 

 This sandy country is very absorbent of rain, whieh latter, after pene- 

 trating the porous beds, finds its way down to the clay level and 

 makes a seepage along the valley bottoms and thereby creates 

 swamps. A swamp vegetation has grown in many places suffi- 

 ciently long to form a black peaty mass many feet in thickness. 



On the rises, the glacial sandstones, highly charged with grit 

 and pebbles, are sufficiently indurated to make good road-metal, 

 and are used throughout the district for this purpose. 



The present lines of drainage are superimposed on the older 

 Permo-Carboniferous system, with which they have no relation. 

 Glacial sandstones form the present watershed between the east and 

 west, and the older valleys of Permo-Carboniferous erosion have 

 not been sufficiently unloaded to show the main lines of drainage 

 pertaining to the older system. 



Mount Observation, an inlier of Cambrian rocks, roughly 

 divides the great glacial basin into two parts. On the southern side 

 is the Mount Compass and Nangkita sub-basin, which has its 

 drainage outlet by the Black Swamp, and on the northern side is 

 the sub-basin of Giles Creek and the River Finniss. With the 

 exception of that part of the River Finniss which flows through 

 Mount Observation, in a deep gorge, the whole of the drainage of the 

 area is practically over glacial deposits. 



In the lower portion of the Finniss basin the glacial sandstone is 

 compact, fine-grained, and free working, and has been extensively 

 used for building purposes locally, and, to some extent, in Adelaide. 



Near the northern limiits of the area, on the Bull's Creek road, 

 4| miles from Strathalbyn, and within the valley of Giles Creek, an 

 important exposure of glacial till is seen in a road-cutting. The 

 erratics are varied, of large size (up to four feet in diameter), and 

 many of them glaciated. 



The evidences for the glacial origin of the deposits in this wide 

 area may be briefly stated : — 



(fl) The beds are continuous and lithologically identical with 

 those on the western side of the area, in which the 

 glacial features are most pronounced. 



{b) The presence of erratics foreign to the district. These 

 are not usually so large as those frequently met with in 

 the Inman valley and other localities on the western side, 

 but they are characteristic. 



