SEDIMENTARY ROCKS OF THE LOWER SHOALHAVEN RIVER. 327 
formed beaches of coarse shingle. These would naturally 
dam back the surface drainage from the island or perhaps 
peninsula lying to the east. Thus lagoons were formed, 
in which ferns and horse-tails became embedded and pre- 
served. Similar lagoons to these may be seen in many 
places on our own coast at the present time. At times 
the sea would break through these barriers and a layer of 
sandstone would result. As the land sank these inunda- 
tions became more frequent, until at last the plain became 
entirely submerged with marine sediments deposited above. 
But before the final inundation took place, slight changes 
of level allowed the old conditions to be resumed for short 
intervals. Thin lenticular beds of shale were then formed 
near the landward boundary of the old plain, but at last 
these too were finally submerged beneath the Permo- 
Carboniferous sea. But by this time the coastline had 
retreated away to the south and west, and naturally finer 
sediments began to be deposited; these were the Edmondia 
sandstones and grits, the Grassy Gully sandstones and the 
Burrier mudstones, which will now be considered in detail. 
Sub-Division II—The Wandrawandian Series.' 
(a) Grits with Edmondia.—This formation is charac- 
terised by the abundance of the shell Edmondia nobilissima, 
de Kon., in some horizons. Many good outcrops are to be 
found in the neighbourhood of Yalwal Creek, where they 
form the lofty escarpments which cap the mountains 
bordering both Yalwal Creek and the Shoalhaven. They 
consist of sandstones, grits and conglomerates, and dip 
slightly to the east. At Yalwal Creek they cap the fresh- 
water beds, but further to the west they lie directly on 
the conglomerates, from which they are in some places 
1 Professor David calls these the Wandrawandian Sandstones. As the 
series evidently embrace other rocks besides sandstone, the word series is 
here substituted. 
