136 MUD ISLANDS, PORT PHILLIP BAY 



complicated along the northern shore of the Nepean Peninsula 

 by tectonic movements. Hills (1940) draws attention to the 

 shell-beds up to 5 or 6 feet above ordinary high sea-level between 

 Sullivan's Bay near Sorrento and "The Rocks" at Dromana. 

 He points out that ''The gradual fall in elevation of the raised 

 beaches towards The Rocks indicates that their emergence was 

 caused, at least in part, by tectonic movements." If the raised 

 beaches referred to were formed at the Postglacial Optimum, 

 these tectonic movements have occurred since then. The Post- 

 glacial Optimum is estimated to have occurred about 4,000 years 

 ago (c/. Brooks, 1922), or, as suggested by the solar radiation 

 curve (c/. Zeuner, 1945), about 10,000 years ago. Assuming Mud 

 Islands to have remained static, with a progressive fall of sea- 

 level based on the shorter estimate, they were uncovered between 

 1,000 and 1,500 years ago, but on the longer period between 

 2,500 and 3,500 years ago. If the tectonic movements evident on 

 the north shore of the Nepean Peninsula extended to Mud Islands, 

 an upward movement would tend to increase slightly these estim- 

 ates and a downward movement to decrease them. 



The author of this paper has stated elsewhere (Keble, 1946) 

 that the Port Phillip Sunkland on which the Mud Islands group 

 is situated is oscillating, but subsidence is much in excess of up- 

 lift. 



Some Features of The Mud Islands Group 



The sandy mud of the generally level surface of the islands and 

 forming the bottom of the lagoons, suggests a former sea bottom 

 not unlike that of the deeper portions of the Inner Basin. The 

 section at the guano rocks gives some idea of the geological history 

 of the islands. 



a. Dune sand accumulating at present and contemporaneous with the un- 



consolidated dunes at Sullivan's Bay. 



b. Guano. 



c. Sandy mud which has been partly built up into the dunes (a) 



d. Shell limestone, loosely cemented Recent shells predominating:. 



e. Soft limestone with Recent shells. The limestone layers dip southwards. 



f. Pleistocene dune rock comparable with the dune rock at Sullivan's Bay and 



elsewhere on the Nepean Peninsula and also at Queenscliff. 



The guano has been completely removed but there is no doubt 

 as to where its position was in the section. The workings suggest 

 that it was not extensive and a foot or two thick. Maclvor (1879) 

 comments on it as follows: "Flat (or Mud) Island Guano. — This, 

 a Victorian guano, found upon a small island in Hobson's (sic) 

 Bay ... is poor in fertilising constituents, and therefore unsuited 



