MUD ISLANDS, PORT PHILLIP BAY 135 



curvate islands enclosing a relatively large tidal lagoon — the 

 Inner Lagoon — under a foot deep ; this opens to the south-west into 

 a smaller lagoon, the Outer Lagoon, which has a wide opening to 

 the south. This wide opening is being rapidly filled with sedi- 

 ment, the Low Bank being formed across it. The bottom of the 

 lagoons is sandy mud. The islands, except near their outer 

 fringes, have a fairly level surface, about 5 feet above low 

 water mark, of sandy mud similar to the bottom of the lagoons. 

 Along the outer (western) shore of Western Island, sand-dunes 

 are now being piled up and have reached a height of about 12 feet 

 near the northern and southern ends of that island. All the 

 islands support a low, thick scrub in which there are occasional 

 open spaces where rookeries are found; the largest is on the 

 Middle Island and Boatswain's Island. 



Between the eastern island and Middle Island, and flowing over 

 the south-east portion of the Inner Lagoon, Ross shows a waterway 

 with an outlet between the southern extremity of Eastern Island 

 and the south-eastern extremity of Boatswain's Island. 



Development of the Mud Islands Group. 



In a previous contribution (Kehle, 1946), it lias been pointed 

 out that the so-called channels through the shoals in the southern 

 portion of Port Phillip Bay could more accurately be called tide- 

 ways, formed by the outgoing tidal stream of the Inner Basin of 

 Port Phillip finding an outlet into King Bay or Bass Strait, and 

 the incoming tidal stream through The Heads finding its way 

 into the Inner Basin. 



The Great Sand has been prograded to the north-east and south- 

 west from a dune rock platform, portions of which are exposed 

 at the south-east end of Boatswain's Island and the northern end 

 of "Western Island. This sand has been shaped, and the Mud 

 Islands group given its atoll-like configuration, in the first 

 instance by both the incoming and outgoing tidal streams. The 

 main incoming tidal stream comes through the Heads and along 

 the South Channel, finding its way through Pinnace Channel 

 and the channels east of it into the Inner Basin ; it also sets to a 

 lesser extent through Symonds' Channel. The outgoing tidal 

 stream coming from the Inner Basin flows over the banks as 

 well as along the channels. 



Age of Mud Islands. 

 There is evidence around the shores of Port Phillip of the 

 world-wide raised beach, 15 to 20 feet above sea-level, but it is 



