KP1 - Sand Hummocks - Barrier Spits

This information has been developed from the publications:

Sites of Geological and Geomorphological Significance in the Western Region of Melbourne (1986) by Neville Rosengren
Sites of Geological and Geomorphological Significance on the Coast of Port Phillip Bay (1988) by Neville Rosengren.
Sites of Geological and Geomorphological Significance in the Shire of Otway (1984) by Neville Rosengren.

Geological heritage sites, including sites of geomorphological interest and volcanic heritage sites, are under regular revision by the Geological Society of Australia, especially in the assessment of significance and values. Reference should be made to the most recent reports. See the Earth Science Heritage (external link) section of the Geological Society of Australia website for details of geological heritage reports, and a bibliography.

Location Access Ownership	Kirk Point - 820855. Former Shire of Corio. Coastal embayment and associated wetland four km north of Point Wilson. Twenty Nine Mile Road. Crown land.	KP1 - Shelly barrier spits, Sand Hummocks.
Site Description	The Sand Hummocks is a shallow, largely infilled tidal embayment and lagoon formed by the sea flooding a gently sloping depression on the surface of the Werribee Plains. The lagoon is partly enclosed by two long narrow barrier spits that are surmounted by beach ridges and crossed by tidal entrances of variable position and size. The inner margin of the site is defined by a low bluff that extends up to one km inland from mean high water mark. The bluff is an abandoned marine shoreline cut at the maximum limit of the Holocene marine transgression before the infilling of the embayment by sediment and salt marsh. In places the bluff is marked by an abrupt interface 30 to 40 cm high separating the embayment from the basalt plains, but elsewhere it is degraded to a sloping surface that merges with the lower salt marsh terrain.	KP1 - Shelly barrier spits, Sand Hummocks.

KP1, Site of Geological and
Geomorphological Significance

In front of the bluff lies a complex zoned salt marsh developed on the sand, shells and mud that have infilled the embayment. In the upper (inner) zones, the marsh includes unvegetated salt and claypans, open shrublands dominated by Halosarcia halocnemoides (Grey Glasswort), herb fields of Sarcocornia quinqueflora (Beaded Glasswort) and Frankenia pauciflora (Southern Sea-heath), and sedge lands of Gahnia filum (Chaffey Saw-edge). Seaward of this the lower marsh zones are dominated by close shrubland of Sclerostegia arbuscula (Shrubby Glass-wort). Included in this, and coinciding with a small change in the level of the marsh surface, is an arcuate zone 8 to 10 m wide without Sclerostegia dominated by a low Sarcocornia quinqueflora. The salt marsh is dissected by a well developed network of tidal drainage channels, some of which extend inland to terminate in the salt pans of the upper marsh zone.

Sandy ridges enclosed in the salt marsh zones are relict spits or cheniers emplaced by wave action before the salt marsh reached its present extent.

The shallow tidal lagoon at the Sand Hummocks lies behind two elongated barrier spits that parallel the trend of the coastline and lie 300 to 500 m offshore. The spits are built of shells, carbonate sand and some basalt pebbles. For the most part they are narrow and surmounted by beach ridges built up to 1.5 m above high water mark. The southern spit, anchored at Pelican Point, extends northward for 1.5 km and is separated from the northeastern spit by the shallow tidal entrance to the lagoon. The northeastern spit, of similar length but less regular form, is anchored near the Murtcaim Drain. Wave action has modified the distal ends of the spits, while around the entrance, tidal ebb and flood currents have built a large sand shoal (flood tide delta) in the lagoon and a smaller ebb tide delta into Port Phillip Bay. Apart from the main entrance, a breach developed in the southern spit in 1978 due to storm overwash and there is a small tidal entrance near the proximal end of the northeastern spit. The barrier spits are dynamic features and in recent years the outer edges have been eroding and new washover beaches have developed on the southern spit.

Significance

State. The lagoon and barrier spits has not counterpart elsewhere in Port Phillip Bay and is an unusual feature in the context of the entire Victorian coast. It is an outstanding example of a small tidal lagoon system not modified by land drainage but subject to storm overwash and migration of tidal entrances. The system includes many features, such as relict erosional and depositional landforms, sandy, muddy and biogenic sediments that are relevant to the study of the Holocene sea levels in Port Phillip Bay.

Management

Class 1. Peripheral parts of the system have been modified by activities related to the MMBW farm. No further drainage, reclamation, roadworks, alteration of tidal flows, buildings, extraction of materials, or emplacement of artificial structures should be permitted on the are of the site. Land drainage should not be channelled into or through the salt marsh area. The area should be managed as wildlife reserve in its present state to allow the dynamic of the system to operate without modification.

References

Bowler, J.M. (1966). Port Phillip Survey 1957-1963: Geology and Geomorphology. Memoirs Nat. Mus.Vic. 27:19-67.
Jurkowski, I. (1980). The Sand Hummocks. B.Sc.(Hons.) thesis. (Unpub). Geog. Dept. Univ. of Melb.

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