Landscape, Catchments and Waterways
The catchment area of Arcadia and its two bays, (Geoffrey Bay and Alma Bay), is approximately 290 hectares.
Petersen Creek is the main waterway flowing to Geoffrey Bay and Alma Creek flows into Alma Bay.
Both creeks are important natural corridors traversing the urban landscape and connecting the National Park to the foreshore and waters of the Great Barrier Reef.
> SEE: The Arcadia page in the Magnetic Island Catchment Story
The granites of Magnetic Island determine its overall landscape, with granite tors exposed on summits and steep slopes, and valleys eroding along deep joints and fractures.
The rockiest parts of the island are largely protected in Magnetic Island National Park and have been little affected by development or disturbance.
The headlands around Arcadia show these features in microcosm, with windswept hoop pines (Araucaria cunninghamii) growing among boulders where they are protected from fire.
Arcadia occupies a small coastal plain enclosed by low granitic hills. Despite its small size, there are a number of distinctive landforms which have particular soils and vegetation.
While the original vegetation has been greatly disturbed by urban development, remnants of the original vegetation can still be seen (and some areas are recovering).
Away from the beaches most of Arcadia is built on a land surface that is as least as old as the last ice age about 100,000 years ago.
Lying mostly between 10m and 20m above sea level, these deep sandy and gravelly soils are the heavily weathered remains of rock and soil washed from the hills.
While the surface soils are dark, the subsoils are red from oxidised iron. This surface is best seen on Hayles Avenue, and is highest on Mirimar Crescent. It has subsequently been dissected by Petersen Creek.
This old leached surface once had a dense woodland of bloodwood (Corymbia clarksoniana) and Moreton Bay ash (C. tesselaris) with a diverse lower tree storey and grassy ground cover. Scattered original trees, and progeny of these trees, are still common.
The seaward margin of this old surface was eroded during the last sea level maximum about 6000 years ago. The low erosion scarp formed then, is now the inland boundary of a series of coastal sand dunes built up at the time while sea level stabilised.
These low dunes dominate the eastern end of Arcadia and Geoffrey Bay, and are mostly less than 5m above sea level.
In Alma Bay, exposure to the south easterlies built the beach sands higher and the boundary with the older red soils is buried.
At Geoffrey Bay, beach she-oaks (Casuarina equisetifolia) still line the beach, and the dunes once had an open woodland of poplar gum (Eucalyptus platyphylla), bloodwoods, Moreton Bay ash and paperbarks (Melaleuca nervosa and M. viridiflora), with scattered under story trees. Some of these under story trees remain, but many of the larger trees have been lost and have not been replaced by their progeny - possibly due to the heavier concentration of human use in these beach areas.
Depressions within and behind the dunes had large weeping paperbarks (M. leucadendra), some of which remain.
Photo: (Top) Beach she-oaks line the foreshore. (Bottom) Large paperbarks can still be found.
There are also small areas of alluvial soils associated with the floodplains of the creeks, the most extensive areas being on the lower reaches of Petersen Creek.
They are dominated by tall weeping paperbarks, but the under story is currently compromised by weeds.
Behind the Bright Avenue shops there is a small area of vine thicket growing in a depression behind the sand dunes, fed by small watercourses. This is a rare community that is nationally endangered. (We call it Geoffrey Bay Scrub).
The lower reaches of the main watercourses are tidal and have small stands of mangroves and other salt tolerant plants. They are best developed on Petersen Creek, which is isolated from the sea by beach build up in the dry season, but is opened when the creek runs in the wet season. Alma Creek mangroves were cleared in 1969/70 but are now regenerating