Natural Values

Lay of the Bay: Basic Geography

Roebuck Bay is an embayment (about 30 by 20 kilometres) south-west of the Dampier Peninsula on the coast of north-western Australia. The west of the bay merges with the Indian Ocean. Eroded orange cliffs with narrow sandy beaches line the northern shores and mangrove systems occur east and south of the Bay.

Defining the boundaries of the bay is a challenge. Maps typically show the sea meeting the base of the beaches and mangroves of Roebuck Bay. However, at low tide the sea edge is separated from these points by kilometres of mud and sand intertidal flats. On the lowest tides more than 150 square kilomtres of intertidal flats are exposed and these habitats and their extraordinary wildlife dominate.

On the highest tides, the sea does not stop at the mangroves; it flows right through them, into the salt marshes and clay pans beyond. The tidal inundation of these habitats has profound effects on their ecology, so they can be regarded as part of the bay. Also included are the saline grasslands of Roebuck Plains, Cable Beach and the shores of Broome peninsula.

Geology

The oldest exposed rock type around Roebuck Bay is the Broome Sandstone, deposited in shallow water 120-150 million years ago. Dinosaurs roamed the region in those days, leaving behind some remarkable footprints that are still preserved today in the sandstone. The best-exposed examples, both of the Broome Sandstone and of the dinosaur footprints, are at Gantheaume Point.

Changes in sea levels since dinosaur days resulted in the deposition of sediments that now cover much of the region. They include the Bossut Formation variable and poorly cemented sedimentary rocks that form well exposed and attractively weathered outcrops behind the narrow beaches of northern Roebuck Bay. Sand eroded from the Bossut Formation (and other sources) was reworked, shifted by the winds, weathered and somewhat cemented, eventually forming the deep red, sandy soils we now call pindan. Pindan soils form low crumbling cliffs along the northern shores or Roebuck Bay but, for the most part, pindan country is rather flat. It occupies large areas to the north and south of Roebuck Bay and has distinctive vegetation dominated by acacias.

Why don’t pindan soils surround all of Roebuck Bay? And why, for that matter, is there a bay at all? The answer lies 200 kilometres away, where the mighty Fitzroy River flows into King Sound near Derby. For many millions of years it flowed into what is now Roebuck Bay, and it left behind both the structural layout of the bay, and an extensive plain of fine alluvial and estuarine sediments – Roebuck Plains, a region of magnificent natural grasslands that abuts eastern Roebuck Bay.

References: (September 2003), Life along land’s edge Wildlife on the shores of Roebuck Bay, Broome. Rogers, DI. Piersma, T, Lavaleye M, Pearson GB, de Goeij P.