Pure Shell Ridges

Multiple shore parallel ridges composed of pure shells (Fragan eragatum) occur at the head of a long indented bay known as Hamlin Pool at Shark Bay, Western Australia (Figs. 10 and 11). This species of shell fish normally grows much larger in size in open ocean conditions but here at Hamlin Pool the hyper-saline conditions, which also promote stromatolite growth here, result in these shells only growing to a maximum of 1-1.5 cm across. Beaches are entirely composed of these shells in this immediate region and as a consequence so too are the beach ridges. The majority of the ridges rise to 4 m AHD but a few in the middle of the sequence rise to 5 and 6 m AHD. Figure 11 shows that the ridges date to before the Holocene transgression and ages on these pre-Holocene ridges exceed the radiocarbon limit. Hence they are presumably Pleistocene in age and likely last interglacial. These

The Last Interglacial Map
Fig. 10 Location map of Hamlin Pool, Shark Bay, Western Australia

pre-Holocene ridges are composed of a variety of different species of shells and they are also present in a variety of sizes unlike the Holocene ridges with their mono-specific shells of the same size. Clearly environmental conditions have changed here since the Holocene transgression compared to the last interglacial and one assumption is that the present day hyper-saline conditions did not exist here during the last interglacial. The ridge stratigraphy also suggests that an period of aridity lasting approximately 1,500 years also occurred during the mid-Holocene between approximately 2,200 to 3,700 years B.P. The evidence for this suggestion comes from the eolian capping of very fine grained silt over the ridges dating from approximately 3,700 to 5,500 yrs BP and not before this time. There was a 1,500 year gap between deposition of the last (most recently deposited) ridge at 3,700 yrs BP and the next ridge which is not covered in a layer of fine silt at 2,300 yrs BP. Presumably here, which is part of the most arid section of Australia's coast, tropical cyclones of any substantial size (intensity) did not make landfall for this 1,500 yr period which also coincided with exceptionally dry conditions.

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