Saturday, 10 May 2008

Portland stone

Between the ooid lime sand dunes discussed above there was some development of patch reefs (i.e. small reefs). They were made of the oyster - Ostrea and the bivalve Plicatula,which like oysters cements itself to hard surfaces or other shells. There were also red algae of the genus Solenopora and bryozoans (sea-mats) involved in the construction of these small reefs. They contain cemented internal sediments and were bored by Lithophaga, a boring bivalve, by sponges and by worms (Bosence, 1987). Patch reefs up to 5m high and 10m across occur in the quarries in the centre of the Isle of Portland (Townson, 1975; Bosence, 1987). Small ones are present at Portland Bill (SY 680705). In the reefs the shells are vertically stacked and make up most of the framework (ie. the structure of the reef). They are encrusted by the bryozoan Hypororpora portlandica. In places the red alga Solenopora portlandia contributes to the reef framework. This alga shows some seasonal banding with small-celled summer layers visible under the microscope (Wright, 1985).

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