Castlepoint fossils

At Castlepoint, a rocky ridge (the Castle Point Reef) is formed of limestone beds that were deposited in a submarine canyon in water more than 300 metres deep, 2.2 million years ago. The limestone is composed almost entirely of fossil material, mostly barnacles which grew on the walls of the canyon. When they die, barnacles usually disaggregate (fall apart) and the individual plates of which they are made become scattered on the sea floor. It was the accumulation of many millions of these plates over time that formed the shell beds which now have become cemented into the limestone. The barnacles grew on hard substrates, including clam shells growing in the deep waters. Sediment debris slid down the steep canyon sides in submarine landslides along with shallow water fossils including Mesopeplum. So the limestone has fossils from very different environments and this had geologists puzzled for many years. Rapid tectonic uplift of the whole region raised the rocks above sea level and erosion has now exposed the once-buried canyon deposits.

As you walk out along the ridge towards the lighthouse, look carefully at the rocks exposed in the rocky faces. The limestones are largely composed of fossil barnacle plates and some complete barnacles can be seen still attached to the scallop shells, such as Zygochlamys, on which they grew (see photo). Some beds are packed with fossil clams and others contain large gastropods (marine snails) that have come down the canyon sides from shallower sites.
If you look at rock faces in the inter-tidal zone you will see modern barnacles (much smaller than their ancient relatives) growing on the limestone, some of them growing on the fossil barnacles!