Tag: Selwicks Bay

Selwicks Bay

Selwicks Bay

Dipping back in my A-level field notebook and in those pre-digital days, I found that colour prints could be more useful than slides, as I could stick them in my notebook. Here I’ve indicated a fault in the wave-cut platform of Selwicks Bay, Flamborough Head.

Flints in Chalk

flints
Flints in chalk

Flints are exposed in the chalk of the wave-cut platform south of the fault. Flints like these may have formed when the silica-rich skeletons of sponges and other creatures formed a gel on the seafloor which was drawn down into burrows in the chalk ooze – hence the shape of the nodules.

Buttress of Contorted Chalk

RockWATCH group guided around the features of the bay by geologist Richard Myerscough.

We looked at a buttress of contorted chalk south of the fault. The chalk contorted by the fault has been re-cemented by calcite-rich fluids circulating through the rock and depositing veins of calcite.

Strengthened by this cement the chalk is harder than that surrounding it and it has withstood erosion and formed a buttress.

Contorted Chalk with Calcite Veins

calcite vein

This vein is exposed on the wave-cut platform in front of the buttress.

Tilted layers near at the fault plane
Fault breccia: chalk crushed by movement along the fault
Fault and crush zone, Selwicks Bay, Flamborough Head

Fossil Sponge from Flamborough

This fossil sponge was collected from the beach at Selwicks Bay, Flamborough Head, on a Rockwatch weekend in the early 1990s.

It was embedded in a fragment of chalk that had fallen from the cliff, so it dates from the Upper Cretaceous Period, 95 to 65 million years ago. I drew it for my 1996 book Yorkshire Rock, A Journey Through Time but since then the chalk that surrounded it has split into shards. The collar of the smaller sponge has disintegrated too.

How I imagine it might have looked, growing in the chalky ooze of the Cretaceous sea bed.

Sponges were common as the age of the dinosaurs drew to a close. They had a sac-like body with a central cavity known as the cloaca.

The nearest species that I can find is Laosciada, a mushroom-shaped lithistid, a kind of demosponge, informally referred to as a calcisponge. It lived in deep water, between 100 and 400 metres.

The skeleton of the collar of this sponge is made up of interlocking spicules, which look like little pyramids in close-up. They’re made of silica which, remineralised, forms the layers of flint found in the chalk.