2005 - Whitby Naturalists

THE GEOLOGICAL RECORDER'S REPORT
Nodules And All That
The grey nodules (morc properly called concretions) that we find
sticking out of the Jurassic clifrs that buttress Whitby are often rhc
source of our best fossils. If we look up at the sea cliffs we can see bands
of thesc oval grey shapes running alorig the exposure. These nclduie
beds must extend some distancc inland, for they formed on the floor o{
the vanished Tethys Ocean in the da-vs before the Atlantic and North
Sca were rwinkles in the Earth's eye.
Thc Tethys was a wann sea, for it once occupied that part of the globe
whcre the Indian Ocean now sits: the continents which bordered the
Tcthys have also brokcn up and re-formed into what we have today.
Somc scraps of the Tethys rcm:lin; The Black Sea, The Caspian and
the Aral Sca are all said to be remnants. But I wander ofr the pointl
Concretions build when elements set free by thc decay of organic
matter react rvith minerals in the deposited mud of the sea floor.
Layers of new compounds grow .slowly around the organic 'seed'. The
'seed' may be a chunky ammonite or a group of trny molluscs or a mere
scrap of she1l and flesh. Usually it is the hydrogen sulphide set free from
decaying proteins which, by reacting with free ferric ions in the warer,
begin to form thc concretion. The nodule will continue to grow until
the right chemistry is availablc. Eventually that layer of seafloor will be
buried and subjecred to pressurc. Fossils which havc not given rise to
nodule growth will be crushed flat whilst those fossils within nodules
are protected because of the great hardness of the concretion.
Cliff erosion drops them onttl the shore; the sea wears thenr down and
brcaks them open so that we find thcrn in various statcs of decay
scattered around our feet; we pick thcm up, tum ttrem over and over
and may notice the trace of a fossil. They are heavy because they are
rich in Iron Pyrites. Strike them with a hammer and you might smell
LO