Then we all trooped off to Coldknuckles Quarry for some fieldwork.
At the bottom of this quarry is a deep layer of sand known as The Yellow Sand. It is the remains of Permian sand dunes formed about 250 million years ago when England's climate was dry and tropical. It is mined and used as building sand. It can be seen in its dune layers, cross-bedded from the changing winds of the desert. At the top of the sand is a thin grey line of Marl Slate which forms the boundary between the sand and the Magnesian limestones. It is about halfway up the picture and above it are the layers of limestone.
It's the Marl Slate that has the fossils in it and the quarrymen dig it up and pile it out of the way as they go down for the sand. Lots of folks come round and fossick in the dumped slate, including me.
We then went down to Blackhall Rocks to study more of the limestone in the cliffs, but the tide hadn't gone out far enough to see the best bits.