The ammonites' graveyard

This is a truly spectacular slab of rock!

Ammonites are among the few fossils that just about everyone has heard of - and a good number of people with no interest in invertebrate fossils know one when they see one, especially of it is cut along its vertical axis to show its structure.  Ammonites lived at the same time as the dinosaurs, and were cephalopods - that is to say, they belonged to the same group of organisms as octopods, nautiloids, squid and cuttlefish.  There were a huge number of different species of ammonite, which makes them very useful for dating the rocks they are preserved in.  The ones in this slab are not the same species that Mary Anning used to find at Lyme Regis, and they are not the same species that you find in the Oxford Clay or in Yorkshire.  For the fossil-minded these are Promicoseras planicosta; this photogenic slice of ammonite-rich limestone  - a variety known as Marston Marble - hails from Somerset in England's West Country.  

Incidentally, this variety of stone (which dates back to the Lower Jurassic) is not actually a 'true' marble in the sense that geologists use the term - limestone metamorphosed by heat and pressure - but stonemasons use the term to describe any limestone that takes a good polish. This one certainly does that! Stonemasons were using some 'geological' terms long before geologists were.  This beautiful specimen is in the Sedgwick Museum, Cambridge, UK - do go and see it for yourself!

It is not often that a polished slab of stone photographs decently with just a smartphone, but I was really pleased with this photo.  The main issues I find with this sort of shot are (i) holding the phone absolutely steady, and (ii) the fact that cameras tend to focus straight through polished surfaces onto something – often the photographer! – being reflected.  This slab is inside a glass case, which tends to add an additional complication in terms of reflections, but I was lucky this time.