The water lapping the shore is warm and relaxing, and the Tethys ocean – which later in the year will be wracked with hurricanes that will scour the sea bed and dump new sediment along the beach – is currently calm. The sand along the shore of this small tropical island, one of many strung like pearls along the southern margin of Central Laurasia (modern day southern Europe), is white and surprisingly cool to the touch. The beach is littered with bits of coral, bryozoans, ammonites and crinoids. Reaching down and clutching a handful of the sand to closely examine, little white grains and bits of shell trickle through my fingers. The sand is not made of finely ground quartz and other silicates (like the beaches of the UK will be, 165 million years from now) this beach is made of carbonate shells. Little delicate structures of limestone built by organisms to use as a home or a skeleton. There are also ooids, little grains of limestone that precipitate from the water then roll about on the sea bed until they look like tiny little eggs. A short way inland there is a commotion in the trees, pterodactyls erupt into the air, squawking indignantly while something large grunts and thuds its way towards the beach. I decide to move on.
The next beach is different, it sits on a broad lake in what will become Alberta, Canada 90 million years in the future. It’s not as warm as the tropical island, but still much warmer and closer to sea level than modern day Alberta. Occasionally the lake is flooded by the Boreal Ocean (ancestor to the modern Arctic Ocean) and becomes inhabited by clams. When the sea retreats, the clams are stranded in an increasingly saline lake. The beach is composed mostly of clam shells and little else. They have become brittle and hard and shatter with a crackle when I step on them; they have lost the typical bone white colour and appear grey and greasy. The faint smell of sulphur ghosts on the breeze from nearby hot springs. To the west, small island continents are colliding with the west coast of North America and pushing up the Rocky Mountains. Water from the surface seeps down fault lines into the deep Earth. Here it is heated and rushes back to the surface, saturated with minerals. It is these fluids that have metamorphosed the clam shells- bit by bit, the carbonate of the clam shells has been replaced by silica.
I hear the thudding again and realise it is one of my office mates, not a ravenous dinosaur. I am no longer on an ancient beach. I am in my office here in Oxford, at my desk, peering down a microscope. On the stage are little slices of rock, each as thin as a human hair. One is fossilised beach from a tropical island in the south of Jurassic France, the other from a different beach in now cold and snowy Canada. By using the petrographic microscope, which uses polarised light, I am able to tease out the secrets of ancient rocks and the environments that formed them.
The images below are taken from these samples. Fig. 1 shows a cross section through a sea urchin spine. Different generations of carbonate growth- from life, from burial and from long after burial – are tinted different colours by the polarised light. Fig. 2 shows how the formerly carbonate clam shells have been replaced by silica, in the form of chert, chaldedony and quartz, which has been stained by iron and organic matter.
Optical petrography is a primitive technique by modern standards, but it is still useful and important in understanding how rocks form. If nothing else, the images can be breath-taking and it’s not every day you can spy on a world that has not existed for hundreds of millions of years.
EVIDENTLY SCIENTIFICAL 
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