Fossil Beach

Lyme bay, in Dorset, England, sweeps from Lyme Regis in the west to Chesil Beach in the east. At Charmouth, not far from Lyme Regis, Jurassic limestone cliffs tower over the beach. The area is renowned for its fossils, and most days you can find groups of people on the beach, armed with hammers, chipping away at rocks, looking for ammonites.

But there are not only ammonites to be found along this shore, but also ichthyosaurs, crinoids, brachiopods, and belemnites. Just west of Lyme Regis, the layers of grey shales that tumble down onto the stoney shore can be prised apart with a knife or a screwdriver. Mostly, when opened up, there's nothing. But sometimes there are the faint impressions of coiled ammonites, or bivalves with frail, iridescent shells.

But perhaps the easiest fossils to find are belemnites. These conical fossils were once the internal guard shells of squid-like animals that swam in the Jurassic seas, some 200 million years ago. The soft parts of the belemnite bodies have vanished, leaving only the conical guards in the Belemnite marls. As these marls in the cliffs erode, the belemnites embedded in them are washed down onto the beach, where they in turn gradually get eroded by waves and sand until unrecognizable. You don't even need a screwdriver to find belemnites: they're scattered all over the beach.

To find them, you have to tune your eyes to register small, mostly grey, conical objects. It helps if you know what they look like, and this page has near-actual-size belemnites down the left margin on 5 mm grid paper. Once you've tuned your eyes to notice small conical things, you find that, just gazing at a sea of pebbles, the belemnites almost jump out at you.

It is a curious ability, to be able to tune vision to notice a particular sort of shape and colour. Gardeners use it to recognize weeds - brambles, nettles, etc. They first get a firm visual image of the shape of the plant, its leaves and stem, and then they go round finding them and pulling them up. The same goes for blackberry pickers, peering into hedgerows. It took me ages to find my first belemnite, because I didn't really know quite what I was looking for. But when I finally stumbled across one, I had a firm visual image, and began to rapidly find more.

It helps to look in the right places. The beach grades up from sand at the water edge to large pebbles beneath the cliffs. I guess this grading happens because, if a mix of pebbles and sand is shaken up, the smallest pebbles and grains tend to drop between the larger ones, producing a grading from fine sand at the bottom to large pebbles at the top. On the beach, shaken by the tide, the highest part is up by the cliffs, and the lowest is lower down by the water edge, and so the grading appears almost horizontal. And because most belemnites are larger than grains of sand, and smaller than large pebbles, they're usually found among stones of a similar size, midway up the beach. But then, mostly what you'll find this way are eroded and broken belemnites which have been washing up and down the beach with the tides. The best belemnites are the ones which have just been washed out of the belemnite marls.

And one place to look for these are below the belemnite marls of Black Ven, a huge landslip between Charmouth and Lyme Regis, where sometimes great gray mud slides can flow right down the beach into the sea. The belemnites shown at the top left were picked out of the gray mud of the dissolving marls on the beach below Black Ven - and are the most perfect examples that I have. The others, broken and rounded by erosion, were picked up elsewhere on the beach.

The rapid erosion of exposed belemnites in Lyme bay is an indicator of just how fragile fossils are. The limestone cliffs around Lyme Regis have been pumping out fossils for tens of thousands of years, and 99.999% of them have been ground into sand. The fossil record consists almost entirely of fossil outcrops on weathered rock faces, just like the belemnites on the margin of this page. We literally are barely even scratching the surface of actual fossil reserves, buried in the cliffs around Lyme Regis.

And then it has to be recognized that probably only a tiny minority of living creatures ever get to be fossilized. Most must end up being torn apart by predators or scavengers, or their remains dispersed by wind and water, or dissolved by acids. It can only be a very few that die and are immediately covered by some protective layer of silt that are preserved as fossils. And even then they have to survive the folding of the earth's crust, and avoid subduction at continental plate boundaries. The belemnites in the cliffs above Lyme bay are the ones which weren't eaten by Jurassic sharks, or whose remains weren't pulped by waves on some rocky shoreline, or dissolved by carbonic acid.

What is amazing is not how little we have discovered of the fossil record, but how much. And of what we have discovered, how much has since been lost, destroyed, or stolen? In many senses the fossil reserve is safer where it is, buried in the cliffs in Lyme bay, than it is in our museums and private collections.

This isn't really about Idle Theory. But it's somehow instructive to stroll along a sunny beach, listening to waves breaking on the shore, and picking up the remnants of creatures that lived and died some 200 million years ago.

Idle Theory

Author: Chris Davis
First created (and all belemnites collected): 26 Sep 2002