Country diary: a vertical timeline is clearly visible in the cliffs

Country Diary on 30 November by Emma Mitchell : Image of cliffs at Walton on the Naze Photograph: Emma Mitchell

I walk past the arcades and beach cafes. The smell of vinegar on chips is sharp in my nose and makes my mouth water but I haven’t come to Walton on the Naze for seaside snacks. Higher ground is visible beyond the beach, partly shrouded in mist, and I strike out towards it.

Finds from the Red Crag including Glycimeris with a whelk drill hole. Photograph: Emma Mitchell

Along this part of the coast, a vertical timeline is clearly visible in the cliffs. Just beneath the topsoil are gravel layers laid down by the river Thames as it flowed across central Essex 600,000 years ago. Under the gravels is the Red Crag: a rust-coloured, iron-rich, sandy layer that is 2.1m-3.6m years old (late Pilocene to early Pleistocene). Laid down by a cool sea, this deposit is rich in marine fossils, as can be seen a metre or two from the main path bordering the beach.

Bivalve and gastropod shells stained by the iron protrude from the grains. The first one I encounter is a dog cockle or Glycymeris clam: a flattish, almost perfectly round bivalve up to 6cm across. Modern species of Glycymeris are eaten in several European countries. I find several early Pilocene specimens, one with a neat, round hole in its pointed end made by a predatory whelk drilling into its shell and eating it alive.

At the base of the cliffs is a soft, dark older layer: the London Clay formation, dating back to the Lower Eocene, 49m-56m years ago. Where beach huts and a promenade sit now were subtropical seas teeming with life. I move onto the beach and stoop to search among the piles of pebbles.

Sharks’ teeth from the London clay. Photograph: Emma Mitchell

I’m a seasoned beachcomber and could happily spend hours immersed in a quest for seaglass or Baltic amber. This time I’m seeking fossilised shark teeth scoured from the London clay. I find one, burnished black, almost as though varnished; even after 50m years, it is scimitar-sharp. I wince in sympathy for its prey.

The largest species of shark in the fossil record cruised a warm sea here. The megalodon was at least 16 metres long and snacked on whales; several of the huge teeth that gave it its name (they can be 12cm long) have been found at Walton on the Naze. I keep searching.