What is the question I get asked most often when I give talks on local geology? Any guesses? Perhaps: ‘Why are there fossil seashells on the tops of hills?’ or ‘Does fracking pollute underground water?’ or ‘Is Whitby jet only found in Whitby?’ Well, I get all these but the runaway winner is ‘What does Dogger mean, and is it the same as Dogger Bank?’

This comes up because the Dogger is a crucial marker bed along most of the Yorkshire coast – so I mention it a lot in my talks. What do I mean by ‘marker bed’? There’s no strict definition but the Dogger is a thin (from 1 to 12 metres thick) layer of very distinctive rock that marks the boundary between the Lower and Middle Jurassic strata. It is visible along the coast from just north of Runswick Bay, south as far as Saltwick.

All the dales in the North York Moors are cut through a layer of hard Middle Jurassic sand and gritstone into the soft shales of the Lower Jurassic beneath. This change shows up brilliantly in every dale from Westerdale to Glaisdale and Bilsdale to Farndale. Why? Because the Middle Jurassic rock is infertile and is therefore covered in heather moorland, while the Lower Jurassic supports pasture. And the Dogger marks the boundary, visible as a straight line between moorland and green pasture.

The Dogger is rich in iron, particularly at Rosedale where it was heavily mined on both sides of the dale. Look closely and you’ll see how the old mine workings are at the boundary between moorland and fields.

So what does Dogger mean? It seems that Dogger Bank is named after the Dutch name for a two-masted cod-fishing boat, while ‘dogger’ is a general English word for, well, a lump. So because our marker bed is full of lumps it is called the Dogger.