Which do you prefer: stacks of bread and butter, or piles of books, or maybe even layers of sponge cake? I’ve used all of these and more to show how the rocks of the Yorkshire coast fit together. That’s what they are, just layers of rock – let’s call them strata – with the oldest at the bottom and the youngest at the top.
What makes this coast so brilliant for geologists is that each stratum of rock extends over great distances. This means you can trace a rock like the Dogger all the way along the coast from Runswick Bay to Ravenscar; the same Hambleton Oolite seen north of Helmsley forms the top of Oliver’s Mount 30 miles away.
And this stacking up of strata makes it easy to trace changes in conditions through time. If a bed of dark shale sits on top of a pale rock then you’re looking at a reduction in oxygen – due to a deepening of the sea or a change in its chemistry. This all works so well because most of the rocks were laid down on level sea floors that extended over miles in every direction – which in turn produces nice level and extensive strata.
But not all the rocks on the coast are like this. In the Middle Jurassic period this area changed from a deep sea to a vast delta – a coastal area with a shifting array of streams, sandbanks, forests and beaches. In contrast to the level, uniform seabed this was a varied and changing environment.
And so, not surprisingly, Middle Jurassic rocks vary from place to place. From Ravenscar to well south of Scarborough the cliffs and scars are a complex mix of river sandstones, beach deposits, shallow sea limestones and gritstones. The rock platforms show a fascinating series of features from the Jurassic delta. River channels, meander belts, even the shifting patterns of streams are preserved in fantastic detail.
The Yorkshire coast is a natural theme park for geologists, having variation from place to place as well as up and down the strata is just one more remarkable feature of this natural wonder.