Green hills laced with drystone walls, impressive rocky cliffs, and limestone pavements with their unique and delicate flora are all icons of the Yorkshire Dales. It is a landscape unapologetically dominated and influenced by the limestone that underpins it. At around 350 - 330 million years old the lowermost limestone units are up to five times more distant in time from Tyrannosaurus rex than we are from that tyrant lizard, and yet these are still not the oldest rocks in Yorkshire! Lurking beneath the limestone is something far older still, from a time before creatures colonized the land. These rocks tell the story of the death of an ocean and the birth of mountains! Before we dive too deep we need a bit of geographic context, and prepare yourself for a shock. During the time period we are interested in here (the Early Ordovician) Yorkshire was in the south. Very south. The southern hemisphere, in fact. I’ll let you digest that thought for a moment… We find ourselves in the ocean off the northern margin of a tiny little continent named Avalonia. Across the sea to our east lies Baltica, another small continent that will eventually become much of north-west Europe. To our north lies Laurentia (North America, Greenland, Northern Ireland and Scotland). Between these continents lay the Iapetus Ocean. Now you’ve got this picture in your heads let’s look at the rocks! Let us begin in around the village of Ingleton and the Waterfalls Trail along the River Twiss. At around 470 million years old, these are Yorkshire’s oldest rocks. These rocks, known as the Ingleton Group, are a type of rock known as greywackes (pronounced grey wacky), which is not only the best name for a sedimentary rock but it is quite appropriate as they are tilted and folded to as much as 70 degrees at Thornton Force waterfall! Like most of the rocks featured in this blog they were deposited by massive underwater landslides (turbidity currents) cascading off the margins of continents and bringing silt and sand into deep oceanic waters (check out our blog on Pendle Hill for more on turbidity currents!). Remember those three little contents all sitting pretty in the Southern Hemisphere? Well things were on the move: Avalonia (remember that included Yorkshire!) was heading north (yay!). This wasn’t the best thing for the Iapetus Ocean though which was being squeezed between the continents. The sediments of the Ingleton Group were pushed up from the sea bed, bent, buckled and eroded. This all took a considerable amount time, and no rocks were being laid down in the region whilst it was happening. Our story picks up again in the Ashgill (Ordovician, around 440 million years ago), with the deposition of the Coniston Limestone Group. These sediments were deposited in much shallower water and locally is quite fossiliferous. On into the Silurian! And the Iapetus is still closing! The rocks of Yorkshire again record deep water sediments: more turbidite sequences, only this time there are more fossils. In the depths of this closing ocean the low oxygen muds preserved some very delicate fossils.
This type is called Monograptus, ‘mono’ meaning ‘one’ because they have only a single thread of cups in a row. Monograptus was quite abundant during the Silurian and some surfaces can be covered in them, like the above block from a quarry in Horton-in-Ribblesdale. Eventually the Iapetus ocean closed completely, all three continents crunched together in a mountain building event called the Caledonian Orogeny. It’s this mountain building event that gave birth to the Scottish mountains, although they were much larger mountains when they were first formed. Over time the high areas of land produced in Yorkshire by this event were eroded and new seas flooded this ancient land. It is now the Carboniferous, and the classic limestones of the Yorkshire Dales which we all know and love are being formed. The time gap between these rock units has created a type of geological boundary called an unconformity, which can be seen in the valley sides and quarry faces of Ribblesdale, Crummack Dale and Chapel le Dale near Ingleton. At sites like these you can truly get a glimpse of the enormity of geological time, and peel back the layers of the landscape to reveal the dramatic histories of lost oceans! Author: Jed Atkinson
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