An Overview of Lake District Geology: Part 1

To understand geology – or rather, to picture the way the world once was at a time so long ago that it’s almost incomprehensible to the human mind - you have to have a really great imagination. You have to be able to forget the way things are in front of you, and instead be able to visualise a world that may look unlike anything you’ve ever seen. This is never more the case than in the English Lake District, where what you see today is the end result of a complex history of rock deposition and mountain building episodes that by turns takes you through deep sea, shallow tropical seas, subduction zones and volcanic island arcs at the meeting of two tectonic plates. On top of that, much of this action took place far south of the Lakes' current location; somewhere near or above the current equator in fact, when the south of the UK was on an entirely different landmass to northern parts of the UK, and we gave those landmasses different names.

Landmasses

To explain the geology of the Lake District would take a long time (and for that there are very good books, albeit a little technical) and much of the detail is only likely to be of interest to experts, aficionados and zealous amateurs with too much time on their hands. But to understand the main bits of the overarching geology and how things have come to be can be enough to deeply enrich your enjoyment of the mountains and take you to a deeper understanding of them that will enliven your walks, and always give you something more to look at than just the views. Knowing a little about their history will also silence the naysayers who think mountains are not mountains unless they are Alpine. Not so!  In this first post on Lakeland geology I’ll cover what has generally happened over the last 500 million years (!) and give you a sense of why certain areas in the Lakes look as they do, and why parts look different from each other.  Being able to read the landscape in broad terms is often the most exciting bit of geology, and the precursor to being able to pick up a rock and talk about its origin. I have tried to bring this in under 1500 words and with silly pictures to help!

For ease of understanding we can break the main Lakes up into 3 zones of interest that run roughly SW-NE. These zones are broadly underlain by a huge dome of granite that peeks out at the surface in several places (for example, Shap where beautiful pink granite is mined) but is covered in most areas by the 3 zones we are about to look at. In later posts I will go into a bit more detail about each zone so that you can look for rock features yourself whilst our walking!

Lake district geology simplification

Furthest North and representing the oldest rocks in the Lake District are the Skiddaw Group – the collective name for a group of rocks that were formed through slow and gradual deposition of sediments into a sea environment, starting 500 million years ago. These would have been 1000s of metres thick and accumulated over millions of years when nothing much else of note was happening. This is where you first need your imagination; removing the mental image of the Lakeland fells, and instead imagining sediments being carried through rivers and down slopes into the sea! No mountains at all. Over time these layers of sediments were compressed into beds and turned to rock  (‘lithification’). We’ll return to them later. 

Looking North to the gently sloping mountains of the skiddaw massif and surrounding peaks. Lake district uk (R.Allen 2015)

Looking North to the gently sloping mountains of the skiddaw massif and surrounding peaks. Lake district uk (R.Allen 2015)

The next major group of rocks is the Borrowdale Volcanic Group (BVG). These rocks aren’t restricted to the Borrowdale valley, but this is where the ‘type examples’ of rock are found and best documented. Once you’ve got to grips with the idea of no mountains in the Lake District yet (and indeed no lakes!) you really need to get your imagination working. Rocks of the BVG, as the name implies, are volcanic in origin and were erupted from low-lying volcanoes that formed an island arc similar to the Indonesian volcanoes we know today (see below!). These volcanoes developed around 450 million years ago (in spasms of activity over millions of years) as a result of tectonic activity driven by currents in the earth’s mantle, which moved the continental and oceanic plates together that had previously been moving apart whilst the Skiddaw Group (as we know them now) were being deposited.  As they moved, the sea that lay between land masses sat on the converging plates, began to close and the oceanic plate was forced under the continental plate. As it dragged sea water down with it over time, this water needing to escape, did so through vents with magma….and these were what we now know today as the BVG. Some appearing as effusive lava that flows and glows orange as we imagine lava to, and other material blasted out of the earth as explosive debris and pyroclastic flows. Think Mt Pinatubo or Mount St Helens.  Over time these hardened into igneous rocks which would be durable to erosion compared to the Skiddaw Slates. These rocks account for the highest peaks in the Lakes, and are the reason for the rough terrain and craggy edges. But again, hold your thoughts there! We need to quickly cover one last group before we even get to the mountain building bit. 

subduction zone

The Windermere Group is the third zone of note, that formed after the volcanic period about 420 million years ago and these are comprised of more sedimentary rocks deposited in a sea environment. Again we have to imagine an end to the volcanic period, and a return to more gentle deposition over slopes and into the sea. This to-ing and fro-ing of activity over hundreds of millions of years, is partly why Lakeland geology is so hard to get your head round: the land we know today is so old that it has been through millions of years of movement both in terms of upward and downward movement (by turns submerged below water and exposed at the surface) but also horizontally; the landmass having started below the equator! This is not just the story of the Lake District, but also the Scottish Highlands, Snowdonia in Wales and the Appalacians in the United States which all have similar (though not identical) histories. 

the craggier peaks of bowfell and surrounding peaks which are comprised of the harder volcanic rocks of the BVG. In the distance are the foothills of the windermere group. (R allen 2015)

the craggier peaks of bowfell and surrounding peaks which are comprised of the harder volcanic rocks of the BVG. In the distance are the foothills of the windermere group. (R allen 2015)

So, we know our overarching rock types and we know they were created in an environment nothing like we see today, and nowhere near where we are today! The exciting bit then really started happened at approximately 400 million years ago when ALL of the above rocks were smashed together as the oceanic plate was fully subducted and the continental plates on either side of it met. This was the Caledonian Orogeny (Orogenesis, meaning mountain birth) which thrust all of the rocks we’ve looked at to a level higher than the Alps sit today. At this point the Skiddaw group was baked by underlying volcanism again (to give Skiddaw Slate), bedding was bent into all directions and everything from all the groups got moved about and mixed up.

mountain building


But that’s not the end because we’re still only at 350 million years and at this point the whole lot was worn down again when mountain building ceased. Think of the Alps or the Himalayas disappearing almost completely. In fact, what was by now just hills again went back under the sea – a warm tropical sea full of life! I could talk at length about the following 347 million years and how the area became a dune system, and how at 250 million years we were still only at the latitude of the Sahara, but let’s cut to the chase now and rejoin the story at 2 million years ago. 

the lake district shows some devastatingly beautiful glacial features - here in the middle is the glacial valley of honister and fleetwith pike (R allen 2015)

the lake district shows some devastatingly beautiful glacial features - here in the middle is the glacial valley of honister and fleetwith pike (R allen 2015)

At this point the ‘Lake District’ was just an area of gently sloping hills but was finally out of the sea for what would be the last time. The earth was entering into a phase of glacial cycles, and the land that was to become the lakeland area gradually covered with ice sheets and glaciers. The melting of these ice sheets, and the movement of the glaciers that sat within the hill valleys is what created, eventually, the Lake District we know today with all of it's u-shaped valleys, aretes or 'ridges' that are popular for walking, and of course the plentiful lakes and tarns! Almost everything we see today in broad landscape terms is the result of how those 3 distinct zones of rocks were created, moved and changed over time. The softer, sedimentary Skiddaw Group and Windermere Group were eroded into sloping, softer hills such as Skiddaw and Blencathra (and the lower foothills around Windermere and Kendal, respectively). The harder zone of rock created at the centre of volcanic activity are today’s Sca Fell, Bow Fell, Crinkle Crags, Great Gable to name just a few, and are much more resistant to erosion and thus show steep crags and deep valleys where glaciation couldn’t do as much damage and instead could only cut downwards. The Lake District as we know it today is therefore a product of it's mountain building phase, but crucially of it's earlier volcanism, and later glaciation.

In future (shorter) posts I will return to the glacial beauty of the Lake District in more detail, we'll look at how the Westmorland Green Slate that you may have on your roof or dining table came to be, and I’ll show you some beautiful areas where you can see rocks of each 3 groups in situ. Until then, happy landscape reading!