Sep 15, 20226 min read

How The Lake District Was Formed...In Three Minutes

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For those interested in a quick summary of the half a billion years of geological history of The Lakes

I was walking in the Lake District the last few days, hence the radio silence. The cover photo is of my 14 year old pooch Midnight at the end of a jetty on Windermere wondering if she should go swimming for the first time ever. She didn't. You can't teach an old....

Before I get back to writing about inflation, more inflation, and even more inflation, I thought some Chimp readers might be interested to know a bit more about the geological history of The Lakes. Understanding the geology and geological history of regions of natural beauty like the Lake District has for me made walking in them even more enjoyable.

520 million years ago (520 Ma) what is now England and Wales was close to the south pole on the edge of the supercontinent Gondwana - what is now Scotland was near the equator 4,500 km away on the edge of the continent Laurentia - what is now the US - on the other side of the Iapetus Ocean (Fig 1).

The Iapetus Ocean had been widening, but when this widening stopped, Iapetus ocean crust started during the Late Cambrian around 500 Ma to be subducted at its S/SE and N/NW margins underneath, respectively, Gondwana and Laurentia. The subduction on the Gondwana margin tugged at the slice of the supercontinent that comprised what is now England and Wales. This tugging induced rifting underneath Gondwana, and eventually a continental slice that contained England and Wales separated from the supercontinent. We call this continental slice or microcontinent Avalonia, and the ocean that formed between it and Gondwana, the Rheic Ocean (Fig 2).

In the Late Cambrian Early Ordovician (485-470 Ma) as the Iapetus was closing and Avalonia was moving towards Laurentia, vast quantities of mud were washed down Avalonian rivers and ended up at the bottom of the Iapetus on a relatively deep water shelf. This thick layer of mud would eventually become the four km layer of slate that covers the northern part of the Lake District and that we call the Skiddaw Slates (Fig 3). When mud is lithified it becomes the sedimentary rock shale, which then turns, given the right pressure and temperature conditions, to the metamorphic rock slate. If deep water mud sediments are very pure - i.e., do not contain sand etc - you end up with good quality slate as indeed is the case with the Skiddaw Slates.

As Avalonia closed in on Laurentia in the Middle Ordovician (470-458 Ma), the subduction of the Iapetus ocean crust underneath Avalonia caused extensive volcanism that formed a continental arc behind the deepwater sediments (Fig 5). This volcanism would first have been more effusive, forming mainly lavas, then later more explosive, forming mainly pyroclastics/ash. The rocks that formed from these volcanic products we now call the Borrowdale Volcanic Group that makes up the central part of the Lake District. The Lower Borrowdale Volcanics are comprised of rocks formed from lavas and tend to sit in the northern part of the group, while the Upper Borrowdale Volcanics made from ash are more to the south. I came across the rock in the photo (Fig 6) near Rydal Water which is in the Upper Borrowdale Volcanics. I believe it is a banded tuff formed from volcanic ash. The layers represent different eruptions and the waves are formed because the ash ended up in low energy water, at the bottom of the sea or a lake perhaps. I was delighted that the rock appeared to be where it should have been!

Avalonia and Laurentia were colliding in the Late Ordovician (458-443 Ma) and Silurian (443-419 Ma) in what is called the Acadian Orogeny - continental collisions cause orogenies, mountain building events. The orogeny produced a foreland basin behind the volcanic region, into which rivers flowed (Fig 7). These rivers carried sand, silt, clay etc that eventually turned to rock that we now call the Windermere Supergroup in the SE region of the Lake District.

So, the Lake District is mostly comprised of three distinct geological regions: the Skiddaw Slates towards the NW, the Borrowdale Volcanic Group in the middle, and the Windermere Supergroup towards the SE. However, there was quite a lot of other stuff that was going on at the time and since.

For example, there is a seam of what is called the Coniston Limestone between the Borrowdale Volcanics and the Windermere Supergroup (Fig 4).

There are also areas of granites towards the SW and the NE (Fig 4) that would originally have formed from magma in chambers deep below the surface and which have since been exhumed and exposed at the surface as rock above was eroded - it was these magma chambers that fed the volcanoes that formed the Borrowdale Group. Magma that cools at depth - i.e., plutonically - does so more slowly than lava erupted at the surface - i.e., volcanically. It therefore forms rock like granite that has coarse grains/crystals that are able to grow as magma cools very slowly.

Then there are the 400 million odd years since the formation of the Windermere Supergroup during which things were happening. The closure of the Iapetus had been an early part of system of tectonic plate movement that led to the formation of the supercontinent Pangaea around 335 Ma that comprised most of what are now the main continents (Fig 8). Around 200 Ma, Pangaea began to break apart, and the North Atlantic began to open. Scotland and the rest of the British Isles had by that time become attached to each other on the edge of the Eurasian plate. The Eurasian plate separated from the North American plate as the North Atlantic opened.

Briefly...

There is an outcrop of Devonian (419-359 Ma) Old Red Sandstone in the NE of the Lake District. Then there are Carboniferous (359-299 Ma) limestones and coal measures on the northern, eastern, and southern margins on the region. These were followed by the so-called red beds which are found to the NE beyond Penrith and in the SW coastal region and which were formed in the Permian (299-252 Ma) and Triassic (252-201 Ma). There are no rocks of Jurassic (201-145 Ma) and Cretaceous (145-66 Ma) age to be found in the region but it is believed that during the Triassic, Jurassic and Cretaceous the entire Lake District region became covered by 700-1750m of sediments that have since been eroded away during what is known as the Cenozoic Era (66 Ma to date). The most recent period of the Cenozoic is known as the Quarternary (2.6 Ma to date) during which repeated glaciations produced the distinctive U-shaped valleys and other features of the region.

The other thing worth mentioning is that while the Acadian Orogeny was occurring on the margin of Avalonia, the Grampian Orogeny was occurring on the margin of Laurentia. This orogeny is what formed the mountains of NW Scotland (Fig 9).

That's it. I hope when you (next) visit the Lake District it might be even more enjoyable. If you are really interested in geology, I highly recommend the part time (4y or 6y) BSc Geology degree at Birkbeck College, University of London.

Fig 1: Palaeogeography during Late Cambrian (c. 520 Ma)