28 May 2022

Local History: The landscape which shaped the Derry that we know today

Local historian and genealogist, Brian Mitchell outlines the forces of nature which created our local landscape

Local History: The landscape which shaped the Derry that we know today

The Foyle Basin began to look as it is today seven million years ago.

Local historian and genealogist, Brian Mitchell, has written a book called 'Derry: A City Invincible'. In the coming weeks, we will be publishing extracts from the book. In this first article, Brian outlines the forces of nature which created our local landscape.

Lough Foyle and the Foyle Basin reflect hundreds of millions of years of earth movements and moulding.
Encompassed within the Antrim Plateau which culminates in the sheer cliffs of Binevenagh in the north; the rounded peat-covered summits and deeply dissected flanks of the Sperrins in the south; and the rugged series of hills and mountain ranges of the Donegal Highlands to the west, the Lough Foyle basin is a geological time scale.
The Donegal Highlands and Sperrin Mountains represent the western end of a thick belt of sedimentary rocks, deposited in a sea trough, which stretched from what is now Ireland to Scandinavia.
About 800 million years ago, under the weight of accumulating sediments of sand (sandstone) and black muds (shale), the trough began to subside.
Then 500 million years ago this rock sequence, now 15 miles thick, was intensely folded, heated, crystalised and uplifted into rugged mountain ranges aligned north-east to south-west.
This period of Caledonian mountain building reflected the collision of moving plates on which the earth’s crust is welded.
These plates are constantly being regenerated by volcanic activity at mid-ocean ridges, spreading out and finally being consumed at ocean trenches, generating earthquakes and chains of volcanoes in the process.
By the Carboniferous period, 325 to 370 million years ago, the old mountains of the Caledonian period had been worn away to lie beneath the waves.
A warm, shallow sea, similar to the Caribbean today covered Ireland which now lay across the equator. In this sea, limey muds (limestone), sand and muds were deposited.
What is now Lough Foyle represents a downfold or syncline of the ancient Caledonian rocks which became filled with sandstones and shales of Carboniferous age.
Lough Foyle today submerges this basin of Carboniferous sandstone.
Throughout this period the Foyle Basin was in the middle of a vast super-continent called Pangaea, destined to fragment into Africa, the Americas, Eurasia, Australia and Antarctica. Its surface changed continuously.

At various times the Foyle Basin was intersected by sea-filled troughs, submerged by shallow shifting seas, crossed by mountain ranges and subjected to climatic conditions ranging from desert heat to equatorial rain and arctic cold.
About 150 million years ago Pangaea began to break up and drift apart. Eighty million years ago the North Atlantic Ocean began to form, as America and Greenland were pushed apart. By 60 million years ago the Atlantic was beginning to open right next to Ireland, as the British Isles separated from Greenland.
This split was heralded with intense volcanic activity, as basalt lavas flooded out to form the Antrim Plateau, whose western limit now overlooks the Roe Valley and Magilligan.
At the same time, the earth movements which formed the Alps (as Africa collided with Eurasia) caused the downfaulting and sinking of Lough Foyle along existing north-east to south-west structural lines.
The River Foyle, in following the axis of this downfold, also flowed in a north-east direction. The Donegal Highlands and Sperrins, long eroded, were uplifted once again.
By seven million years ago the Foyle Basin, owing to extensive erosion and drainage development, was beginning to look as it does today.
If the general structure was now established, it was the quaternary ice advances, commencing about 2 million years ago and ending 12,000 years ago, which sculptured much of the present detail in the landscape.
In fact, much of the detail results from the final retreat of the ice.
During the Ice Age the Foyle Basin experienced climatic fluctuations which caused an alternation of glacial periods, during which the Donegal Highlands and Sperrins were submerged by considerable thicknesses of ice sheets, and interglacials during which temperatures were as high or higher than today.

During the last phase of the Ice Age the Irish ice sheet entered the Foyle Basin through the Glenshane Pass and down the Foyle Valley, while an ice sheet from Scotland advanced to the mouth of the Foyle.
A variety of drift material was deposited, by both the ice sheets and by their meltwaters as the ice sheets decayed, to clothe and soften the landscape.
Towards the close of the Ice Age a large glacier persisted in the Foyle Valley after the northern slopes of the Sperrins had become ice-free.
The River Faughan, dammed by this glacier, became a massive lake.
Likewise, the River Roe, in the stretch from Dungiven to Limavady, became a great lake in front of the southern limit of the Scottish ice. In these lakes extensive thicknesses of sand and gravel were deposited.
Glacial drainage channels were carved out, acting as overflow channels for the ice-dammed lakes. The River Faughan was forced to turn northwards along the eastern margin of the valley glacier.
As the Foyle glacier downwasted and retreated southwards its meltwaters carried large quantities of sand and gravel, which were deposited as extensive outwash terraces along the shores of Lough Foyle.

On the lower reaches of the Faughan, at Ardlough, kettle holes were left behind as masses of ice, buried under the outwash deposits, melted.
With the ice gone, this outwash material became a 50 feet terrace along the shore of Lough Foyle, as the land level rose in adjustment to its lighter, ice-free load.
The island of Derry owes its isolation to the Foyle glacier, as meltwaters flowing beneath it carved out the deep channel to the west of the hill.
Culmore Point and Magilligan Point had their origins in post-glacial times.
They are both sand spits. The latter is an enormous flat triangle of river-borne alluvium and wind-blown sand.
When man first reached the Foyle Basin, perhaps in about 6000 BC, this was the landscape which confronted him.
Only one piece of detail is missing.
He would have found a land forested everywhere.
As the climate improved, with the retreat of the ice, forests of willow and birch, followed by hazel, pine, alder, oak, elm and ash replaced the sparse alpine flora.
This is the landscape which man in the Derry area had to deal with.
Compared to the geological forces which created the Foyle Basin, man seems rather puny and very inexperienced.
Man has been around these parts for 8,000 years which, in geological terms, is just a blink of the eye.

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