Frontiers 2021

The mystery of the oaks

Written by Steve Webster

Here are two extracts from scientific papers of recent years. One of them must be wrong. The first one is a map showing how Quercus (oak) gradually reappeared across Britain and Ireland after the Ice Age. It’s been built up from various finds, and pictures a northwards spread of oak through contour lines, with each contour showing the further limit of oak at that particular time.

The spreading of the oaks

The spreading of the oaks

This map (Reference 1) was first published in 1989, when most of the evidence in this article was not available, and reproduced in a paper published in 2005. This paper was recommended to me by an official source in 2020, when I asked what was the current understanding of the advance of oaks across Britain after the Ice Age.

Here is the second extract from a published paper, this time from 1992.

Quercus [oak] pollen reached frequencies of 14% at Dallican Water at 6,800 BP … Quercus was therefore probably also present at Catta Ness.

… Betula [birch], probably Betula Pubescens, increased at about 9,500 BP …Quercus and Ulmus at 9,200 BP … ”

(Reference 2)

Two contrasting pictures

Current convention represented by the map from Reference 1. Oak is supposed to have entered Britain and Ireland from the south. The oldest oak pollen on mainland Britain has been found in Cornwall from c. 9,500 BP. So it’s argued that oak would then have headed north. It would have done this slowly, since acorns rarely travel far from the parent oak, and an oak takes 20 years or more from germination to reach sexual maturity and produce more acorns.

Yet oak was leaving traces of its presence in Shetland over 9,000 years ago. Shetland is so far north it doesn’t even appear on the map above. How was oak in Shetland hundreds of years before oak reached the middle of England?

Let’s take a look in some other islands north and west of Britain – Ireland, Orkney and the Outer Hebrides.

A wonderful quality of pollen is that although naturally intended to work fast, so a plant can make seeds in the summer available, pollen can in certain conditions last for thousands of years, intact enough for a trained eye to see what kind of plant the pollen came from. Another helpful quality of pollens is that they come from different species in an amazing variety of shapes and colours.

One of the best places to find ancient pollen is in the mud in the bed of a freshwater loch. Pollen sinks. Here is such a loch on what is now a treeless island in the Aran Islands off southwest Ireland.

An Loch Mór on Inis Oírr

An Loch Mór on Inis Oírr

These various graphs (Reference 3) show the pollen from different types of tree in the mud of An Loch Mór on Inis Oírr, drawn out of the loch bed by the coring tool. The pollen is dated by the layers, with the oldest pollen in the deepest mud on the loch bed.

The timeline runs along the bottom of the diagram. The jagged red line shows temperatures detected from analysis of another set of cores, the Greenland ice.

We can follow the various diagrams for individual species of tree, to see how they vary over time, starting with the oldest cores extracted, from pollen that fell on the loch c.13,500 years ago.

Quercus (oak) is seen in the deepest oldest mud drawn out of the loch bed.

The authors are cautious with the evidence. You can see the lines for Quercus, and for Corylus (hazel), are dotted for the first 3,000 years. This denotes the authors’ interpretation that this pollen comes from a remote source and is not firm evidence of oak and hazel growing here in flagrant disregard of convention. They state that the amount of oak and hazel pollen in this early time is insignificantly small, although I have to say the lines don’t exactly show me that, and they conclude:

Near the base of B1b (c.10.5 ka, CS a) values increase to consistently >1%. This is indicative of a modest local population of oak in a landscape dominated by hazel and pine.

In other words, even if we write off the first 3,000 years of evidence we have to recognise a local presence of oak 10,500 years ago.

Inis Oìrr became an island somewhere in the timeline above. The sea channel between the island and the Irish mainland reaches 50 metres deep now, so 13,500 years ago it would have been a valley, perhaps full of trees, more sheltered than the hill above the valley which is now the island.

The authors don’t try to identify their suggested remote source of pollen. A mature oak can release a colossal amount of pollen, and it can travel a long way, but according to convention even 10,500 years ago there was no oak north of what is now the English Channel.

This apparent remote source consistently deposited oak and hazel pollen on this far corner of southwest Ireland from 13,500 years ago, but nowhere in mainland Britain, apparently.

For example, cores going back 12,000 years at Hockham Mere and Stowe Bedon in southeast England (Reference 4) did not register this “remote source” oak pollen before the clear signal of the arrival of oak a little over 9,000 BP.

A pollen survey from southeast Ireland goes further back in time than any other I’ve seen in Britain and Ireland. Here is the diagram with the results.

Thomastown bog

Thomastown bog

This diagram is laid out according to the depth of the mud, with the oldest deepest mud at the bottom. So to get the information about any particular species, we look for its name and then follow the timeline upwards.

In the deepest mud recovered, from around 16,000 years ago, we see juniper, willow, pine and birch. We see two early glimpses of oak in times when other trees seem to have been producing more pollen. Maybe this oak pollen was blown in from somewhere. Oaks were plenty and fertile in mild climate eras – the prevailing winds in Ireland blow from the west, where An Loch Mór was – or maybe oak began to get locally established but failed in colder times. Around 13,000 years ago we begin to see a more persistent signal, then it shuts off, along with all other tree pollens. There is a silent period. No tree pollen, from any source, is falling on Thomastown bog.

This is the Younger Dryas, when around 12,900 years ago land west and east of the North Atlantic was abruptly plunged into over a thousand years of deep freeze. A new ice sheet grew in the northwest of mainland Scotland, and carved out Glencoe as we know it. Where the ice reached its southern limit and dumped rock transported out of the Highlands, Loch Lomond would eventually fill behind that heap.

Britain was still connected to the European landmass. Clouds out of the western ocean dumped their moisture in snow on the western ranges of modern Scotland, Cumbria and Wales, and inland became arid and cold. In Norfolk birch survived, and some pine. Further north reverted to tundra, mammoth and reindeer country.

In the west, I believe, then as now the climate was milder and moister. The pollens retrieved from An Loch Mór and Thomastown bog show what might be expected if we consider proximity to the ocean was helpful to woodland survival in the Younger Dryas – such as the persistence of oak on the west coast while it died away inland with the onset of the cold at Thomastown, 16 kilometres from the east coast of Ireland today. At An Loch Mór Fraxinus (ash) after the earliest sign of ash I’ve seen in Britain and Ireland, disappeared rapidly in the Younger Dryas. Other trees like alder thrived. The alder signal is too strong to discount as from a remote source, and where alder did so well it suggests it was wet.

The picture from the Western Isles

Between Ireland and Shetland are the Outer Hebrides, the Western Isles. In the Younger Dryas they formed one huge island, around 300 kilometres south to north and extending much further west than now.

Pollen records shown in Reference 6, from two freshwater lochs, one now on Lewis, one now on North Uist, go almost 8 metres deep in the mud, back to when both lochs were on the same island.

 Loch a’Phuinnd and Loch Buailaval Beag on Lewis

Loch a’Phuinnd and Loch Buailaval Beag on Lewis

The results from Loch a’Phuinnd on Uist show tree pollen reaching back over 12,000 years, back into the Younger Dryas. We see birch, pine, willow and juniper. Not much, but it’s there, long before any sign of pine has been found on the north British mainland. The most sheltered places on the Western Isles then are today the narrow steep-sided glens of sea lochs, initially carved out by glaciers which would have left them rich in mineral nutrient.

And again, deposited at higher elevations in the mud of the beds of Loch a’Phuinnd and Loch Buailaval Beag on Lewis, oak pollen, over 9,000 years ago.

The earliest sign of pine ever found on mainland Scotland comes from c.9,500 years ago. It was not found in the south of Scotland, where it might have been expected. It was found at Loch Maree, near Gairloch, due east of the Western Isle of long ago. In the Western Isles we see pine pollen thousands of years earlier. What might be found in cores from the beds of sea lochs remains to be seen.

North to Orkney

So what about Orkney? An insight is provided in the next paper, Reference 7.

Quoyloo meadow

Quoyloo meadow

Corylus avellana [hazel] probably arrived in the area at around [10,400 BP]. At about [8,900 BP] the abundance of arboreal taxa increased and extensive woodland cover developed around both basins. The woodland was predominantly BetulaCorylus avellana scrub, with some Salix and a fern-rich understorey vegetation. At about [7,400 BP], the woodland cover at Quoyloo Meadow was disturbed and reduced in extent, with Corylus avellana being particularly affected….

The Saksunarvatn ash layer was recorded, coinciding with the arrival of Corylus avellana in the islands at around 9-2 ka BP. Extensive woodland developed at about 8,000 BP, predominantly Betula and Corylus with Salix …  Quercus and Pinus sylvestris occasionally present.”

Saksunarvatn volcano in the Faroes erupted c. 10,200 years ago.

So in this natural bowl in the western hills of Mainland, the largest of the Orkney Isles, Corylus avellana (hazel) is seen over 10,000 years ago, and oak a thousand years later.

Orkney may not have become an island until around 10,000 years ago, maybe later. The Younger Dryas was also milder on the western coast of the mainland than further inland.

A photo of a 12,000-year-old willow leaf found near Arisaig on the west coast of Scotland in the course of postglacial geology fieldwork was posted on Twitter by Dr Natasha Barlow in autumn 2018. There is evidence of human activity on Islay c. 12,000 years ago.

People at those times would have seen snow all year round on mountains inland. In all these places, and the Orkney peninsula – if such it was in the Younger Dryas – the pollen evidence shows that trees were already there, and some survived.

Oaks to the islands

The early presence of trees on Shetland, Orkney, the great Western Isle and Ireland raises another problem. Mervyn Lloyd Jones, on whose farm in Wales the 500-year-old Brimmon oak stands, announced a couple of years ago that he was making his yearly collection of acorns from the tree to grow new oaks. He said he would drop them in a bucket of water, and discard those that floated. Only acorns which sink are viable. I asked him if he’d ever tried it in sea water, and a little later he came back and told me he had tried it, and it worked the same.

Viable acorns sink in sea water.

So how could oak have reached all these islands in the west?

The BRITICE CHRONO project to determine the timing and pattern of retreat of the British-Irish ice has found evidence the ice started receding from the west far earlier than previous estimates – at a time when it was still extending southward across modern England.

The terrestrial cosmogenic nuclide (TCN) test looks at minerals in the surface of rocks to determine how long they have been exposed. The high-energy impact of cosmic rays on the Earth can create particular nuclides of certain metals. So by looking for those nuclides, and counting how many of them are seen on a rock surface, it is possible to work out how long that surface has been exposed to the atmosphere. So when ice-melt exposes the surface of a rock, the TCN test can provide a date for it.

The BRITICE project (Reference 8) found rock at Cape Wrath, up at the far northwest corner of mainland Britain, which they were satisfied hadn’t been dragged and dumped there by the ice, and which apparently became exposed to cosmic rays – and thereby free of year-round ice cover, at c. 25,000 BP. That is 2,000 years before what is considered the global last glacial maximum.

Various sites on Lewis became ice free c. 20,000 BP; the Aran Islands and the southwest of Ireland 18,000 BP.

Birch beginnings

The first known humans to find Iceland found a third of it covered in forest, almost all birch whose DNA suggests the seeds flew there from Europe. As large areas in the west of the future Briain and Ireland became free of Ice, birch would not take long to arrive.

I’ve watched birch grow over a metre a year on a disused coal mine waste heap in Yorkshire, a grey lump most weeds wouldn’t grow on composed of crushed mudstone and clay mined involuntarily with the coal and separated in surface screens in the 19th century. I planted a few birch on there and they loved it.

17,000 years ago so much ice still sat on the northern continents that the global ocean level was around 110 metres lower than now. You could perhaps have walked from Iberia, in far southwestern Europe, up the west and over Ireland and the Outer Hebrides if you chose, or travelled all the way on land now undersea, to Shetland. You might have encountered a lot of birch forest on the way, along with mineral-rich heathlands of juniper and crowberry, with a wealth of life in a nutrient-rich sea. There was plenty to invite the advance of life north from its Iberian Ice Age hideout. The remains of some of those forests may be on the seabed today.

Another wonderful tool we’ve recently acquired to look into the deep past in intimate detail is DNA analysis. The DNA of 98% of oaks in Britain and Ireland today shows they are of Iberian ancestry.

The rising sea

No one knows when Shetland, the single Western Isle or Ireland became islands. Orkney may have remained a bulge at the end of a long narrow peninsula off the northern mainland a few thousand years longer than the others. The retreat of the ice from the west was surely accelerated by attack from the sea, ironically aided by the fact the colossal weight of ice over millennia had pressed the land down far below its present level, exposing the ice to the Atlantic.

What happened to the land and sea after the ice left – the relative sea level – is a subject of intense ongoing study. One thing that is clear is it was intensely variable in different locations. It’s a story being pieced together from ancient signs of marine or freshwater life known to live within certain depth ranges, along with signs seen in the land such as gouge tracks where an iceberg has been carried in the sea with its base dragging on the seabed.

Further evidence is provided on the seabed between the Outer Hebrides and the west coast of mainland Scotland in the form of isolation basins. These were low lying areas flooded by sea as the ice crumbled, then separated from the ocean as the land rose in the following millennia.

A paper published just this year (Reference 9) looks at isolation basins from the time of meltwater pulse 1A. This is a period seen in evidence around the world when the global sea level rose perhaps 18 metres between c. 14,500 – 14,000 BP. The paper shows that the ice-melt sources for the rising sea level were primarily the northern ice sheets. The saltwater lakes that developed became isolated from the sea around 14,500 BP or earlier, and the rising sea in the next 500 years did not reconnect them. The land was rising faster.

So islands may have formed and then reconnected with neighbouring land, land bridges come and gone, saltwater lakes existed for a few centuries, in the shifting maze that was the way north for life that finally became marooned by the rising ocean on the islands of Ireland, the Western Isle, Shetland and Orkney. On the west coast of Scotland due east of the Western Isles today you can see beaches where the shingle reaches 5 metres above present high tide, as the land has continued to rise since Ice Age meltdown finally slowed to a trickle c. 5,000 years ago.

In the Western and Northern Isles on the other hand we see no such features. Instead a car ferry negotiates the shallow sea between Harris and Uist where the islands separated around that time. Just between Harris and Skye, a short boat crossing apart, the observed effects of isostatic land uplift are dramatically different. That might reflect the fact that between them is a fault line created in a collision of two continents south of the equator about 400 million years ago. The Western Isles are geologically near the western end of a shard left stuck behind when the continents tore apart again around 70 million years ago and Greenland went west. Skye was created in the volcanic convulsion that tearing apart sparked off.

That fault line became the gorge where the ice streams studied by the BRITICE project ran north and south, and is now a deep channel along the east coast of the western isles.

To the west, between the Western Isles and the continental shelf edge more or less 70 km away, the sea is shallower. Land extended far west of the current shoreline for a few thousand years.

So, it’s a complicated picture still nowhere near being defined. Life found a way to move through this landscape, I believe.

Life moving north

One of the early challenges facing life moving north into the future Britain and Ireland was what is now the English Channel, or Manche. A river ran out of the east fed by the Thames, Rhine, Seine and so on. The river carried a lot of silt and clay, more rock paste, and out west could spread out in mud flats and meanders. Yet acorns sink, and squirrels don’t care to cross water, so it helped that the ice was still in control further east.

The river channels froze sometimes, dried up at other times, perhaps changed course sometimes so you might find yourself north of a river you’d been south of. Or if a stand of oaks got established on the southern shore of a wide marsh full of shifting channels, birds such as jays who like acorns might cross the marsh from nest sites in birch woods on the northern shore, carry acorns across and drop a few among the birches. It was not an impassible barrier to life advancing north with a thousand years to negotiate it.

11,500 years ago, after the Younger Dryas, that was no longer the case. A deep sea inlet ran up the centre of the modern Channel, reaching approximately as far east as modern Brighton. Any life advancing north from Iberia which could not swim, fly or float had to go far upriver east to find a way across or around it and into what would become the island of Britain.

If oaks had taken this path, we might expect them to have left traces as they tracked west across southern England to reach Cornwall. That does not seem to be the case.

I propose that oak reached as far north as Shetland before c. 14,000 BP. A thousand years later, when the Younger Dryas struck, oaks survived in the west in deep sheltered glens, now sea lochs and channels between islands. A diverse forest protects the individuals within it. After the Younger Dryas, the climate warmed. By 9,500 BP it may have been warmer than late 20th-century climate. Oaks spread across these islands, and reached those more exposed locations where we find oak pollen from that time.

Another palynological study (Reference 10), by M. J. C. Walker, found the earliest oak pollen near the west coast of Cumbria, not far south of the border with Scotland, from 9,000–9,450 years ago.

Earliest signs of oak pollen in the Peak District of Central England, according to Reference 10 appear less than 8,700 years ago.

Seascale is almost 100 kilometres north of the Peak District, yet oak appears around 500 years earlier.

It is also further west, and near the coast.

At least as far as oak is concerned, all this evidence suggests to me the reforesting of Britain after the Younger Dryas did not come from the south.

It came from the west.

A glimpse of aspen

The scenario I’m proposing here is a radical departure from existing convention about the deep past of Britain and Ireland. If it’s true, there ought to be more evidence to corroborate it apart from some anomalous tree pollen. I will examine some of that evidence in future.

Before we leave this glimpse of a forest which may have lasted in the west over 8,000 years until its decline in the last 6,000 years, let’s thank the people who peer through microscopes at 50mm slices of mud or peat, identify and count & record what they see.

They do not see gorse or broom pollen. Gorse and broom make tubular flowers which the insect has to clamber inside to reach the nectar. Gorse and broom don’t waste their pollen on the wind.

They rarely see aspen pollen in Britain and Ireland. Our summers through most of the time since the ice left have not been warm enough.

Aspen stands spread through runners underground and share nutrient through a runner network. As individual trees die above ground the network lives on and sends up new trees. Aspen stands survive in the west in amazingly exposed barren situations, and provide another mystery.

The aspen stand below is on a ledge in a gully out of nibbling reach on the east coast of Lewis, north of Stornoway. There is no other tree for miles around apart from another aspen stand on another ledge a few hundred metres up the road. These aspen must have first arrived as seeds, and must have arrived in something like a forest. An aspen seedling would not survive in such a place now.

How long has this stand of aspen lived here, since a warmer climate era brought a seed to this place?

An aspen on the east coast of Lewis

An aspen on the east coast of Lewis

On a tiny islet in a loch on Lewis or Harris you may see a bonsai forest of native trees, in ungrazed heather honeysuckle sprouting, on an exposed ledge a solitary birch or rowan, on bare moorland a juniper splayed across a rock.

Pic 7 Pic 8

And once in a while in the west you may come upon a surviving scrap of temperate rainforest. It is an enchanting experience.

I believe such scenes were abundant in Ireland, the long-ago Western Isle, Shetland and Orkney over 10,000 years ago. In a future article I will explore what other life those places might have held at that time.

Pic 9 Pic 10


1. Route, speed and mode of Oak Postglacial colonisation across the British Isles: Integrating molecular ecology, palaeoecology and modeling approaches. 10 authors. Botanical Journal of Scotland, January 2005.

2. Holocene history of Environment, Vegetation and Human Settlement on Catta Ness, Lunnasting, Shetland. K.D.Bennett, S,Boreham, M.J.Sharp, V.R.Switsur. Journal of Ecology, June 1992.

3. Post-glaciation plant colonisation of Ireland: fresh insights from An Loch Mór, Inis Oírr, western Ireland. Michael O'Connell, Karen Molloy, National University of Ireland, Galway.

Irish Naturalists’ Journal, January 2014.

4. Competitive interactions among forest tree populations in Norfolk, England, during the last 10000 years. K. D. Bennett, 1986.

5. A multiproxy (micro-XRF, pollen, chironomid and stable isotope) lake sediment record for the Lateglacial to Holocene transition from Thomastown Bog, Ireland. Jonathan N. Turner, Naomi Holmes, Stephen R. Davis, Melanie J. Leng, Catherine Langdon and Robert G. Scaife. Sheffield Hallam University Research Archive (SHURA).

6. Late Quaternary Vegetation History of the Western Isles of Scotland. J. A. Fossitt. New Phytologist, January 1996.

7. Vegetation history of Orkney, Scotland; pollen records from two small basins in west Mainland. M. J. Bunting. New Phytologist, July 1994.

8. Pattern, style and timing of British–Irish Ice Sheet advance and retreat over the last 45 000 years: evidence from NW Scotland and the adjacent continental shelf. Tom Bradwell et al, 2021

9. A reconciled solution of Meltwater Pulse 1A sources using sea-level fingerprinting. Yucheng Lin et al, 2021.

10. A Lateglacial pollen record from Hallsenna Moor, near Seascale, Cumbria, NW England, with evidence for arid conditions during the Loch Lomond (Younger Dryas) Stadial and early Holocene. M. J. C. Walker. Proceedings of the Yorkshire Geological Society, 2004.

11. Radiocarbon-dated Holocene pollen and ostracod sequences from barrage tufa-dammed fluvial systems in the White Peak, Derbyshire, UK. David M. Taylor, Huw I. Griffith, H. Martyn Pedley and Iain Prince. The Holocene, 1994.

About the author

Steve Webster

Steve Webster is a crofter and a gardener on the west coast of Harris.