The Limeslade Bay Erratic discovery

Upon a shore where waves in tempest churn,

A hulking mass of stone, unyielding, lay,

Its surface rough, by ocean's wrath in turn

Made stark and bare, where storms hold their sway.

Within its heart, a tumult seemed to bide,

A hollow drum where roars and rumbles deep

Did echo forth, as though history's tide

Had found a voice within its stony sleep.

And yet, for all the sound and fury pent

Within its depths, no meaning could be found,

No whispered lore, no message heaven-sent,

Just empty noise upon the wind-whipped ground.

So lies the stone, a monument to strife,

A hollow shell where echoes mock at life.

https://www.pembroke-today.co.uk/news/giant-glacial-erratic-hailed-as-missing-piece-of-bluestone-puzzle-505195

A giant bluestone erratic just discovered near Mumbles, on the south Gower coast, has been hailed as one of the most important glacial discoveries of the last century since it proves beyond doubt that the Irish Sea Glacier was capable of carrying large monoliths of dolerite rock from Pembrokeshire up the Bristol Channel towards Stonehenge.

Two years today after this wonderful discovery of a large erratic deposited on a rocky coast by an ice floe we still haven't been allowed sight of the analysis which may reveal where it was plucked from to be dropped in Limeslade Bay. We continue to await with interest. 

The Ups and Downs of Ice Flow

 I am told there is some confusion being spread about glaciers and ice sheets flowing uphill. Contrary to the impression some people have got they obey the simple law of gravity, they flow downhill.

But when they are constrained in a passageway either by mountains or other large amounts of ice of course the flow can rise up and over obstacles because of the pressure from the weight of the ice above. The total mass is flowing downhill even if parts of it are going up.

A simple experiment will show it how works, find a short flexible but reasonably stiff string of beads. Provided the are constrained in tube or valley they can easily be pushed over obstacles and will happily bump over ridges on their way downhill.

The key is that their lateral movement is controlled. Now consider the broad dry Bristol Channel during the last ice age, there are no mountains and, as it is at the edge of the ice sheet, no deep layers of ice to constrain any flowing ice. It will follow the downward slope. Try pushing your beads across an incline with a lip at the far edge. Down the slippery slope they will go, not deep into Somerset.

Recent Ice Dropped Boulders

Icebergs which may have dropped debris, including boulders, sailed to our latitudes in the last little ice age, it wasn't just Frost Fairs on the Thames. 

Iceberg at the western harbour head of Delfshaven on January 2, 1565 during the cold wave in the Little Ice Age - Cornelis Jacobsz van Culemborch 1565 - (The grounded iceberg was measured at more than 6 meters high and almost 70 meters long). https://museumrotterdam.nl/collectie/item/11113
 

So maybe some of those ice dropped boulders on our coasts haven't been there that long.  

As Bing imagines it.

Bristol Channel Boulders - No Ice Required

On 30 January 1607, around noon, the coasts of the Bristol Channel suffered from unexpectedly high floodings that broke the coastal defences in several places. Low-lying places in Devon, Somerset, Gloucestershire, and South Wales were flooded. The devastation was particularly severe on the Welsh side, extending from Laugharne in Carmarthenshire to above Chepstow in Monmouthshire. Cardiff was the most badly affected town, with the foundations of St Mary's Church destroyed. It is estimated that 2,000 or more people were drowned, houses and villages were swept away, an estimated 200 square miles (51,800 ha) of farmland inundated, and livestock destroyed,wrecking the local economy along the coasts of the Bristol Channel and Severn Estuary. https://en.wikipedia.org/wiki/1607_Bristol_Channel_floods

But there was another legacy of the flooding, thought to be a tsunami caused by an underwater earthquake, it moved boulders on the edges of the Bristol Channel.

Bryant, Edward A., and Simon K. Haslett. “Catastrophic Wave Erosion, Bristol Channel, United Kingdom: Impact of Tsunami?” The Journal of Geology 115, no. 3 (2007): 253–69. doi:10.1086/512750.

A very detailed paper which explains the positioning of many Bristol Channel boulders as tsunami relics. Calling them all glacial erratics may be simply wrong.

Glacial Gradients and Gravity

There is a suggestion that the glacial gradient from South Wales to Stonehenge was sufficient for ice to flow along it:

From https://brian-mountainman.blogspot.com/

Just a reminder that during the last ice age the Bristol Channel was above sea level and was a deep wide valley. Glaciers flow downhill so rather than the ice floating up the channel and landing on the Somerset coast from south Pembrokeshire it would have followed the gradient. 

Figure based on: "Location and bathymetry of the Bristol Channel. Contours are depths in metres relative to mean sea level" from "Impact of Tidal Energy Converter (TEC) Array Operation on Sediment Dynamics" - Scientific Figure on ResearchGate. Available from: https://www.researchgate.net/figure/Location-and-bathymetry-of-the-Bristol-Channel-Contours-are-depths-in-metres-relative-to_fig4_260052339

Unpredictable weather not unprecedented again.

 A year ago The National Trust warned of the dangers of the new norm of "tumultuous weather" - https://www.nationaltrust.org.uk/services/media/weather-and-wildlife-2022 This year the National Trust is: "sounding the alarm for UK wildlife as the loss of predictable weather patterns and traditional seasonal shifts causes chaos for nature." https://www.nationaltrust.org.uk/services/media/weather-and-wildlife-2023

As a worrier myself, and as this endangers ancient sites, I was intrigued enough to quickly look to at the historical records to see if the unpredictability was unprecedented. 

My approach was to graph the difference between one year's temperature, or rainfall, record and the previous year's. 

I used the UK's Met Office records. https://www.metoffice.gov.uk/hadobs/hadcet/data/download.html   and  https://www.metoffice.gov.uk/hadobs/hadukp/data/download.html 

So for instance 2022's mean temperature was 10 deg C and 2021's 10.3 so I record that as a drop of 0.3 for 2022. Obviously the larger the gains and drops the more "unpredictable" the weather is. 

Last year's post is here: https://www.sarsen.org/2022/12/is-this-years-tumultuous-weather-set-to.html and has links to data sources and results.

I have updated the data and there is no change in the pattern, the amount of unpredictability by my definition seems to be same. There doesn't seem to be any evidence that the amount of unpredictability is unprecedented or unusual so far. Against a background of rising temperatures this is good news and long may it continue.

If anyone is interested in the Data or more graphs please get in touch.

 

Debunking Pigs From Scotland

 Last year I did a round-up of the Isotope evidence for "neolithic pigs from Scotland"  - https://www.sarsen.org/2022/10/strontium-values-reappraisal-south-west.html . 

I missed another paper that came out a fortnight later:

Applying lead (Pb) isotopes to explore mobility in humans and animals - Evan et al

Dr Gordon Barclay kindly pointed this out to me and so I should update the record:

The paper is open access, so please read it. The key passage is:

Key to this study is our ability to distinguish between northern and southern British Pb sources and this can be achieved because of differences in the underlying geology between these two parts of Great Britain. The Pb isotope composition of rocks and minerals tends to be dominated by major geological tectonic events such as mountain building, which is accompanied by metamorphism and the intrusion of granites, the heat from which drives the re-mobilisation of Pb to create ore deposits.

The junction between these two tectonic plates is called the Iapetus Suture and it runs on a NE–SW line from Berwick-upon-Tweed to the Solway Firth and projects into Ireland.The underlying geology to the north and south of this suture is fundamentally different; the Laurentian basement, to the north, is geologically much older (> 3000Ma–c. 1750Ma) and is depleted in uranium (U) whereas the Avalonia basement in the south is geologically much younger (c. 700Ma), and this means the Pb isotope compositions, related to the basements of the two areas, are different. As this geological boundary essentially defines the modern political border of Scotland with England, it provides a potential method of discriminating between Scotland and the rest of Great Britain.

The conclusion is quite clear, in their opinion: 

"We have tested this application using a sample of Neolithic pig enamel from sites in southern England, some of which, because of Sr isotope composition, could not be excluded from an origin in northern Britain. Pb isotope data from the teeth excludes Scotland as a source but the diverse range of Pb isotope results, combined with other isotope proxies, are consistent with the animals being raised on a variety of lithologies of diverse age and from variable environments."

So is this end of the idea of neolithic links between Scotland and Stonehenge, I wouldn't bet on it.