Ancient cataclysmic floods -- erratics from Lake Missoula drainage events

 

A nice little gallery of erratic pics from YouTube videos on the topic of the Missoula floods.  More info here:

https://www.nps.gov/iafl/index.htm

https://volcanoes.usgs.gov/observatories/cvo/Historical/LewisClark/Maps/map_missoula_floods.shtml

https://en.wikipedia.org/wiki/Missoula_floods

The floods (maybe as many as 100 flood "events" during the Quaternary) were all cataclysmic in the best sense of the word, but naturally we know most about the latest series of meltwater outbursts at thge end of the last (Wisconsin) glacial episode.    Many of the giant erratics are well over 100 km from their source areas.  Many were incorporated in glacier ice and swept away in icebergs following the collapse of each successive ice dam -- these have many of the diagnostic characteristics of typical glacial erratics.  They are often sub-rounded or sub-angular.  But others are angular or sharp-edged, suggesting they they were literally ripped away from the bedrock by the force of the water, armed as it was with abundant fragments of glacier ice.

More on the West Kennet granidiorite mystery

Some of the coherent cobbles found at West Kennet.  They are interpreted as "corestones" or solid rock remnants which have survived while the outer parts of a boulder (or several) are weathered away.

Cunyan Crags -- a possible source area

Here is a new article on the mystery of the strange rotten granidiorite bits and pieces found at West Kennet. The article is not out yet in printed form -- it is dated as 2025.  It's a detailed paper, and although the authors (as ever) tend towards an explanation of the boulder as "yet another possible example of Late Neolithic long distance prehistoric transport" they admit to being puzzled.  Are they correct in indicating that the 77 bits have possibly all come from one boulder derived from the Cunyan Crags area of Cheviot, in Northumberland?  Time will tell.........

Of course, boulders such as this one could have been picked up by overriding ice and carried southwards -- rather like the Whin Sill erratics described by Olwen Williams-Thorpe and others many years ago. But why on earth would anybody want to collect the boulder (if it was just one) from a till exposure or from a glaciated ground surface and cart it off to West Kennet?

https://brian-mountainman.blogspot.com/2021/11/breaking-news-another-igneous-erratic.html

https://brian-mountainman.blogspot.com/2012/01/whin-sill-erratic-cluster-in-stevenage.html

https://brian-mountainman.blogspot.com/2012/04/chunk-of-whin-sill-in-london.html

"Geochemical provenancing of igneous glacial erratics from Southern Britain, and implications for prehistoric stone implement distributions" by Olwen Williams-Thorpe, Don Aldiss, Ian J. Rigby, Richard S. Thorpe, 22 FEB 1999, Geoarchaeology, Volume 14, Issue 3, pages 209–246, March 1999

http://onlinelibrary.wiley.com/doi/10.1002/%28SICI%291520-6548%28199903%2914:3%3C209::AID-GEA1%3E3.0.CO;2-7/abstract

Put this down as one of the mysterious boulders of Southern England and the Channel coasts..........

==================================

Rob Ixer, Richard Bevins, Nick Pearce, Duncan Pirrie, Josh Pollard, Alex Finlay, Matthew Power and Ian Patience. 2025 "Exotic granodiorite lithics from Structure 5 at West Kennet, Avebury World Heritage Site, Wiltshire, UK." Wiltshire Archaeological & Natural History Magazine, vol. 118 (2025), pp. 1–18




ABSTRACT


Seventy-seven pieces of very weathered pyroxene-bearing granodiorite corestone excavated from trenches 2, 3 and 9 within Structure 5 of West Kennet in 2019 and 2021 and varying from small pebbles to >500grms cobbles, have a total weight of 22kg. Detailed petrographical and geochemical analyses of typical samples show them to share an unusual (for Britain) and distinctive mineralogy and petrography and also suggest they are all from a single outcrop/subcrop. The essentially unaltered pyroxene-bearing granodiorite carries ‘large’ skeletal zircon crystals, which are a determinative characteristic. Petrological comparisons with similar British granodiorites show that its origin is to be found within the large, 60km and lithologically highly diverse Cheviot Igneous Complex of Northumberland, more than 450km from West Kennet. Three Cheviot samples were selected for comparative analysis, one chosen for its petrographic similarity to the corestones, as suggested by previous workers, a second, close to the first and also to significant Neolithic activity at Threestoneburn Stone Circle, and finally a third based on petrography and notable topography, namely Cunyan Crags.  Only the last sample shares a sufficient number of similarities that there warrants further investigation in that area. The corestones are highly exotic with regard to their find spot and although it is difficult to conceive of any practical use for them, West Kennet provides yet another possible example of Late Neolithic long distance prehistoric transport, a distance of between 450km if taken from outcrop and 150km if collected from secondary glacial drift sources, although North Sea coastal glacial tills as a source for the stones appears unlikely and from East Anglia very unlikely. The original Cheviot Hill location remains unidentified but is being actively sought.

The Courtmacsherry "glacial" raised beach

  I have been looking again at the evidence for the Courtmacsherry raised beach, and am still very puzzled.  The dating of it is hugely dependent upon the reliability of OSL dating, and I'm a bit worried about some of the other grounds cited for it being formed at a time of high RSL in MIS 3 or 4.  I have nagging doubts, and remain to be convinced that it is not Ipswichian in age, like most of the other raised beaches in the SW of the British Isles.

I have been reading the excellent article by Colm O'Cofaigh et al (2012), and have concerns about the following:

Extracts from p 175

Undoubtedly there is extensive evidence for the beach along the
south coast of Ireland forming in a cold-climate environment (see
above), and indeed some of the evidence which underpins this
interpretation was first noted by Wright and Muff (1904). This
includes the presence of erratics within the raised beach gravels
and interbedding of the beach gravels with periglacial slope
breccias. Previous investigations of the sand facies that overlie the
raised beach at sites such as Howe's Strand and Broadstrand have
interpreted it as ‘blown sand’ (e.g. Synge, 1978). This is inconsistent
with the sedimentology of these sands which exhibit well defined
hummocky and swaley cross-stratification consistent with
a shallow marine rather than aeolian setting. Thus the facies
sequence of raised beach/HCS sands/SCS sands indicates
submergence following beach formation. Periglacial slope deposits
and isolated angular clasts of local bedrock within these sands
indicate the maintenance of a ‘cold’ depositional environment
during this submergence.

We suggest that high
relative sea level and the submergence indicated by the facies
sequence of raised beach deposits overlain by shallow marine sands
are related to glacioisostatic depression during ice sheet build-up on
land combined with still high eustatic sea level prior to the LGM. The
interpretation of high RSL during ice sheet build-up is consistent with
observations of erratics in the beach gravels (proposed byWright and
Muff to record deposition from floating ice) and the presence of
periglacial slope breccias interbedded with the beach and overlying
shallow marine sands.

Colm Ó Cofaigh, Matt W. Telfer, Richard M. Bailey, David J.A. Evans, 2012
Late Pleistocene chronostratigraphy and ice sheet limits, southern Ireland
Quaternary Science Reviews 44 (2012) 160-179.

First, the evidence of erratics within the raised beach gravels is NOT a ground for claiming that the beach is a cold-climate feature. Second, how do the authors know that the slope breccia is a genuine cold-climate deposit rather than one associated with rockfalls such as one sees all the time on a temperate coastline subject to wave attack?  Third, the transition from shoreline deposits to shallow-water marine deposits suggests a transgression or rising sea level, but that does not of itself prove glacio-isostasy or the advance of a glacier from the north or from anywhere else.  Raised beaches may be formed at a time of falling sea-level, or rising sea level, or indeed oscillations over a few metres over thousands of years.

The raised beach deposits are apparently devoid of organic materials -- which is a pity!  A shell fauna such as that seen in the Gower raised beaches would be rather valuable in sorting this matter out........

I'm struck by the similarity between the southern Irish sites and West Angle, at the mouth of Milford Haven.  Here there are interglacial sands, silts and clays which have been overridden and partly destroyed by advancing ice which left behind a characteristic Irish Sea till.  I have dealt with West Angle in many posts on this blog (use the search box), and I do not see any features there that might support the idea of an Early or Mid Devensian glaciation or a high RSL on the Pembrokeshire coast coinciding with a global sea-level at about - 80m. This of course implies isostatic depression of the coastline by around 80m, which in turn implies an ice load imposed by a glacier c 300 - 400m thick.  That's a big ask for the Early or Mid Devensian........

The surveyed exposure at West Angle, showing a sequence of supposed interglacial marine and estuarine deposits overlain by till.

Quaternary events on the North Devon coast

Prof Nick Stephens's map of ket Quaternary features in the South West.  Many of the features on this map are now known to be incorrect, but note Lake Maw -- centred on the Somerset Levels, withy an overflow to the south

Been looking again at this major contribution:

Bennett, J. A., Cullingford, R. A., Gibbard, P. L., Hughes, P. D., & Murton, J. B. (2024). The Quaternary Geology of Devon. Proceedings of the Ussher Society, 15, 84-130. 

https://ussher.org.uk/wp- content/uploads/benettetal1584130v2.pdf

I think we are getting pretty close to a consensus here:

Quote:

Glaciation appears to have modified the landscape south of the Bristol Channel relatively little, although this is not surprising given that the ice from both the Irish Sea and South Wales did not penetrate far beyond the present north Devon and Somerset coast. Presumably the relatively easily deformed marine deposits flooring the Bristol Channel allowed the ice to flow along or across the valley until reaching the resistant rocks on today’s northern coast of Devon. Here, river valleys parallel to the coast and also truncated may in fact be ice- marginal landforms (Rolfe, 2015; Gibbard, et al. 2017). The BRITICE project (Clark et al., 2020) takes a broad view of the glacial limits in Britain but suggests that the pace of ice retreat from the Celtic Sea may have varied according to bed topography. In addition, Scourse et al. (2021) propose short- lived events of ice advance down the Celtic Sea that may have affected the north Devon coast and the further possibility that the ice margin had advanced as far as the shelf break at 25.5 ka.

Lake Maw is still a puzzle!  In my correspondence with Philip H and Philip G we have pondered on Lake Maw.  Use the search box to look at earlier posts.  If ice really did cross the Bristol Channel and hit up against the clifflines of N Devon and N Cornwall, in the Late Wolstonian or Late Devensian (and maybe at other times too!) then an ice barrier must have blocked the Bristol Channel and impounded a considerable meltwater lake in the Severn Estuary  -- referred to by the older workers (including Frank Mitchell and Nick Stephens) as Lake Maw.  But in that case, where are the lacustrine clays that might be expected in this lake?  

A similar lake -- Glacial Lake Teifi -- filled a broad depression in the Teifi Valley when Irish Sea ice moved in from Cardigan Bay from the W and NW.  No shorelines have been identified (which means that its surface level was anything but stable) but there are abundant laminated lake clays in many locations, as described by Fletcher and Siddle (1998) and Etienne et al (2006). At the time of its greatest extent it was more than 10 km wide and 40 km long.

doi:10.1016/j.quascirev.2005.03.019

The interesting thing about this lake is that it formed during the ADVANCE of the ice from the Cardigan bay, rather than during its dissolution.  It appears that the ice advanced over the lake deposits, creating complex structures and depositing Irish Sea till and glaciofluvial sediments as it did so,

The full sequence of events is not yet elucidated......

High glacial sea levels?

Newgale storm beach -- the most prominent feature of its type in Wales.  Erratics galore.....

 In several journal articles recently I have encountered variations on this statement:  "The raised beach contains erratic pebbles, demonstrating that at the time of its formation glacier ice was active in the vicinity."   How weird is that? 

Let's just look at the present-day storm beaches and quiet water beaches of Pembrokeshire:  Poppit, Ceibwr, Parrog, Newport, Abermawr, Druidston, St Brides, Manorbier, Dale, West Angle,  and so on and so on.  Hundreds of beaches.  They ALL contain erratic cobbles.  The percentages are not very high -- maybe up to 10% in places.  But I defy anybody to name a beach in Pembrokeshire that does NOT contain far-travelled erratic materials. 

The explanation is simple.  Beaches are generally built of clasts derived from adjacent bedrock outcrops and exposures of unconsolidated sediments.  Those sediments include till, glaciofluvial sands and gravels, slope breccia and silts and clays derived from past glacial and periglacial episodes.  They are ubiquitous -- found on all of the coasts regardless or orientation of configuration.  Because they are unconsolidated, they are easily destroyed by wave action across quite a wide tidal range, and incorporated into beach materials.  Because of the Holocene sea level rise, and because the sea has been advancing at the expense of the land, there is an "accumulator effect" seen at its most spectacular in Newgale, where the storm ridge has marched eastwards for many thousands of years, incorporating glacial and glaciofluvial deposits from locations now several miles offshore.

So let's move to the raised beaches. There are plenty of them -- for example at Poppit, Ogof Golchfa, Abermawr, Broad Haven South, Ragwen Point, and Ballum's Bay.  

Cemented raised beach at Ragwen Point

I don't think I can recall a single raised beach in West Wales which is free of erratic material, and TN George, DQ Bowen and many others have made exactly the same point about the raised beaches of Gower.  There is no reason to think that the processes operating during the Last Interglacial were any different from those of the present interglacial, even though the climate may have been slightly warmer and the sea level might have been slightly higher.  There were clearly abundant glacial and water-lain deposits lying around to provide at least some of the materials required for beach building.

So why should it be assumed -- anywhere -- that because there are erratics in a raised beach there must have been an active glacier ice edge not very far away?

Coastal erratics -- St Brides Haven, Saunton and Whitesands

 

Dolerite erratics resting on a broken rock platform of old red Sandstone at St Brides Haven in Pembrokeshire. They are washed by high tides. There is no stratigraphic context, but they were most likely transported into this location by the ice of the Irish Sea Ice Stream in the late Devensian glaciation.

The Baggy Point "giant erratic" and the pink granite erratic at Saunton.  These are sealed by sandrock at the base of the local Pleistocene stratigraphic succession -- and this means they were probably in place on the rock platform before the Ipswichian interglacial.  Like the St Brides erratics, these big boulders are probably washed out from ancient glacial deposits, just like the rounded "boulder bed" boulders seen at Whitesands in Pembrokeshire.

Erratic boulders resting on the rock platform at Whitesands.  Here the raised beach is not visible, and the boulders are sealed in slope breccia assumed to date from the Early or Mid Devensian.

The MIS 4 (Early Devensian) glaciation

 

New Quay, Ceredigion.  Some rather interesting exposures....

It's interesting to see the debate about how the MIS 4 (Early Devensian) might have affected the western parts of Britain.  If you put "Early Devensian" into the search box on this blog, you will see that over the years I have revisited this issue over and again, seeking to explain details of the local Quaternary stratigraphy with reference to modelling studies and also field evidence from the south coast of Ireland, north Wales and elsewhere.

I have also highlighted this issue in some recent posts, arguing that it is a mistake to pull in evidence and suppositions about the "giant erratics" of the Bristol Channel and to pretend that they are related to MIS 4 glacial events.  I now think that these boulders are simply the last remnants of ancient glacial deposits, visible by chance on "washed" rock platforms near present sea level and also found at higher altitudes (60m, 80 m and even higher) above the modern cliffline.

But the evidence from southern Ireland (Courtmacsherry and Fethard, for example) is important and persuasive, and we have to accommodate it in whatever narrative we develop.

At the moment, in West Wales, we have not much to go on,  apart from evidence of two (at least) glaciations -- one before the Ipswichian Interglacial and the other later, in the Late Devensian.  Key evidence of a pre-Ipswichian glaciation comes from Black Mixen, near Lydstep, and Ceibwr on the north Pembrokeshire coast.  Look them up using the search box.

The only evidence of two recent (Devensian) tills separated by evidence of a melting interlude comes from New Quay -- and I am sure that one of the tills is the Irish Sea Till and the other is a till derived from the Welsh Ice Cap.  Could one of the tills date from MIS 4 and the other from MIS 2?  I'm not sure about that, since that would require a "non-glacial interval" of around 40,000 years.

Many researchers in the New Quay - Llanon area have been confused by the sequence of deposits. At New Quay I recorded a lower Irish Sea till and a local till above it -- the two separated by evidence of a warmer non-glacial interval.  Others, including Eddie Watson, recorded at Llanon a "basal" Welsh till overlain by Irish Sea till.  There is much discussion in the literature about a "Llansantffraid soil" which might be interglacial or interstadial, and abundant cryoturbation and other features indicative of intense permafrost conditions. During his lifetime I disagreed on many things with Prof Dai Bowen, but I do agree with him that there seems to have been an oscillating ice front on the Welsh coast in the New Quay - Llanon area during the Devensian........ but which till deposit (if any) dates to MIS 4?  And can it be correlated with the MIS 4 glacial deposits of Southern Ireland?

My interpretation (1965) of the stratigraphic sequence on the south 

coast of Ceredigion.

Quote from one of my earlier posts:

It's interesting that Etienne et al (2005) refer to another exposure at Llannon where a shelly Irish Sea till overlies "subglacial tills of Welsh provenance". This site is also described in the "Quaternary of Wales" volume edited by Campbell and Bowen, on p 65. They refer to a Welsh till overlain by an Irish Sea till, and argue for a complex interaction between Welsh Ice and Irish Sea ice along the coast of Cardigan Bay. They do not quite resolve the issue of the intense periglacial disturbance of the surface of the Welsh till, which seems to have taken place prior to the laying down of the Irish Sea till. Was there a prolonged periglacial episode with deep permafrost between the two glacial phases?

I am also very sceptical about the raised beaches of West Wales being interpreted as "cold climate" features just because they have erratics in them.  As far as I can see, all of the raised beaches contain erratics of various sizes, and that is not surprising since they are formed from whatever deposits happen to be available for wave attack and the removal of fines.  Raised beaches are essentially "lag deposits" reworked by the action of tides and waves.  Besides, faunal and microfossil evidence suggests that the bulk of raised beaches are genuine representatives of warm or interglacial climates.

This is all rather interesting.  I am looking at my West Wales stratigraphy to see whether there are sites that will be susceptible to "alternative" explanations as to the chronology and the severity of the glacial / periglacial climate.  One problem is that most of the sections I have examined over the years are in coastal valley locations, where a thick accumulation of pseudo-stratified slope breccia does not necessarily indicate a long period of climate dominated by permafrost and freeze-thaw processes. Rockfalls and scree development continues in such locations even when the climate is relatively mild and one has to be very careful about "climatic indicators".....

Much thinking going on........

==========

PS.  Let's not forget that there has been a considerable debate in Eastern England about a pre-LGM glaciation (assumed to be from the Early Devensian) that was more extensive than the LGM.   Our late colleague Allan Straw was instrumental in proposing this to other specialists:

http://www.emgs.org.uk/Mercian/Mercian%20-%20Individual%20papers/Mercian%20Geologist%20volume%2019%202016-2019/Mercian%202019%20v19%20p207%20MIS%204%20glaciation%20in%20eastern%20England,%20Straw.pdf

See also Prof Peter Worsley:

Worsley, P., 1991. Possible early Devensian glacial deposits in the British Isles. 47-51 in Ehlers, J., Gibbard, P.L., & Rose, J. (eds.), Glacial Deposits in Great Britain and Ireland. Balkema: Rotterdam.