Knock and lochan terrain -- the Teifi Pools again

In a post the other day about the Teifi Pools area, I showed some shots of the landscape and described it as typical of areas of areal scouring close to -- or beneath -- the ice shed of either an ice sheet or an ice cap.  In the case of the Teifi Pools the ice shed during the Devensisn glaciation -- and probably others as well -- migrated back and forth over a distance of maybe 20 miles, but the map below (by7 Henry Patton and others) shows its approximate position:

The white-coloured "spine" down the centre of the ice cap shows the highest part of the ice dome, possibly the area of lowest basal ice temperatures (cold or polar ice rather than temperate ice) and the area of most sluggish ice movement and hence lowest erosional rates.  However, because conditions were never static, and as the ice shed moved to east or west, there WAS some erosion, entrainment and debris removal, albeit on a less organized scale than in the flanking areas where streaming was increasing and where eventually the ice discharge was through distinct glacially eroded channels.   We are talking about the area between Tragaron and Rhayader.

The landscape in this ice-shed area is often referred to as "Knock and Lochan" topography, after Prof David Linton who studied certain glaciated landscapes in the western parts of Scotland.

There's a good description here:

http://www.landforms.eu/shetland/Knock%20and%20lochan.htm

Let's just look at the landscape again:

On the lower photo the Teifi Pools are top left.  You can see that there is quite a strong structural control here, which makes me rather reluctant to refer to this as a "glacially streamlined" landscape.  But on the other hand it is clearly scoured, with the eroded rocky ridges standing proud, and the eroded softer rock depressions filled -- in some cases -- by lakes.

When he first used the term "knock and lochan terrain" I don't think Linton had any idea of just how widespread it is across the surface of Planet Earth.  Many millions of hectares are to be seen across all landscapes affected in the past by ice sheets -- in Greenland, North America, Arctic Russia, Scandinavia, Antarctica -- and in many other areas such as Iceland, the highlands of Asia, and parts of South America where smaller ice caps have been based on plateau surfaces.  Here are a few examples(note that the scales vary):

Assynt, Northern Scotland

Isle of Lewis, Scotland

Northern Baffin Island

Kangerlussuaq, west Greenland

Glama Plateau, NW Iceland

Hardangervidda, Norway

Kealkill megalithic complex, Ireland

Ah -- what a fabulous image.  It was featured on Austin Kinsley's Facebook page -- worth sharing.  It shows the Kealkill megalithic complex near Bantry Bay, in Ireland.

Stonehenge 1944

Fabulous image from Austin Kinsley's Facebook page.  Many thanks. It's a highly unusual and fascinating photo taken in 1944 as a trial, when various techniques of aerial photography were being tried out.  Apparently when this was taken the strobe lighting came from an aircraft flying overhead at altitude of 1500 feet.  Very moody......

Stonehenge - more on the Devonian (?) sandstones

The petrographic differences between the four studies samples.  Note that the old "type section 277" has just been examined visually, and that the other three -- including the new type section -- have been subjected to QEMSCAN analysis.  That may have resulted in analytical error -- but the authors are confident that the differences are significant and that they will withstand scrutiny.  

I have been looking again at the 2019 paper by Ixer et al on the carbonate-cemented micaceous sandstone samples collected over the years at Stonehenge:

Alternative Altar Stones? Carbonate-cemented micaceous sandstones from the Stonehenge Landscape
by Rob Ixer, Richard Bevins, Peter Turner, Matthew Power and Duncan Pirrie
Wiltshire Archaeological & Natural History Magazine, vol. 112 (2019), pp. 1–13

https://www.academia.edu/37882770/Alternative_Altar_Stones_Carbonate-cemented_micaceous_sandstones_from_the_Stonehenge_Landscape


ABSTRACT
The six-tonne recumbent Altar Stone is perhaps the most enigmatic of all the Stonehenge bluestones, differing markedly from the others in size, tonnage, lithology and origin. It has therefore had more than its fair share of speculation on all of these aspects and many questions remain: was it always recumbent, was it a singleton or half a twin, where did it come from? Clearly it is not from the Preseli Hills hence the debate as to its geographical origins for over a century. However, any provenancing of the Altar Stone must rely on a detailed and accurate lithological and petrographical description. New descriptions of material labelled ‘Altar Stone’ held in museum collections and a re-evaluation of suggested Altar Stone debitage using automated scanning electron microscopy and linked energy dispersive analysis using QEMSCAN technology suggests that modification of the published petrographical descriptions is needed. A new ‘typical Altar Stone’ description is provided including the presence of early cementing barite and a better characterisation of the clay content. These new data should continue to narrow the search for the geographical origin of the Altar Stone, one that is expected to be at the eastern end of the Senni Formation outcrop, an outcrop that reaches as far east as Abergavenny in the Welsh Marches.

The real significance of this paper is that having studies assorted thin sections (only 4 were suitable for comparative analysis) the authors now feel that the famous thin section 277 may NOT have come from the Altar Stone at all, and that thin section SH18-196 should now be taken as the "type section" for it.  The reason is that the petrography of sample 277 is out on a limb, with various striking differences with samples FN593, HM13-3y/1 and SH18-196.  Those three have close similarities -- close enough in the view of the authors to have come from the same stone.  

What this means is that the carbonate-cemented micaceous sandstones have come from at least two different locations -- probably within the Devonian outcrops of south and west Wales.

There are a further 8 samples of similar sandstones which have not been analysed in this study, and no matter what the geologists may say, there is always a possibility that at least some of these have come from additional sites.  

Note that these are all different from the Lower Palaeozoic sandstones which have just been studied for this paper:

‘No provenance is better than wrong provenance’: Milford Haven and the Stonehenge sandstones,  by Rob A. Ixer, Richard E. Bevins, Duncan Pirrie, Peter Turner and Matthew Power.
Wiltshire Archaeological and Natural History Magazine 113, 20220, pp 1-15.

https://www.academia.edu/41105834/Mill_Bay_Milford_Haven_and_Stonehenge

https://brian-mountainman.blogspot.com/2019/11/new-paper-on-stonehenge-sandstones.html

See also:

https://www.academia.edu/32048879/LOWER_PALAEOZOIC_SANDSTONE_DEBITAGE_FROM_THE_STONEHENGE_LANDSCAPE_The_petrography_geological_age_and_distribution_of_the_Lower_Palaeozoic_Sandstone_debitage_from_the_Stonehenge_Landscape

Wind direction and landscape change

This is a Bing satellite image and accompanying map to the same scale -- from the wonderful "side by side" web site created by the National Library of Scotland.

This is part of the main upland ride in the Brecon Beacons -- click to enlarge.

You can see here what a profound effect orientation and wind direction have on landscape change during the ice age.  Notice that the main cwms or cirques are oriented broadly NE.  The group of cirques and small glacial troughs in the NE quadrant of the photo were formed because this was the lee side of a ridge, where snow could accumulate and get converted into firn and then glacier ice, during the cold phases of the Ice Age when prevailing winds came from the W and SW.   The long escarpment running from top left to bottom right has been fashioned in a similar way, with a number of small cirques coalescing on the downwind flank of the upland ridge. 

In contrast, look at the valleys in the SW quadrant of the image -- these are "normal" river valleys with no trace of cirque formation.  In the Ice Age most drifting snow was blown up over the ridge to accumulate on the lee (shady) side, leaving the sunny flank largely unaffected. 

Here is another example, showing the Beacons landscape some way to the west, around the famous lakes of Llynyfan Fach and Llynyfan Fawr.   Here the coalescing cirques have a slightly different alignment -- there is a scarp face oriented NNW, but the cwm occupied by Llynyfan Fach is oriented NE, like most of the other cwms in the Beacons.  The scarp alignment may have something to do with the pre-glacial topography.  But note the long river valleys to the south, apparently unaffected by ice action.

  

The "upland glaciation" of this mountainous region must have occurred intermittently right through the Ice Age.  At other times the mountains were completely ice-free, and at the peak of each glacial episode the whole landscape hereabouts was deeply submerged by the ice of the Welsh Ice Cap.

Mega Megaliths

There seems to be a growing interest at the moment in the biggest standing stones, where they are located and what they are made from.  The one above is named Llwyn-y-Fedwen,  just across the River Usk from the village of Llangynidr; it's a huge piece of sandstone, very battered and weathered, presumably found locally as a glacial erratic.  The hammer measures 30 cm.

Above are two photos of a stone named Maen Dic which is about 500 yards south-west of the village of Battle (about 2.5 miles north-west of Brecon).  Made of Old Red Sandstone?

It seems that these large stones form a cluster in the Brecon, Ystradfellte, and Crickhowell area, the majority of which are very large.  Weight estimates c 5 - 8 tonnes?

Above is Maen Llia (OS grid reference SN924191).  Standing just 60m off the minor road between the Senni valley and Ystradfellte, this impressive stone is relatively easy to visit.  Made from a massive sandstone block which stands 3.7m high, the task of moving and erecting it must have been a huge challenge, especially as it is likely that a quarter to a third of the whole stone is below ground.

Cwrt-y-Gollen (The Growing Stone) in Powys -- grid red:  SO23241686.  This is a magnificent slender pillar c 4.e m high. 

Maen Bredwan, near Neath. Grid ref: SS724995.  It is also called Carreg Bica.   This one, in a very exposed position, is reputed to be 4.3m tall.  According to legend, this stone goes down to the Neath River for a drink once a year, on Easter Sunday morning.  So there we are then.

Maen Madoc standing stone, Powys.  OS grid reference SN918157.  At almost 2.7m in height, this imposing stone (pictured right) stands high on the moors alongside the Roman road, Sarn Helen.

The Latin inscription, DERVACUS FILIUS JUSTI IC JACIT translates as Dervacus, son of Justus, lies here. Dervacus was a sixth century Roman name.  Although widely recognised as a Roman memorial stone, it was probably erected in Bronze Age times, and just came in handy.......... 

Parc y Meirw, near Llanychaer, North Pembrokeshire -- the biggest of the remaining standing stones, c 3m high.  This is part of a famous stone row, now incorporated into a hedge.  The field is named "the field of the dead" because a famous battle was fought here some time in the early Middle Ages.  There is also a "Ladi Wen" -- a white lady -- haunting the field.  

 

Rotational slump at Red Cliff, Marloes

Can't resist sharing this, since I know that many of our followers like a bit of geomorphology now and again. 

 Here is the coastguard photo of the rotational slump at Red Cliff, Marloes, on the west coast of Pembrokeshire --SM 788071 to SM 790069. A classic of its kind!  This illustrates how coastlines evolve -- with slow and incremental change maybe for centuries or millennia, and every now and then a catastrophic and instantaneous change of vast proportions, triggered by over-sturation of sediments on a slope, or an earth tremor, or an extreme storm event.

The landslide occurred around 21st November. Luckily nobody was hurt.  Must get over there to have a look........