Pot-Holes
Glacial Prehistory
Pot-Holes
Pot-holes, not to be confused with pot holes on the public highway.......solving
(This is the only close approximation for a name for these particular features that I have found. A novel name might be beneficial.)
One of the last companies I worked for in Archaeology was Cotswold Archaeology who, in 2008/9 were contracted to run the excavation of sites that were found in the path of a National Grid Liquified Natural Gas pipeline running across the Gloucestershire countryside.
The planners of the route of the pipeline had managed to avoid any major archaeological sites leaving some fairly dog-eared ruins for the team that had been assembled for the work, to excavate and record.
There was a rambling spread of roman features, pits, kilns and the like in one location, and nearby there were a couple of large round features, one of which I was set to excavate and record
This was a circular feature of five metres diameter, and I had enough experience to know that it had not been constructed by the hand of man. This was a natural feature, and I had no idea how it might have happened.
As I dug it though I was aware that there were mysterious anomaly features elsewhere that might be capped shafts of which the capping material might, if i were unfortunate, break through allowing me to drop down into an underground caves.
With that in mind I dug more carefully than I might otherwise have done.
My excavation was to suggest that there was no evidence that the feature was man-made, and I dug a one metre wide, one metre deep slot down the middle of the circle. Safety concerns mean that excavation deeper than a metre deep need to have their edges shored up to prevent collapse, so this excavation, clearly a " natural" feature was limited to just the one metre depth.
The soil in the pit was unremarkable, and easy enough to excavate, but the same cannot be said for the second pit on this stretch of pipeline, which was further uphill, and twice the size.
This was also circular, and was huge at ten metres diameter. Only limited excavation of this feature took place largely because the contents were overconsolidated.
Overconsolidated soils are those that have been overlaid by extreme weights, such that all moisture and air has been expressed from the spaces between the grains of rock, of which they are formed . The soil thus formed is very nearly as hard and compacted as any solid rock. Excavation of this material is challenging, and can cause extreme injury to the hapless digger!
It is difficult not to ascribe the cause of this compression of material in the middle of a Gloucestershire field to the deposition of a thick ice sheet on its surface.
Fortunately I managed to avoid most of the hard grind of that task, but the hardness of the ground certainly relieved me of the idea that I might fall through the floor of one of these things.
My curiosity led me to find documentary evidence of backfilled holes like this further east into England, overconsolidated or unconsolidated. These are common across Wiltshire and Salisbury Plain. The names they are given vary, Swallow Holes, Sink Holes, Dolines, to name a few.
Only when we get to Bedfordshire do we get a site where features like this are fully excavated.
There, a group of over 40 Mesolithic pits were discovered in Linmere, Houghton Regis.
The pits were from 2.1m to 5m in diameter and from 1.0 to 1.7metres deep. All had steep sides, some flared out at the bottom into a wider base, and with a mix of concave and flat bases. Six pits produced animal bone and two produced struck flint. The majority of the animal bone assemblage derived from just two pits and was dominated by aurochs. Other identified species comprised red deer, roe deer and pig. Four pits yielded five fragments of animal bone that were suitable for radiocarbon dating and all returned late Mesolithic dates between 8,500BP and 7,500BP.
The pits found by Albion and MOLA were spread across an 8ha area and were clustered around the three palaeochannels.
They "appeared to lead down to the Ouzel Brook, suggesting that they may have been created by seasonal springs or run-off from the higher ground to the south. An association between the palaeochannels and the large pits seem highly likely as, despite extensive investigations in the area, no such pits have been found away from this part of the Ouzel Brook."
The full teport for the site is:- Pit-trap Hunting, Place-Making and Cosmic Reciprocity in Mesolithic Britain: the Excavation of a Substantial Pit Cluster at Linmere, Houghton Regis, Bedfordshire. By Joshua Pollard, Mike Luke, Yvonne Wolframm-Murray and Bryony Cairncross
Drawn sections for some of the pits are presented below.
I have called this blog Pot-Holes because it is likely that the majority of these features across England are, what i understand to be called pot-Holes. Not pot holes in the public highway caused by frost and repeated road traffic, but an erosion feature found in some river beds.
They are formed when sediment and other material carried by a river scour the bed. Where depressions exist in the channel floor turbulent flow can cause pebbles to spin around and erode hollows through abrasion/corraision. As the holes get bigger even bigger debris can become trapped in the pothole and this material further supports erosion.
From my analysis of melting ice sheets over Britain I calculate that in two melt periods, 15,000BP and 11,000BP sufficient meltwater reached the seas around Britain to raise sea levels locally for ,perhaps, a hundred years.
My calculations, which are bound to be guesswork, suggest that the amounts of water draining across England during these periods of deglaciation would have averaged a cubic kilometre of water every day. More in summer, less in winter.
These palaeovalleys on the Cotswold Hills Salisbury Plain and Bedfordshire would have been repeatedly flushed through with metre deep, sludge filled, torrents of meltwater from the Welsh mountains and the mountains of northern Britain.
In rivers pot-holes can be small but the amount of meltwater released by a melting ice sheet will have been huge, and larger erosive features, like those at Houghton Regis would have been common.
Those pot-holes on top of the Cotswold Hills will have been formed in two deglaciation periods with at least one glacial period between them. The huge size of the larger pit suggests that it was close to the source of a meltwater event, and the overconsolidated material in the pit suggests that it was overlaid by an ice sheet of significant thickness and weight. The smaller pit, not far away, down the palaeovalley from that had unconsolidated soil in it is likely to have been cut in the most recent period of deglaciation.
Comparing the size of pot-holes in current river beds with the that of those in ancient palaeovalleys is cause for some doubt about the theory of the formation of the features. Additional information about the two situations might help to explain this huge difference in size.
In modern river valleys the amount of water passing over these features is relatively quite small, and it is also relatively free of sediment. As it is the sediment, Gravel, and miscellaneous rocks in the water flow that cause erosion to the river bed the amount of erosion occurring is limited.
In the older palaeovalleys, now defunct, the amount of water flowing down the valley would have been huge, and the amount of sediments mixed up in it would have been equally huge.
At the speed and volume of this water flow the manner of its movement would have been violent and turbulent easily causing riverbed features most of which are lost due to present day land development and usage.
Jeffery Nicholls
South Ronaldsay
Orkney
Glacial Prehistory is a spinoff from my research into prehistoric Britain, the Orkney Riddle.
The Orkney Riddle (index here) is a series of blogs promoting the notion that Neolithic people walked from Scotland to Orkney to build the Ring of Brodgar.
Email jiffynorm@yahoo.co.uk
#Orkney #archaeology #Neolithic #prehistory #British #SkaraBrae #NessofBrodgar #BIIS ##Britice-Chrono #EarthSciences #LastGlacialMaximum #IceSheets #RelativeSeaLevel #Glaciers
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