These photos show the impact that permafrost conditions can have in a landscape of chalk substrate and chalky till.
Above: low level aerial photo of ice-wedge polygons near Colne, Hunts. These are formed on terrace gravels of the Great Ouse River. The gravels are supposedly of Devensian age, which means that the polygons themselves must be of late-glacial age -- maybe 13,000 - 10,000 years old.
Below: remnants of pingos on east Walton Common, Norfolk. The soil is on cryoturbated chalk. The complex pattern of hollows and ridges shows that there must have been many "segregated ice masses" here during a prolonged period of permafrost conditions.
Why do features like these not exist in the chalk downlands further west? Williams and others have suggested that the permafrost was maybe not so intense or prolonged as it was further east. So in Norfolk and Huntingdon, on low-lying land with very low gradients, there was very little summer thawing and a thin active layer, with ice wedges and segregated ice masses able to form and survive and grow. Further west, under greater influence from moisture-bearing westerly winds, maybe the temperature was a degree or two warmer, with less intense frost action. Also, because the altitude is greater (mostly over 100m on Salisbury Plain, as against land surfaces mostly under 30m in the east) the water table was deeper beneath the surface and maybe large areas were affected by "dry permafrost". Because of steeper slopes in and around the coombes, solifluction processes (leading to the accumulation of thick "head" deposits) were relatively more important.
11 comments:
Brian,
The 'explanation' put forth in your post about the marked differences in periglacial markings of the land surface between East and West England argues that the weather in the West may generally have been a few degrees warmer than that in the East.
Setting aside the question of whether that would account for the markings or lack of markings, is such temperature differential true now as well? How does the weather differ from East to West?
And how does such temperature differences concur with the Irish Sea Glacier and its encroachment in Salisbury Plain and even further south to Scilly Islands?
Can we have it both ways? Can we argue for glaciation of the southwestern regions of the UK and also for the milder temperatures there that would leave fewer permafrost markings?
The plot thickens!
Constantinos
It's difficult to generalise about climate patterns -- mean annual temperatures are not that different in Norfolk and Wiltshire, but daily and seasonal temperature ranges in Norfolk may be a bit greater -- as you would expect on the "continental" side of the UK as distinct from the "oceanic" side. There is also much greater weather instability on the west side -- and greater precipitation and wind speeds.
It's difficult to extrapolate back to what weather / climate patterns (and regional variations) might have been during the glacial episodes. That's one of the greatest challenges which the ice sheet modelling guys have to face. This is because ice sheets tend to "mop up" moisture coming from the west -- and as they grow, and get higher and bigger, they can create a distinct rain shadow effect on the lee side, increasing aridity and enhancing the effects of permafrost.
Another problem we have is that the permafrost features we see today in the eastern chalklands are apparently very young indeed. Some may be as young as 10,500 years old (Younger Dryas or Zone 3). They are already becoming more indistinct as the climate continues to warm and as "interglacial" warm conditions alter soil horozons etc. So what chance is there that permafrost features dating from the Anglian glacial episode (450,000 years ago) will have persisted anywhere in the UK? Very little indeed......
Re milder conditions and glaciation: no problem there. All of the big ice sheets have developed not in those parts of the planet that are necessarily COLDEST, but where there is a cold climate AND an abundant supply of precipitation in the form of snow. So there is no problem, from a glaciological point of view, with the British Ice Sheet being skewed or pushed over towards the west, with greater expanses of glacier-free land (and greater expanses of permafrost) in the east.
Bit of a glitsch there -- Blogger posted my same comment 4 times... sorry about that...
Brian,
I appreciate the wealth of information you are providing in your blog, as well as the thoughtful and informative analysis. All this, however, raises more questions and more need for fair-minded consideration. Hope you don't mind if I delve into some of the points you raise more deeply.
You write,
“ Another problem we have is that the permafrost features we see today in the eastern chalklands are apparently very young indeed. Some may be as young as 10,500 years old … “
So from what you say, we do have permafrost features and freezing conditions in the eastern chalklands around 8,500 BC. This is also my understanding. Can we assume the same general weather conditions existed in the western chalklands as they did in the eastern? I don't mean glaciers, etc. Just the sustained prolonged freezing temperatures associated with an Ice Age.
You ask,
“Why do features like these not exist in the chalk downlands further west? “
That is an important question and begs for an explanation. Certainly we can agree that something very different was true in the western chalklands than in the eastern. We have long sustained periods of freezing conditions throughout, yet only the chalklands in the east show many permafrost signs while the chalklands in the west show very few. The only such markings being reported are at The Avenue. And Robert tells us that excavations just 100 m from there at the Parking Area do not show such permafrost features.
As I recall from my visit there this summer, the Avenue is lower from the Parking Area. So if Robert wants to argue that the area was inundated with water, that would put The Avenue under while the Parking Area above water level. But that is just contrary to the permafrost markings, which require open and exposed land surface. Am I right?
In my view, what kept the western chalklands from forming permafrost features is that these areas were covered and protected. And I only know of two things that could have done that: ice and water. But if the conditions were freezing for long periods, water would also freeze. So that leaves only ice!
The western chalklands do not show the same permafrost markings because these were under ice. Since there is no evidence of glaciers in Salisbury Plain, the only explanation is that Salisbury Plain was a solidly frozen lake.
Can you think of any indisputable evidence that can dispute this?
Constantinos
The story we are getting from the chalklands is essentially the same as the story I picked up on in 1962-65, when I did my doctorate in West Wales. For the Devensian there was a prolonged period of permafrost / periglacial conditions, with some climate oscillations. Then there was a short episode of glaciation -- not intensive enough or long enough to erode away all the solifluxion material that had accumulated on slopes. Then the ice melted away. to be followed by a shorter period of heavy permafrost, with more solifluxion and some formation of frost cracks / ice wedges etc.
I couldn't work it out accurately at the time, but I assumed that this period affecting the landscape was the cold episode called Zone 1 and the other cold episode called Zone 3 combined. In all honesty I couldn't pick up the Allerod / Zone 2 warmer interval.
So western Britain did certainly have cold enough conditions for a while to lead to ice wedge formation etc -- ice wedges are taken to be an indicator of VERY cold permafrost conditions.
Why are there no signs of ice wedges and patterned groundon Salisbury Plain? I have already pondered oin this -- but maybe the signs are there and have not been picked up, simply becaiuse of a lack of research. (As you may have gathered, I'm not all that convinced by those "periglacial grooves in the Anenue.....)
I will go with the ideas that a prolonged period of "protection" by extensive snow patches during the period 12,000 - 10,500 yrs BP might have something to do with it. an insulating blanket of is a nice idea. But I won't go with the frozen lake idea. Lakes don't grow from the bottom up ---- you have to have a contained or dammed body of water that then freezes from the surface down. There are no landscape signs of such a convenient basin in the right place at the right time.
Happy Christmas to you!
Thanks for your response Brian. I do appreciate your thoughts.
From what you say, we are coming closer together on this. Our only difference (and I agree it is a big difference) is whether that 'insulating blanket' was a frozen lake or 'extensive snow patches'. I do agree with your attitude that lack of evidence may simply be lack of research …
It would help me if I knew that Salisbury Plain geomorphologically cannot be a 'contained basin'. From my travels to the southern coast to Portland Bill and along that coastline, the coast consists of high hills and is wooded by more mature forests. The topsoil near the southern coast also seems to be very different and deeper than that at Salisbury Plain. And from the photos you posted of the western coast, there is also a high hill barrier (which is constantly eroded away as you pointed out in your post) that separates the lowlands from the sea water.
Best wishes to you and your family for a Merry Christmas and a Happy New Year!
Constantinos
Thanks for the greeting, Kostas. Hope you and your family have had a happy time.
On the matter of Salisbury Plain, you just have to look at the map -- it's an area of relatively high land, surrounded by lower land. Not a good place for a frozen lake -- or even a liquid one.....
Some Pleistocene lakes have been suggested in the past, in some of the basins and valleys to the north and south of the chalk uplands, but the evidence is disputed.
Thanks for your explanation Brian. That is helpful.
Certainly elevation alone does not determine if a lake basin can exist. You can have lakes even at high mountain ranges. The relative morphology of the land itself determines if a lake is possible to form. Unfortunately, can't figure out such level contours from the maps I've seen. Perhaps also isostatic rebounce may have made the coast lower relative to the uplands (by some 200m as I believe you said at earlier posts). My sense is that the geological history of that area is probably very unique in all the world. For me reconstructing this history is the true mystery of Stonehenge.
I'm just keeping an open mind Brian. I am inclined to go where the evidence takes us. The problem always is that the evidence itself is open to interpretation. So it is different for different theories one considers.
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