In recent days I have been looking at some of the information on North Wales glacial deposits -- and have been reminded of the occurrence of shelly till and sands and gravels in a number of unexpected locations. The most famous is at Moel Tryfan, at an altitude of c 400m, where glacial materials derived from the Irish Sea bed have been famous since the early days of the glacial theory. I have done a previous post on this:
There are other occurrences of high-level glacial deposits laid down by Irish Sea ice at over 300m altitude in the north and middle Welsh Borders. Shelly till occurs at c 300m on Halkyn Mountain near Wrexham and at over 300m on Gloppa Hill near Oswestry. At none of those locations could the ice have transported and emplaced these deposits without flowing uphill. We should not be surprised. Irish Sea ice flowing from the north and encountering the bulk of the North Wales uplands was forced to either flow round them or over them. In the event, where lateral diversions across Lleyn and into the Cheshire Lowlands proved impossible because of the massive buildup of ice coming from other directions, the ice was forced to flow uphill until it could flow no further because of the conflict with ice from the Welsh Ice Cap, which was seeking to flow northwards.
The evidence of uphill glacier flow in this case is of course matched by abundant evidence from every glaciated area of uphill erratic transport. We have discussed this at length in this blog, and there is no point in reiterating previously made points.
The question I want to address here is this: how does a glacier behave when it encounters an obstacle like a reverse slope or a range of hills or mountains? First, let's look at glaciers in constrained situations such as troughs or deep valleys. The fjord glaciers of Norway, Baffin, Chile and Greenland are good examples. In Antarctica many current ice streams flow within clearly defined troughs.
The point is not made often enough that these troughs can only be excavated if glacier ice flows uphill, and transports excavated material up and over the threshold at the trough exit. In the great majority of cases, there is a steep trough head close to the glacier source area, where erosion and downcutting are suddenly accelerated, and a steep reverse slope to the threshold at the outer end of the trough, where erosive power is suddenly lost either because of diffluence or greatly increased surface melting. The long profiles of Sognefjord and Hardangerfjord in Norway are classic textbook examples:
In Sognefjord (which is about 200 km long) the trough floor has been excavated to a depth of over 1500m, but there is an extraordinarily rapid rise in the bedrock floor as the fjord approaches the west coast. It rises from -1000m to -400m in a distance of less than 5 km -- and over a distance of 20km towards the trough exit the bed rises no less than a thousand metres. Diffluence and a loss of erosive power can easily be demonstrated here, as suggested in the diagram. Where the surrounding mountains are less than 800m high, glacier ice has been able to escape from the constraints of the main trough and has managed to cut additional exit routes across old cols in the coastal hilly landscape.
The Hardangerfjord trough is shorter (about 170 km); it has a lower plateau catchment area and a more broken pattern of feeder troughs; an ice was able to escape from the main glacier trough much further upstream. So in this case the deepest part of the trough is only about 50 km from the source area, and instead of a steep threshold step the fjord floor rises gradually if irregularly towards the outer coast. Again the relationships are demonstrable: supplements to glacier discharge have enhanced erosion, and diffluence has reduced erosive power.
In our book "Glaciers and Landscape" (1976) David Sugden and I referred to these and other fjords in our treatment of glacial erosion, but to our shame we never really confronted the issue of what the precise mechanisms of trough overdeepening actually are. Make no mistake about it -- the quantities of bedrock actually shifted from these fjords (admittedly over a string of glaciations) BY ICE FLOWING UPHILL are staggering. The overdeepened part of the Sognefjord trough is 170 km long and has an average width of 4.5 km and an average depth of 1000m or 1 km. (In this calculation we take water depth, and take no account of the glacially eroded sediments that have been left behind in the trough.) If the trough sides were vertical beneath the water line, this would give an estimated volume of excavated and removed rock as 76,500 cubic km of rock. Let's assume that sloping sidewalls reduce that volume by 25% -- and the figure still comes out as over 57,000 cubic km of rock. Let's say that a cubic metre of rock weighs 2.5 tonnes, and we come up with a weight of bedrock removed equal to 14,250,000,000 tonnes. Have I got this right? Anyway, it's a lot of rock -- and remember that all of it has been transported uphill in order to get it out of the trough. Then you can add all the material transported out of the tributary troughs as well -- and all of that has been lifted up and ejected over the Sognefjord threshold........
Nordvestfjord in East Greenland is about the same length as Sognefjord, as measured from trough head to threshold, but it has a much greater area where the trough depth is in excess of 1200 m, and it has a maximum sounded depth of 1508m. We do not know how much lower the rock floor of the trough is; if Sognefjord is anything to go by, there may be more than 200m of unconsolidated sediments. So here the volume and weight of rock exported from the trough will be even greater.
So what exactly are the mechanisms by which these vast quantities of bedrock can be eroded and then transported uphill, over the fjord threshold, and away to the outer coast? All of the well known erosional processes must come into play, accompanied by pressure release and rock bursting on a very large scale. But how have many millions of tonnes of rock debris been carried upwards and over the threshold sills of all of these huge fjord systems? Can basal sliding and internal deformation of ice be invoked as the key movement and transport mechanisms? Maybe, but I'm not convinced. Ice can only move uphill in circumstances such as these if there is massive ablation and mass wastage on the glacier surface, since the glacier itself must maintain a surface gradient all the way from the head of its accumulation zone to its snout. There can be no reverse slopes on the glacier surface, even there may be a huge reverse slope or threshold to be surmounted......
Many authors have pondered on the "enigma" of overdeepened troughs, remarking on the occurrence of two "thresholds", one at the trough head and the other at the lip or rock bar at the trough exit. At each location there must be dramatic change in the internal characteristics and erosive capacity of the glacier. John Andrews, in his book called "Glacial Systems", suggests that positive feedback mechanisms kick in at both locations. At the trough head there must be a sudden acceleration of glacier flow as extending flow begins to operate -- and a dramatic acceleration of abrasion, plucking and block removal. There are classic examples in Vestfirdir -- the NW Peninsula of Iceland. In fact, the trough heads are so spectacular there that Prof David Linton coined the term "Icelandic troughs" to distinguish them from other trough types. I studied these trough heads in 1973-77 on a number of visits to Iceland -- and could never fully work out the processes involved! Basal sliding by ice at its pressure melting point, and rapid internal deformation, can reasonably be assumed.
But what happens at the other end of the overdeepened part of the fjord long profile, as ice comes up against the buffer of the rock threshold? We have to assume, according to the laws of physics, that maximum erosion beneath the fjord glacier (or outlet glacier, as it might more properly be labelled) occurs beneath the equilibrium line and the point of maximum ice throughput, somewhere between the lowest point on the glacier bed and the threshold bar. Below that point, compressing flow kicks in, with decelerating ice movement. In a theoretical examination of this problem, Shoemaker concluded thus: "If the ice is sufficiently thick, radial divergent flow results in erosion rates which decrease in the down-stream direction. This provides an explanation for the formation of the largest class of fjord thresholds which occur at channel widenings." He explains everything in terms of basal sliding velocities and internal deformation, with "quarrying rates" suddenly reduced at the point at which divergent or diffluent flow becomes possible.
Journal of Glaciology, V ol. 32, No. 110, 1986
THE FORMATION OF FJORD THRESHOLDS
By E. M. SHOEMAKER
In spite of a rather complex mathematical analysis, Shoemaker still, in my opinion, fails to explain how a threshold or reverse slope up to a thousand metres high can be formed, maintained and kept clean by a flowing glacier or ice stream. We know that shearing or thrusting can occur in glaciers in the compressing flow zone, and I wonder what role shear planes, and debris transport along them, might play in the formation and preservation of these features........ This would involve brittle fracture, which is not often referred to in the leading texts. On the other hand, John Andrews has suggested that meltwater flow under very high hydrostatic pressure might assist in the removal of broken bedrock. Cold based ice or warm based ice? Maybe oscillations between the two? It's all up for grabs, and shows us how little we know about the processes that operate beneath thick ice when selective linear erosion is going on.
Back to Wales. We now know that ice can move uphill and can erode and transport vast quantities of debris up and over rock thresholds where there is streaming within a confined rock basin. It can also move uphill, eroding and transporting debris, even where there is no confinement and where lateral spreading is possible. We can see the results of that in North Wales, where Irish Sea glacial deposits can still be observed at altitudes up to 400m. These deposits appear to date fromthe Devensian glaciation.
During the Anglian glaciation, there was a very similar situation in North Pembrokeshire, where Irish Sea ice moving in from the north and north-west encountered the varrier of Munydd Preseli -- more or less perpendicular to the direction of ice flow. The ice could not divert eastwards, since there was a blockage in that direction from Welsh Ice, and it could not divert westwards either, because of the pressure of ice flowing through St George's Channel. So it flowed up and over the mountains. That should surprise nobody, and in other posts on this blog I have addressed the matter of the precise mechanics of ice flow, erratic entrainment and long-distance erratic transport.
Having looked at this topic in more detail, with reference to field evidence from North Wales and South Wales, I am more than ever convinced that the above diagram is not far wide of the mark. The only modification I might make is a suggestion that pressure melting, basal slippage and quarrying might have occurred at deeper levels within the ice and that compressive flow occurred on the upslope of Preseli where ice thickness was reduced and where warm-based ice was replaced by cold-based ice. I still think that this episode of quarrying, entrainment, and upward block transport on shear planes occurred for a very limited period of time -- maybe no more than a few centuries while the Devensian Irish Sea Glacier was still expanding.
PS (18 Feb 2020)
Here is another interesting article on trough overdeepening, published in 2016. It is somewhat abstract and theoretical, and could have done with more hard data from actual overdeepened outlet glacier toughs, but nonetheless it is a fine contribution: