I have been exchanging Email messages with various people interested in the Mendips, and am grateful to Alex and Henry for pushing this up to the front of the queue again. In the recent posts about Cheddar Gorge and the other anomalous dry valleys (some of which are humped) we suddenly find an explanation that might make sense. Shall we therefore speculate that the Mendip plateau supported a little ice cap on several occasions, with dimensions of approx 20 miles x 8 miles? From above, it might have looked like the Barnes Ice Cap on Baffin, or the Drangajokull in Iceland. It was probably thin and cold-based, and pretty well incapable of glacial erosion. If anything, it probably protected the landscape from periglacial processes for centuries or millennia.......
One interesting question is whether this little ice cap was "absorbed" into the Irish Sea Glacier when it pressed in from the Bristol Channel. Another interesting question is the nature of the debris that might have rested on the ice cap bed -- might there have been enough movement for the creation of a crude till, or would there simply have been a slight reworking of frost-shattered debris or head? In some polar limestone areas affected by ice, Prof Brian Bird says that the typical regolith is "rubble" -- mostly angular debris with some finer material, but no striated cobbles and erratics, and no clear structure.
It's when an ice cap like this melts that things become interesting. Suddenly we have substantial volumes of ice (even in a thin cold-based ice cap) being converted into meltwater which flows radially and catastrophically away from the centre towards the ice cap edge. The quantities of water involved are orders of magnitude greater than those coming from snowpatches (as in the conventional "periglacial snowmelt" model) -- and gorges and other deep and steep-sided valleys could well be the result. Maybe these valleys were steepened and deepened a bit more, every time there was a glacial episode and every time the Mendip ice cap was regenerated. Much of the meltwater flow might have been initially beneath the ice, close to the ice cap margin -- and then later in open channels.
What about the Rickford Valley and some of the other anomalous valleys that don't conform to the radial pattern? Well, these could indicate that the Anglian ice of the Irish Sea Glacier did impinge upon, and maybe even overwhelm, the Mendip ice cap at the time of greatest ice extent (the GBG or "greatest British glaciation") -- leading to the creation of a small number of subglacial meltwater channels like those seen in North Pembrokeshire and elsewhere.
So there we are then, that's a hypothesis -- with many questions attached to it. Does this hypothesis adequately explain the associations of landforms -- and especially the cave sedimentary sequences -- that we know from Mendip? Over to others for the answers......
9 comments:
Brian,
Little ice here, little ice there and pretty soon we'll have my 'local ice cover theory' covered!
Anyway, one interesting question. While the bedrock at Salisbury Plain is chalk, the bedrock at the Mendips is limestone. Am I correct?
What are the thermal properties of the two? My sense is that limestone strata will crack in a cold/hot interface, while chalk will transmit the heat more evenly onto the ice cover.
This, of course, is 'falsifiable'! As all my hypotheses are!
Any thoughts on that?
Kostas
Yes, the surface rock over much of Salisbury Plain is chalk, and it is limestone in the Mendips. But the geology of the Mendips is actually quite complex.
Both limestone and chalk tend to break up in periglacial environments, as a result of freeze-thaw processes. These are mechanical processes involving the expansion of ice in pores and fissures -- chalk tends to crumble, and limestone tends to break down in larger fragments. Temperature changes can also lead to stresses in the rock and lead to cracking and breaking, simply because of the differential expansion and contraction of constituent minerals.
I do think that you are doing a very useful job!
Eroding the present paradigm
but glacially slowly.
A problm of the Mendips is that much of it is a resurrected landscape-with Triassic valleys Burrington Coombe for example infilled with wadi deposits which I guess when loose can look like glacial material??
GCU In two minds.
Thanks, Anon. Very interesting. What's the evidence for Burrington Combe being a resurrected Triassic valley?
http://www.bgs.ac.uk/Mendips/assets/pix/Burrington_geo_map_large.jpg
The rather strange map shown on the BGS site shows the gorge as not being in the same place as the Triassic conglomerate -- so the gorge would appear to be a much more recent feature, cut entirely in the Carboniferous Limestone?
Brian,
So we have that “... chalk tends to crumble, and limestone tends to break down in larger fragments”.
This difference in chalk and limestone may very well explain some of the differences between Salisbury Plain and the Mendips.
A 'crumbled' chalk bedrock will more easily and directly absorb and drain meltwater. While a 'crusted/cracked' limestone strata will form 'drainage channels'. But not those created by 'meltwater turbulence' and so not conforming to the geomorphology of the land.
What are your thoughts on this?
Kostas
It's a matter of observation that limestone drainage channels tend to be steep-sided and quite deeply incised into the landscape, whereas chalk drainage channels tend to be less steep-sided, with gentler long profiles and cross profiles. So yes, these differences must be down to geology -- but not necessarily related to the nature of the regolith or surface layer. That's another matter....
Brian,
Your thoughts are always relevant and important to me. Thanks.
So it is true then that “ … limestone drainage channels tend to be steep-sided and quite deeply incised into the landscape, whereas chalk drainage channels tend to be less steep-sided, with gentler long profiles and cross profiles”.
If such drainage channels were in both instances made through the action of 'meltwater turbulence' (whether on the surface or under an ice cap), how could this turbulence (same for chalk and limestone) account for such differences? Why should meltwater turbulence open up drainage channels in the limestone strata which are “steep-sided and quite deeply incised into the landscape”? And furthermore, why should these be so sharply zigzag rather than more smooth through the action of water turbulence? It seems that something else is responsible for the formation of these drainage channels.
My 'working hypothesis' is that it is the 'cold/hot thermal cracks' in the limestone strata but not in chalk bedrock. The drainage channels incised in the chalk, on the other hand, are most likely due to 'meltwater turbulence'. And so they do tend to be more smooth with “gentler long profiles and cross profiles”.
What do you think …
Kostas
Hi, I came across your blog while searching the net for websites that tackle about Limestone. Anyway, just want to say that it's been nice dropping by your blog.
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