The strange valleys of the Mendips

 An old map showing some of the key features of the Mendips (Dr Allan Frey, Bristol Univ, 1965).  Click to enlarge.  Both the deposits and the valleys can give us guidance on the Pleistocene history of the area.

Since my visit to the Cheddar Gorge the other day I have been doing some digging -- in an attempt to discover where the water might have come from, for the cutting of the gorge.  I've been looking at old papers by Brian Hawkins, David Gilbertson, Geoff Kellaway and many others, and have come to the conclusion that there is something strange going on at Cheddar and elsewhere.  I'm investigating these strange goings-on from a distance (I'm now back in Pembrokeshire) -- but I know that others who know the Mendips well are also investigating, and I look forward to seeing what discoveries they eventually announce.

I have also been looking at the GCR volume (big and expensive!) on the Quaternary of SW England, and have been rather disappointed to find that the geomorphologists who contributed to that learned tome are increasingly preoccupied with the micro-interpretation of sediment sequences, and hardly ever mention landscape and landform assemblages.  That, in my book, is what proper geomorphology is all about....... so who's doing it these days in the Mendip - Bristol - Bath area?   Somebody, I hope......

If we look at the map above, we can see that there are anomalous valleys all over the place -- some shown here as "Trias floored gaps" and others as gorges and deep combes.  Cheddar Gorge is the most spectacular, but there are steep-sided valleys on all flanks of the Mendips, some associated with thick alluvial fans of spreads of debris beyond the points where they leave the hills.  These valleys are not "normal" in the sense that they are either deeply incised, or difficult to explain by reference to very small catchments, or else simply cut across cols with no normal valley profile, or else take tortuous routes that seem to defy the normal logic of rivers!

Another of Allan Frey's maps, showing the route followed by students on a geography field trip in the 1960's.  But notice the gaps, gorges and cols all over the place -- the NW corner of Mendip is very interesting indeed....

On the above map Allan Frey has shown all the key valleys of the NW corner of Mendip, and the tight contour map shows the col breaches in particular, including those at Hele Coombe (south of Winscombe), Webbington (the gap followed by the motorway), and the complex valleys in the Shipham - Burrington - Rickford area.    The aerial photo below shows the western area, seen from above the Cheddar reservoir.

In a small and forgotten paper published in 1971, Hawkins and Kellaway reported on a Bristol /Bath field meeting in 1970 (PGA, 82 (2) pp 267-292) and devoted some space to the "anomalous dry valleys" here and to the north.  They mention Dolebury Channel, Burrington Combe, and the Rickford Valley as being possible glacial meltwater spillways, and they mention a number of other valleys further to the north and closer to the coast -- including the Providence Valley near Bristol, the Flax Bourton gap, and three channels across the western end of the Failand Ridge.  There are other gaps and gorges near Avonmouth.  Three other channels are also shown on the western end of Broadfield Down, and one between Broadfield Down and Dundry Hill.  We are talking therefore about more than 20 anomalous dry valleys within a fairly small geographical area.   See the 3 maps below:

Three maps from Hawkins and Kellaway -- above, map showing postulated ice directions of the Irish Sea Glacier which flowed in from the west; middle, Providence valley; below, Rickford valley.

Speaking for myself, I do not know the geomorphology of this area well enough to know what the meaning of all of these "anomalous valleys" may be -- but given the history of glaciation in the area, we should suppose them to be related to glacial meltwater on some way.  (That is a far more reasonable explanation that the one normally used in the literature -- namely that the valleys are "relics" of some ancient drainage pattern let down upon the present-day landscape.)  Hawkins and Kellaway do get themselves into a spot of lather by assuming that the channels are "spillways" or overflow channels related to the drainage of pro-glacial lakes during the deglaciation of the area.  But that means that they have some of the valleys flowing in a direction which must have been opposite to that of the ice surface gradient -- and that is not very likely at all.  Much more likely that the channels are subglacial channels, with orientations directed more or less by the direction of ice flow.  Some of the channels are indeed humped -- and that is a good diagnostic sign.

So there we are then -- a suggestion that there are many valleys in this area which are of subglacial origin; and another suggestion that the valleys themselves can be used as strong indicators of glacial action across quite a large swathe of country in the Bristol - Bath - Mendip - Somerset area.  Over to those who know the field details far better than I do.......