Our Very Own River Avon
Standing on Churchill Bridge and watching the slow passage of the river below and climbing to Beechen Cliff and looking across the wide valley to Lansdown it is impossible to imagine the river removing all the material which has disappeared.
Doctor Brian Hawkins gave a series of lectures this Spring on the subject of the River Avon which was attended by a number of the members. He also led a field trip to look at some of the upper reaches and to consider the various sources, about which more later. He also asked us to give an explanation as to why the Avon having started flowing down the dip slope, deflected at Malmesbury to change direction by some ninety degrees to flow southwards along the strike of the Jurassic rocks to Bradford-on-Avon and then to turn to flow westwards against the dip of the rocks and discharge into the Bristol Channel.
It is strange that, living so close to these problems, it has never until now presented itself to me as a subject which deserved a great deal more attention than it has received. However I find that in 1925 P. E. Martineau in a short paper to the 93rd Annual Meeting of the British Medical Association headed it “NOTE ON THE EROSION OF THE AVON VALLEY”. He writes of several sources in the Cotswolds and that it began as life as an east-flowing stream before the tilting and denudation of the chalk plain. He explained that the tilting and denudation of the chalk plain, enormous power of the glacial period brought into the district the cut down its own of the Severn which captured the Welsh rivers, Avon, its own river bed and allowed the cutting back of the Avon.
The Regional Handbook gives a description of the Avon and refers to the anomalous courses. It gives the reason that they were not laid down on the present land surface but represents the superimposed drainage on land subsequently removed by denudation.
John Haddon in his book, Portrait of Avon, goes along with the explanation given in the Regional Handbook but includes an idea put forward at the end of the last century, described but recently reintroduced, namely, a large lake in Wiltshire during an Ice Age with an overflow channel. The last Ice Age, which says that the Severn was 70 feet below its present level which would have permitted the Avon to cut back.
R.S. Barron in his book, The Geology of Wiltshire, refers to “Lake Trowbridge” and a possible overflow to form the Bradford Gorge as suggested by Harmer in 1907. In the recent past this particular area has been one which has been prone to flooding and we remember Maud Heath’s Causeway to prove the point.
To return to our own observations it seems that the fixing of the true source of a river is a contentious matter. It is simpler to say that the source of a river is the water table lying below the catchment area and that when the table is high the water for the River Avon flows from, say Joyce’s Pool at Ditterton but under drought conditions the same watercourse is fed lower down from the constant supply at Crow Down Springs. The same can probably be said about the Sopworth water and the supply from Handcock’s Well near Luckington.
With regard to the two major changes of direction of the Avon, I feel that a considerable barrier must have occurred to prevent it from joining up with the Thames and sending it south and then west. Could this have been the Trowbridge Lake which has been mentioned as having been formed under glacial conditions? Was there an overspill from the Lake with sufficient energy to cut the Bradford Gorge and flow westwards to capture the Bybrook at Bathford and so to Bath and beyond? It is possible that the Avon is now flowing in the valley cut by a former river which had been east flowing. If this is so, its flood rainfall to suggest that this may have been assisted by a fault zone in the area? It is noted that Lansdown is some 150 feet higher than Combe Down although of corresponding strata, indicating some sort of fault zone between them.
Members may wish to ponder these matters and to investigate this very interesting river. If they do they will be able to enjoy as we did the beautiful Cotswold villages such as Sherston, Easton Gray and Sopworth. In trying to unravel the behaviour of our very own Avon over the millions of years we might very well ask ourselves why a river is said to run through a particular town or village whereas the river was there in the first place and habitations were erected on the banks millions of years later.