I happened to be reading through
The Saga-Book of the Viking Society for Northern Research a few weeks ago, and came across an interesting article that seems pertinent to this discussion. The paper, by the Reverend E. Maule Cole, appears in the fourth volume of the journal, which covers the years 1904-5, and would therefore have been available to Tolkien even in his undergraduate years. [1]
At the time of writing, Rev. Cole had been the vicar of Wetwang in the East Riding of Yorkshire for some forty years, and had often been asked to explain the origins of the name. His conclusions would almost certainly have interested Tolkien, and it could be that the interesting dichotomy between the name and nature of the parish might have inspired him to use it in a more appropriate context in
The Lord of the Rings.
In Old English, to quote the great Professor W.W. Skeat, "Wet's wet and Wang's a field, and there you are." But Wetwang in Yorkshire, as Rev. Cole points out, is on a chalk ridge, fifty feet above the bottoms of the dales on either side. It is so dry that in a report on the manor made to Lord Bathurst by his steward in the early eighteenth century, which Rev. Cole quotes in his paper, "Water is here much wanted. There is a pond in ye Town supply'd only by rainwater, wch in dry Summers affords none, and then the Inhabitants are obliged to drive their Cattle three miles for water."
However, as Cole points out, in Old Icelandic there is a compound word 'Vœtt-vangr' or 'Vétt-vangr', from 'vætti' ('witness', 'testimony') or 'va'ttr' (a witness), and the compound is a legal term, basically meaning a place to which one was summoned when accused of an offence; 'vettvang' being the area within a bowshot of the place in all directions. According to Cole, then, the Yorkshire place-name derives from a Norse system of trial by one's peers, which may or may not have been the origin of the English and Scottish systems of trial by jury.
I think that the idea of a place, the name of which can be translated as 'Wet field', but which is actually so dry that the well is useless in hot summers, might have appealed to Tolkien. On the other hand, he may never have read this article and the name of the marshes south of the Emyn Muil may simply be derived from the Old English phrase for a wet field. In either case a certain amount of irony is implied, but I like to think that he had come across the article during the course of his studies in Norse literature and put a reference to it into his story for personal amusement.
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[1] Rev. E. Maule Cole, M.A., F.G.S.: 'On the Place-name Wetwang'.
Saga-Book of the Viking Society for Northern Research IV (1904-5), 102-6