In reply to:
Quite seriously, is there a relationship then between the Welsh cwm and the southern English coomb? The pronunciation can't be that different, and they appear to represent the same type of geographical feature.
well... here's more than you probably wanted to know:
[In OE., cumb masc. ‘small valley, hollow’ occurs in the charters, in the descriptions of local boundaries in the south of England; also in numerous place-names which still exist, as Batancumb Batcombe, Brancescumb Branscombe, Eastcumb Eastcomb, Sealtcumb Salcombe, Wincelcumb Winchcombe, etc. As a separate word it is not known in ME. literature, but has survived in local use, in which it is quite common in the south of England: see sense b. In literature coomb appears in the second half of the 16th c., probably introduced from local use; a century later, it was still treated by Ray as a local southern word. OE. cumb is usually supposed to be of British origin: modern Welsh has cwm (kum) in the same sense, also in composition in place-names as -cwm, -gwm, and in syntactic combination as Cwm Bochlwyd. A large number of place- names beginning with Cum-, especially frequent in Cumbria, Dumfriesshire, and Strathclyde, as Cumwhitton, Cumdivock, Cumlongan, Cumloden, appear to be thus formed. Welsh cwm represents an earlier cumb, OCeltic *kumbos. The OE. word might however be an obvious application of cumb, coomb1, to a physical feature, though there is no trace of any such application of the cognate German words on the Continent; in any case, if the Saxons and Angles found a British cumb applied to a hollow in the ground, its coincidence with their own word for ‘basin, bowl, deep vessel’ would evidently favour its acceptance and common use. This might further be strengthened, after the Norman Conquest, by the existence of a F. combe ‘petite vallée, pli de terrain, lieu bas entouré de collines’ (Littré, 12th c.), cognate with Pr., Sp. and north It. comba, for which also a Celtic origin has been claimed...] {OED}
