And finally follow quite a few forms for the ’red valley’:
§ Caron-dûn ’Red Valley’ (RS:419)
§ Carondoom ’the Red Valley’, {Carondûn, Doon-Caron} (RS:433)
§ Carndoom, Caron-doom, Caron-dûn, Dun Caron ’Red Valley’ (RS:433)
§ Carondoom >> Dunruin (RS:454,464)
See also Narodûm above (1.6). From all these forms one can isolate the word ’valley’ appearing in the variations dun, dûn, doon, doom.
The Etymologies give TUB-, tumbu ’deep valley under or among hills’, N. tum. This already looks similar and one may guess that the forms starting with d- are lenited, but in fact they are not; or at least not all of them, since initially Dunruin and Dun Caron occur.
Looking further, the stem DUB- in The Etymologies looks related to TUB-, meaning ’lie, lie heavy, loom, hang over oppressively (of clouds)’. TUB- yields e.g. Q. tumna ’lowlying, deep, low’, while DUB- yields Q. lumna ’lying heavy, burdensome, oppressive, ominous’. Additionally, there is the stem pair NDUB-, DUB- (<< NDUM-, DUM-) ’lay base, foundation, root, found’, N. dum ’root, foundation’ (VT45:38), while Ilkorin dûm ’twilight’ (DOMO-) differs in sense.
So maybe Tolkien decided to derive ’valley’ from DUB- or NDUB-; or perhaps rather ’dark, ominous valley’. This would explain the forms with final -m, but what about final -n? DUN- ’dark (of colour)’, N. donn ’swart, swarthy’ does not really fit. Was another root change in progress here? Perhaps it is comparable with TALAM- ’floor, base, ground’, N. talaf ’ground, floor’, but later S. talan ’The wooden platforms in the trees of Lothlórien on which the Galadrim dwelt’ (UT:465). According to PE17:52 talan, tâl ’flat space, platform’ is still from TALAM- (CE talam-). In VT47:24 Tolkien notes that Common Eldarin final -m, became -n (with the same subsequent development as n in the descendant languages). If this applies here, we would have *(n)dummē or *(n)dumbē > dûm beside *(n)dum- > dûn. Compare also √TAM ’construct’ appearing as √TAN in Sindarin by influence of √PAN ’arrange, set in order’ (PE17:107-108).
The forms doon and doom are clearly adapted to English orthography, similar to the river Lune; where the actual Elvish name was Lhûn (LotR App.F, RC:773). For carn, caron ’red’ see 1.11.
Dunruin seems to contain lenited N. gruin ’ruddy’ (ROY2-) at the first glance, but in The Treason of Isengard we can later on find Ruinnel ’Redway’ (TI:114), leaving the basic form inevitably as #ruin – see the discussion of Ruinnel below (2.7).
Interestingly, we encounter Carn Dûm later as ’the chief fortress of Angmar’ (LotRI ch.8, App.A, UT:425), but without any hint on how the name is meant to be interpreted.
Now to the other names: