§ Tum Dincelon ’Dimrill-dale’ (= ’Vale of Dim Streams’) (RS:432,466)
§ Nanduhiriath (RS:433)
The Dimrill-dale had been originally put to the northeast of Imladris, before Tolkien moved it to the other side of the Misty Mountains, see also Nen fimred, Nenvithim ’Hoardales’ below (2.1).
It is easier to interpret the later form first – Nanduhiriath apparently consists out of nan, nand- ’valley’ (Let:230, Silm.index) [The Etymologies give NAD- with N. nann, nand ’wide grassland’ and Dor. nand ’field, valley’] + N. dû ’nightfall, late evening’ or ’night, dread of night’ (DOƷ-, DÔ-) + lenited siriath ’rivers’ in the collective plural (see SIR-).
The final from Nanduhirion in The Lord of the Rings contains sirion ’[great] river’ (ibid.). Tolkien explains it as S. nan(d) ’vale’ + dû ’dimness’ + sîr ’stream’ + (i)on which appears in various later-formed names of regions (PE17:37). The ending -ion < √YŎNO ’wide, extensive’ is on another occasion said to be a blend with augmentative or male -on (PE17:43), so Sirion ’the Vale or lands about the River Síre’ or ’the great stream’ (ibid.). Compare note.
The earlier name seems to mean the same with N. tum ’deep valley under or among hills’ (TUB-). The former element of Dincelon is perhaps from the stem DEM-, earlier DIM- (VT45:9) with derivatives like Ilk. dimb ’sad’, dim ’gloom, sadness’, dem ’sad, gloomy’. Compare also Dimbar from The Silmarillion, the first element of which means ’sad, gloomy’ (Silm.index). The final -m could be assimilated to the following c- resulting in -nc- [ŋk], compare Goldogrin dungort, dumgort ’an (evil) idol’, dum ’secret, not to be spoken, especially of bad things’ (GL:31).
The last element is clearly N./Ilk. celon ’river’ form KEL- ’go, run (especially of water)’; hence uninflected genitive: *’valley of a gloomy river’, matching the translation.