tumbo (stem *tumbu-, given the primitive form ¤tumbu) noun "(deep) valley", under or among hills (TUB, SA:tum), "depth" (PE17:81). - In early "Qenya", the gloss was "dark vale" (LT1:269). See tumba.
Quenya
tumbo
noun. deep vale, valley, deep vale, valley, [ᴱQ.] dale
tumbo
(deep) valley
i tumbo tarmacorto
place name. the Vale of the High Mountain Circle
This word was used for “valley” or “vale” for much of Tolkien’s life. In notes from the 1940s Tolkien specified it was a “deep valley with hi[gh] sides though often a wide extent” (PE22/127) and in notes from the late 1960s Tolkien described it as a valley which was “more or less circular, but deeply concave, and had high mountains at the rim” (NM/351).
Conceptual Development: The earliest appearance of this word was as ᴱQ. tumbo “dale, vale” in the Qenya Lexicon as a derivative of the early root ᴱ√TUM(B)U (QL/95). It reappeared in The Etymologies of the 1930s as ᴹQ. tumbo “deep valley under or among hills” derived from ᴹ√TUB (Ety/TUB), and again in the Quenya Verbal System (QVS) from 1948 as a derivative of ᴹ√TUB with the gloss “deep valley with hi[gh] sides though often a wide extent” as noted above (PE22/127).
In Words, Phrases and Passages from the Lord of the Rings (WPP) from the late 1950s or early 1960s it was given as the equivalent of Q. tumbalë “depth or deep vale” (PE17/81). In notes from the late 1960s it was described as a valley that was “more or less circular, but deeply concave, and had high mountains at the rim” as noted above, with a primitive form ✶tumbu (NM/351), the same primitive form it had in The Etymologies of the 1930s (Ety/TUB). Thus it seems this word and its basic meaning was pretty well established in Tolkien’s mind.