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 of the 1940s 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 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.
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.