The adjective ᴹQ. tumba “deep, lowlying” appeared in rough (and ultimately rejected) notes on irregular verbs from the Quenya Verbal System of the late 1940s as a derivative of ᴹ√TUB “fall low, go down” (PE22/127). In a 1961 letter to Rhona Beare tumba was glossed “deep valley” as an element in the Entish phrase Q. Taurelilómëa-tumbalemorna Tumbaletaurëa Lómëanor “Forestmanyshadowed-deepvalleyblack Deepvalleyforested Gloomyland” (Let/308; LotR/467), but I think this is only an approximate translation, and the word is better understood as adjectival in sense: “✱like a deep valley”. As further evidence of this, in notes from the late 1960s the form tumba was changed to a more typical noun form Q. tumbo in the name Q. i Tumbo Tarmacorto “the Vale of the High Mountain Circle” (NM/351).
Neo-Quenya: For purposes of Neo-Quenya, I’d treat this word as an adjective only, and use Q. tumbo for the noun.
tumba noun "deep valley" (Letters:308; SA:tum and TUB gives tumbo "valley, deep valley"); apparently an extended form *tumbalë in tumbalemorna "deepvalleyblack" or (according to SA:tum) "black deep valley", also tumbaletaurëa "deepvalleyforested"; see Taurelilómëa-tumbalemorna...