This root was connected to the name Q. Atalantë “Downfall(en)” as a sort of multilingual pun on “Atlantis”. The first appearance of this root was as unglossed ᴱ√TḶTḶ in the Qenya Lexicon of the 1910s with derivatives like ᴱQ. talta “shaky, wobbling, tottering; sloping, slanting” and ᴱQ. tilt- “make slope, incline (tr.), decline, shake at foundations, make totter” (QL/93). Further signs of this early root can be found in the Early Noldorin Dictionary of the 1920s where Tolkien gave ᴱ✶tḹtá > ᴱN. tlad “hillside, slope” and ᴱ✶tḷtā́ > ᴱN. tleth/ᴱQ. tilta “slanting” (PE13/165).
In The Etymologies of the 1930s the root appeared as ᴹ√TALAT “to slope, lean, tip” with derivatives like ᴹQ. talta-/N. atlanna- “to slope” and ᴹQ. talta/N. talad “an incline” (Ety/TALÁT). The root was mentioned regularly thereafter, mostly in a verbal sense with glosses like “incline, slope, slide down” (PE18/38), “slip (downwards)” (PE18/61), “topple over, slip down” (SD/249), “slip (down)” (PE18/85) and “collapse, fall in ruin” (PM/158). In notes from the late 1950s or early 1960s, Tolkien gave the root a noun sense “ground (bottom)” alongside “fall down” (PE17/150) and in a 1964 letter to Christopher Bretherton gave it the sense “slipping, sliding, falling down” (Let/347).
This root was the basis for “foot” words for much of Tolkien’s life. It first appeared as ᴱ√TALA “support” in the Qenya Lexicon of the 1910s, but its derivatives were broken into two groups, the first with derivatives like ᴱQ. tala “foot” and ᴱQ. talas “sole”, and the second with derivatives like ᴱQ. tala- “carry, bring; weigh”, ᴱQ. talan “burden” and ᴱQ. talma “a weight, measure” (QL/88). This probably represents a blended root ᴱ√TALA and ✱ᴱ√DALA as suggested by Roman Rausch (HPG/§2.1), since there were derivatives in the contemporaneous Gnomish Lexicon that began with both t- and d-: G. tâl “foot” and G. talc “upright” (GL/68) vs. G. dalech “(upright) stone” and G. daltha- “erect, set up”.
In The Etymologies of the 1930s, the root ᴹ√DAL “flat” seems unrelated to any of its meanings from the 1910s, but ᴹ√TAL “foot” was clearly a continuation of the 1910s root, with derivatives like ᴹQ. tál/N. tâl “foot” and an extended root ᴹ√TALAM “floor, base, ground” with derivatives like ᴹQ. talan (talam-)/N. talaf “floor, ground” (Ety/TAL). Both the root ᴹ√TAL “foot” and its extension ᴹ√TALAM “base, root, foundation” were mention in the contemporaneous Primitive Quendian Structure: Final Consonants (PE21/56), whereas ᴹ√TAL “foot” was mentioned in Common Eldarin: Noun Structure from the early 1950s (PE21/70). The extended form √talam- “flat space” appeared again in notes on Words, Phrases and Passages from the Lord of the Rings written in the late 1950s or early 1960s (PE17/52), and both Q. tál and S. tâl “foot” appeared regularly in Tolkien’s later writings.