√DAL root. “bottom, ground”
This root first appeared in The Etymologies of the 1930s as ᴹ√DAL with the gloss “flat” and various derivatives of similar meaning (Ety/DAL). It reappeared again in later notes as √DAL “bottom, ground” written in the late 1950s or early 1960s (PE17/150). In both places, it had a variant form √LAD of similar meaning.
One notable derivative from the 1930s was N. dalath “plain” which appeared in the name N. Dalath Dirnen “Guarded Plain” in Silmarillion drafts from the 1930s (LR/299). But in the second note mentioned above, Tolkien said “X Dalath Dirnen. dalath won't do = plain ... TALAT = ground (bottom), hence TALAT = fall down” (PE17/150). Indeed, in later Silmarillion drafts, Tolkien changes this name to S. Talath Dirnen (WJ/140). This may mean that Tolkien ultimately rejected this root, but the variant √LAD probably survived: see that entry for details.
Reference ✧ PE17/150 ✧ “bottom, ground”
ᴹ√DAL root. “flat”
References ✧ Ety/DAL, LAD