ᴹ√STAL root. “steep”
A root in The Etymologies of the 1930s glossed “steep” with derivatives like Ilk. thall “steep, falling steeply (of river)” and Ilk. thalos “torrent”, the latter used for the river name Ilk. Thalos (Ety/STAL). Tolkien continued to use the name S. Thalos in later versions of The Silmarillion, but the name was translated nowhere else, making its continued connection to the 1930s root uncertain.
Reference ✧ Ety/STAL ✧ “steep”