Quenya 

ruxa-

verb. to crumble

A verb for “to crumble” implied by (active-participle?) ruxal’ “crumbling” in Markirya poem of the 1960s (MC/222).

Conceptual Development: In versions of ᴱQ. Oilima Markirya from around 1930, Tolkien used (plural) ᴱQ. ruste for “crumbling” (MC/214).