An adjective for “forked, cleft, split”, most notably an element in the name Orthanc, derived from primitive ᴹ✶stankā based on the root ᴹ√STAK “split, insert” (UT/400; Ety/STAK). This word dates all the way back to Early Noldorin Word-lists of the 1920s, where ᴱN. thanc had glosses like “cleft, divided, split, cloven, forked, bifurcated” (PE13/154, 165; PE14/66), and in one place was also a noun glossed “cleft, break, breach” (PE13/165).
Sindarin
riss
noun. cleft; cleft, cloven, separate, cleft, [N.] ravine; cleft, cloven, separate
thanc
noun/adjective. forked, forked, [N.] cleft, split, [ᴱN.] divided, cloven, bifurcated; [ᴱN.] cleft, break, breach
ress
adjective. cleft, cloven, separate
A noun for “cleft” appearing as the final element of the name Imladris “Rivendell” = “Deep Dale of the Cleft” (RC/774) or “Canyon of the Cleft” (VT47/14).
Conceptual Development: This noun first appeared as N. rhis or rhess “a ravine” derived from primitive ᴹ✶risse- under the root ᴹ√RIS (Ety/RIS²). The variant form rhess indicates some kind *a*-affection, perhaps because final *e* became *a* after *s*, though usually a double ss preserved e. In the 1940s and later Tolkien sometimes gave the name of Rivendell as Imladris(t), indicating a final element rist (TI/120; RC/774), but normally an -st does not reduce at the end of Sindarin words: compare Angrenost and Eglarest. In notes on Words, Phrases and Passages from the Lord of the Rings (WP) from the late 1950s or early 1960s, Tolkien gave the form as riss < ✶rinsa, an adjective meaning “cleft, cloven, separate” (PE17/87), but here the expected *a*-affection is absent.
Neo-Sindarin: For purposes of Neo-Sindarin I would assume the noun form is riss “cleft”, and the adjective form is ᴺS. ress “cleft, cloven, separate” with the usual a-affection.