Source : Webster's Revised Unabridged Dictionary (1913)

Gritrock \Grit"rock`\ (gr[i^]t"r[o^]k`), Gritstone \Grit"stone`\
   (-st[=o]n`)n. (Geol.)
   See {Grit}, n., 4.

Source : WordNet®

gritrock
     n : a hard coarse-grained siliceous sandstone [syn: {grit}, {gritstone}]