Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
petrōsus, a, um, adj. [1. petra], full of rocks, rocky (post-Aug.): loca, Plin. 9, 31, 50, § 96.
Subst.: petrōsa, ōrum, n., rocky places: maritima, Plin. 26, 8, 29, § 46; Ambros. Laps. Virg. 7, 29: aliud cecidit super petrosa, Vulg. Marc. 4, 5; id. Matt. 13, 5.