striation

/strɑɪˈeɪʃən/

Other forms: striations

If the rock you found on the beach has a deep striation, it means there's an engraved stripe or groove running across it.

If the word stripes seems too fun and informal for your descriptive needs, you might want to try striations, which is tech talk for the same thing. Striation likes to keep company with other technical words like longitudinal and transverse, more commonly known as lengthwise and across. The Latin root of striation, stria, means "furrow, channel, or flute of a column."

Definitions of striation

1. noun

   any of a number of tiny parallel grooves such as: the scratches left by a glacier on rocks or the streaks or ridges in muscle tissue
   synonyms: stria

   type of:

   channel, groove

   a long narrow furrow cut either by a natural process (such as erosion) or by a tool (as e.g. a groove in a phonograph record)
2. noun

   a stripe or stripes of contrasting color
   synonyms: band, banding, stria

   types:

   collar

   (zoology) an encircling band or marking around the neck of any animal

   stretch mark

   a narrow band resulting from tension on the skin (as on abdominal skin after pregnancy)

   type of:

   bar, streak, stripe

   a narrow marking of a different color or texture from the background