TIDEMARK

tidemark

(noun) indicator consisting of a line at the highwater or low-water limits of the tides

Source: WordNet® 3.1

Etymology

Noun

tidemark (plural tidemarks)

a line (of seaweed or differently coloured sand etc) on the shore showing the level of high or low tide

(by extension) any mark showing the limit of some past activity

(humorous) a line of scum left on a bath tub when the water is drained away

Source: Wiktionary