The mark left or reached by the tide at high or (rarely) low water; by extension, the mark left by a river flood. Also, a post or the like set up to mark the rise and fall of, or the point reached by the tide. Also fig.
1799. Scotl. Described (ed. 2), 16. Shells have been discovered … at a considerable distance above the highest tide-mark.
1861. Dickens, Gt. Expect., liv. Red landmarks and tidemarks stuck out of the mud.
1861. J. R. Greene, Man. Anim. Kingd., II. Cœlent., 232. Many … Actinidæ, it is well known, are numerous between tide-marks, the common Sea-anemone tending to encroach upon the line of high water.
1907. Daily Chron., 27 Dec., 4/4. You may still trace the tidemark of the flood by tufts of dried grass and driftwood sticking in the branches above your head.
