a mark indicating the highest point of flood tide or, sometimes, the lowest point of ebb tide
See tidemark in American Heritage Dictionary 4
(tīdˈmärkˌ)
noun
A line or mark on a shore indicating the highest or lowest level reached by the tide.
A trace or indication of past activity: “Just along the highway into town is the last tidemark of the Nazi advance, a brutal tangle of brown sculpture”(Regina Nadelson).