tidemark

Tidemark, or tide mark, is a term used in coastal geography and geology to describe the line on a rock, cliff, beach, or other substrate that marks the highest level reached by seawater during a tide, storm surge, or past flooding. The tidemark often appears as a visible boundary between wet and dry surfaces and may be accompanied by mineral staining, salt crusts, or the concentric growth of marine organisms such as barnacles or algae.

The appearance of a tidemark can vary. It may be a distinct wet-dry boundary on exposed rock, a colored or encrusted layer on sediment, or a marker within sedimentary sequences where marine influence was periodic. In tidal flats and shorelines, the tidemark roughly corresponds to the upper limit of regular tidal wetting and can shift with changes in tidal range, wave action, sediment supply, and climate.

In scientific practice, tidemarks can aid interpretations of past shoreline positions and sea-level history. They provide a reference horizon for correlating coastal deposits, dating sedimentary layers, and understanding transgression and regression events. However, tidemarks must be considered alongside other indicators, since non-marine factors such as rainfall, groundwater seepage, or mineral staining can occasionally create similar boundaries.

Related concepts include high-tide marks, tidal notches, and shoreface deposits. The term is used across geography, geology, archaeology, and environmental history to convey the historical or present extent of marine influence on a landscape.