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How do rising and falling sea levels change ancient coastlines?

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Watch Earth's Continents Move — paleogeography Watch a time-lapse of Earth changing through geologic time, from the Cambrian Era to the projected period of Pangea Proxima.

In the context of paleogeography, fluctuations in global sea level—often termed eustatic changes—fundamentally reshape ancient coastlines by altering the exposure and inundation of continental shelves.1

When sea levels fall, such as during glacial periods when water is locked in ice sheets, the shoreline retreats seaward. This exposes vast areas of the continental shelf to subaerial processes like weathering, soil formation, and fluvial activity, occasionally creating land bridges that facilitate migration. Conversely, when sea levels rise, these previously exposed landscapes are submerged. This "drowning" of the coast often inundates existing river valleys, forming estuaries or, in mountainous regions, deep-water fjords.1234

These shifts are driven primarily by global climatic fluctuations, though tectonic activity and isostatic adjustments can also cause local coastal uplift or subsidence. Over geologic time, the interplay between these sea-level changes and sediment deposition determines whether a continental margin progrades (grows seaward) or retreats.123