An inland sea (also called an epeiric or epicontinental sea) is a shallow, continental-scale body of marine water that occupies broad interior basins or epicontinental platforms and differs fundamentally from ordinary lakes by its partial marine character and large areal extent. Such seas may be entirely surrounded by land or retain one or more hydrological links to the ocean—via rivers, straits, or seaward arms—so that water, organisms and sediments are exchanged to varying degrees with open marine systems. Their bathymetry is typically much shallower than adjacent ocean basins, a condition that, together with the geometry of the basin and the nature of its connections, strongly controls circulation patterns, tidal expression and the development of salinity gradients. Salinity in inland seas is usually intermediate between freshwater lakes and full-strength seawater, reflecting the balance of marine inflow, continental freshwater inputs, and restricted exchange with the open ocean. Because they are true marine basins, inland seas experience tidal forces driven by lunar and solar gravity, but tidal range and character are modulated by basin depth, shape and connectivity. The terms inland, epeiric and epicontinental sea therefore denote a distinct hydrographic category—large, shallow, continental water bodies with partial marine influence that occupy a position between lakes and open-ocean basins.
An inland sea is a principally land‑enclosed body of water—often extensive and frequently saline—located within a continental interior rather than along an open coastline. In functional usage it may be regarded as a very large lake, but in practice inland seas are typically semi‑isolated from the open ocean, connected, if at all, by narrow straits or marine “arms.” This semi‑enclosed configuration distinguishes inland seas from bays, which are openly continuous with oceanic waters.
When defined in geological terms, inland seas overlap with the concepts of epeiric (or epicontinental) seas. Coined by Joseph Barrell in 1917, “epeiric sea” denotes a shallow marine inundation whose seafloor lies within the wave base so that bottom sediments are affected by surface wave action. Such seas commonly form when marine transgression or broad vertical movement of continents (epeirogeny) floods continental interiors. The term “epicontinental” is used synonymously in many geological contexts and is also applied to shallow waters over continental shelves, though in some legal contexts the label carries administrative rather than strictly scientific meaning.
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Epeiric/epicontinental and inland seas are thus understood as shallow, often limitedly connected marine waters situated on continents. Because they occupy continental interiors rather than open coastal zones, they are treated differently from coastal oceanic waters in legal frameworks governing the international law of the sea.
Modern inland seas
In the present Earth system, relatively high continental elevations combined with lower eustatic sea levels have limited the occurrence of extensive true inland seas; “eustatic” here denotes global changes in sea level that control whether continental basins become inundated. Historical misconceptions about inland waters—illustrated by an 1827 map of Australia that showed a fictitious “Great River” and “Supposed Sea”—reflect how incomplete exploration and mapping once exaggerated the frequency and extent of contained marine basins.
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Several contemporary basins occupy an intermediate position between enclosed lakes and open ocean. The Marmara Sea in Turkey functions as an essentially enclosed basin, connected to wider oceanic waters only by the narrow Bosphorus and Dardanelles straits. The Caspian Sea, many hundreds of miles from the nearest ocean, combines substantial salinity with a completely enclosed setting and thus resists a simple classification as either the world’s largest lake or an inland sea. The Baltic Sea is a broad, low‑salinity inland basin often cited as the largest brackish sea—though comparable candidates include the White Sea and the northern Black Sea, the latter containing a deep southern basin that is a remnant of the vanished Tethys. The geomorphic origin of the Baltic basin remains contested, with differing assessments of the roles played by tectonic subsidence versus erosion.
Other large inland water bodies reveal diverse origins and functions. Hudson Bay occupies a depression within the North American shield and was the center of Quaternary glaciation; despite superficial analogies to Fennoscandian basins such as the Gulf of Bothnia, its basin is not attributed to glacial scouring. Japan’s Seto Inland Sea serves primarily as an island‑bounded maritime corridor among Honshū, Shikoku and Kyūshū rather than as a truly isolated sea. North America’s Great Lakes, though freshwater, are treated by the USGS in some management contexts as inland seas because of their extensive spatial scale, internal seiche dynamics, and functional similarity to enclosed marine basins.
Finally, post‑glacial sea‑level rise and shelf flooding demonstrate how inland or marginal seas can be reconstituted: the Persian Gulf was reflooded in the late Pleistocene–Holocene transition (less than ~10,000 years ago), and the South China Sea currently inundates the Sunda Shelf, illustrating how changes in global sea level and continental shelf morphology produce large marginal basins that operate as modern inland seas.
Former epicontinental seas in Earth’s history
Intervals of elevated global sea level produced extensive marine transgressions that repeatedly flooded continental interiors, creating epicontinental seaways wherever low relief and sedimentary subsidence permitted. These shallow, transcontinental embayments were more widespread in Earth’s past than today and exerted strong control on regional sedimentation, faunal dispersal, and paleogeography.
Prominent Cretaceous examples illustrate their scale and influence. The Western Interior Seaway bisected North America from the Gulf of Mexico into present-day Canada, forming a major marine corridor that shaped stratigraphy and biotic migration. At the same time, much of northern France and northern Germany lay beneath shallow chalk seas whose widespread carbonate deposits now typify the region’s Cretaceous lithology. In the Southern Hemisphere, the Eromanga Sea inundated large parts of eastern Australia during the Cretaceous, demonstrating that Gondwanan interiors were likewise affected by epicontinental flooding.
Younger transgressions produced equally significant but more transient seaways. In Oligocene–Early Miocene Patagonia, marine fossils in the La Cascada Formation—showing affinities with both Atlantic and Pacific assemblages—indicate that narrow epicontinental channels intermittently linked the two oceans by cutting through dissected terrain. A particularly consequential example is the Amazon Basin: the proto-Amazon originally drained west to the Pacific, but Andean uplift around 15 Ma blocked that outlet and gave rise to a vast inland marine embayment. Episodes of northern drainage toward what is now Venezuela preceded eventual reorganization to an eastward Atlantic outlet; progressive infilling and profile flattening converted the marine system into freshwater lakes and wetlands, enabling marine-derived taxa to adapt to inland freshwater environments.
The biotic and fossil records preserve this legacy. Amazonia’s freshwater fauna retains marine signatures—more than twenty freshwater stingray species closely related to Pacific lineages and a freshwater dolphin—while Pleistocene and Neogene fossils, including a giant (approximately 14 m) crocodilian from northern Peru, attest to the region’s former large aquatic vertebrates. Collectively, these examples demonstrate how past epicontinental seas reorganized continental drainage, regulated sedimentary regimes, and left enduring imprints on biogeography and the stratigraphic record.