The Great Plains: Once a Shallow Inland Sea

One hundred million years ago, during the Late Cretaceous, a vast shallow sea—the Western Interior Seaway—bisected North America from the Arctic Ocean to the Gulf of Mexico. At its maximum extent, it covered ~1 million square miles (2.6 million km²), submerging what are now the Great Plains, parts of the Rocky Mountains, and the Canadian Shield.

Seaway Characteristics

• Depth: Mostly 100–600 meters (330–2,000 ft)—shallow enough for sunlight penetration
• Climate: Warm, subtropical to temperate; no polar ice caps
• Salinity: Variable—fully marine in open sections, brackish near river deltas
• Duration: ~40 million years (Late Cretaceous, ~100–66 Ma)

Marine Life: A Fossil Treasure Trove

The seaway's chalk and shale deposits (Niobrara, Pierre, Smoky Hill formations) preserve extraordinary fossils:

• Mosasaurs – 15-meter marine lizards (apex predators)
• Plesiosaurs – long-necked and short-necked (polycotylids)
• Xiphactinus – 6-meter "bulldog fish" with fang-like teeth
• Sharks – Cretoxyrhina (Ginsu shark), Squalicorax (crow shark)
• Ammonites – coiled cephalopods, index fossils for biostratigraphy
• Inoceramids – giant clams forming shell pavements
• Hesperornis – flightless toothed diving birds
• Pteranodon – soaring pterosaurs nesting on coastal cliffs

Seaway's Demise & Plains Formation

Around 70 Ma, the Laramide orogeny (mountain-building) uplifted the Rockies. Simultaneously, global sea levels fell due to cooling and reduced seafloor spreading. The seaway drained southward, leaving behind:

• Marine sediments → fertile soils (Pierre Shale → expansive clay; Niobrara Chalk → limestone)
• Flat topography → ideal for grassland establishment
• Mineral resources → bentonite, gypsum, petroleum, natural gas
• Fossil beds → world-class paleontology sites (Hell Creek, Dinosaur Provincial Park)

Modern Evidence

Drive I-70 across Kansas: roadcuts expose chalk layers with fossil clams. Visit Monument Rocks (Kansas) or Badlands (South Dakota)—eroded seaway sediments. The Ogallala Aquifer, underlying the Plains, partly recharges from paleo-seaway formations. The seaway's ghost shapes America's breadbasket.