A Divided America? The Last Time The Sea Bisected North America: Geology Explained

A massive marine transgression during the Cretaceous period that connected the Gulf of Mexico to the Arctic Ocean, flooding the interior of North America (8:45). This seaway was hundreds of miles wide and thousands of feet deep, representing the last time the ocean touched this part of western North America.

A Cretaceous sandstone unit containing abundant oyster fossils that indicates a near-shore, shallow marine environment (5:45). The oyster-bearing sandstone marks the transition from freshwater coastal swamps to marine conditions as sea levels rose.

Thin coal layers found at the base of sandstone units indicate freshwater swamp environments near the coastline (2:45). The presence of coal tells geologists that this area was once a vegetated wetland before being inundated by marine waters.

A clay-rich marine unit deposited in deeper water conditions that overlies the Dakota Formation (9:00). Its gray color, lack of sand, and absence of oysters indicate that the sea had risen significantly, moving the shoreline westward.