The Western Interior Seaway was a large inland sea that split the continent of North America into two landmasses. The ancient sea, which existed from the early Late Cretaceous to the earliest Paleocene, connected the Gulf of Mexico to the Arctic Ocean. The two land masses it created were Laramidia to the west and Appalachia to the east. At its largest extent, it was 2,500 feet (760 m) deep, 600 miles (970 km) wide and over 2,000 miles (3,200 km) long.
A broken concretion with fossils inside; late Cretaceous Pierre Shale near Ekalaka, Montana.
Monument Rocks (Kansas), located 25 miles south of Oakley.
Artist's impression of a Cretoxyrhina and two Squalicorax circling a dead Claosaurus in the Western Interior Seaway
Inoceramus, an ancient bivalve from the Cretaceous of South Dakota.
The Late Cretaceous is the younger of two epochs into which the Cretaceous Period is divided in the geologic time scale. Rock strata from this epoch form the Upper Cretaceous Series. The Cretaceous is named after creta, the Latin word for the white limestone known as chalk. The chalk of northern France and the white cliffs of south-eastern England date from the Cretaceous Period.
Asteroids of only a few kilometers wide can release the energy of millions of nuclear weapons when colliding with planets (artist's impression).