Western Interior Seaway

The Western Interior Seaway (also called the Cretaceous Seaway, the Niobraran Sea, the North American Inland Sea, and the Western Interior Sea) was a large inland sea that existed during the mid- to late Cretaceous period as well as the very early Paleogene, splitting the continent of North America into two landmasses, Laramidia to the west and Appalachia to the east. The ancient sea stretched from the Gulf of Mexico and through the middle of the modern-day countries of the United States and Canada, meeting with the Arctic Ocean to the north. At its largest, it was deep, wide and over long.
Origin and geology.
The earliest phase of the Seaway began in the mid-Cretaceous period when an arm of the Arctic Ocean transgressed south over western North America; this formed the Mowry Sea, so named for the Mowry Shale, an organic-rich rock formation. In the south, the Gulf of Mexico was originally an extension of the Tethys Sea. By Late-Cretaceous times, Eurasia and the Americas had separated along the south Atlantic and subduction on the west coast of the Americas had commenced, resulting in the Laramide orogeny, the early phase of growth of the modern Rocky Mountains. The Western Interior seaway may be seen as a downwarping of the US continental crust ahead of the growing Laramide/Rockies mountain chain. In time, the southern embayment merged with the Mowry Sea in the late Cretaceous, forming the "complete" Seaway, creating isolated environments for land animals and plants. 
Relative sea levels fell multiple times, as a margin of land temporarily rose above the water along the ancestral Transcontinental Arch, each time rejoining the separated, divergent land populations, allowing a temporary mixing of newer species before again separating the populations.
At its largest, the Western Interior Seaway stretched from the Rockies east to the Appalachians, some wide. At its deepest, it may have been only deep, shallow in terms of seas. Two great continental watersheds drained into it from east and west, diluting its waters and bringing resources in eroded silt that formed shifting delta systems along its low-lying coasts. There was little sedimentation on the eastern shores of the Seaway; the western boundary, however, consisted of a thick clastic wedge eroded eastward from the Sevier orogenic belt. The western shore was thus highly variable, depending on variations in sea level and sediment supply.
 Widespread carbonate deposition suggests that the Seaway was warm and tropical, with abundant calcareous planktonic algae. Remnants of these deposits are found in northwest Kansas. A prominent example is Monument Rocks, an exposed chalk formation towering over the surrounding range land. It is designated a National Natural Landmark and one of the Eight Wonders of Kansas. It is located south of Oakley, Kansas.
During the late Cretaceous, the Western Interior Seaway went through multiple periods of anoxia, when the bottom water was devoid of oxygen and the water column was stratified.
At the end of the Cretaceous, a continuing uplift in a mountain-building episode called the Laramide orogeny hoisted the sandbanks (sandstone) and muddy brackish lagoons (shale), thick sequences of silt and sandstone still seen today as the Laramie Formation, while low-lying basins between them gradually subsided. The Western Interior Seaway divided across the Dakotas and retreated south towards the Gulf of Mexico. This shrunken, and final regressive phase is sometimes called the Pierre Seaway.
During the early Paleocene, parts of the Western Interior Seaway still occupied areas of the Mississippi Embayment, submerging the site of present-day Memphis. Later transgression, however, was associated with the Cenozoic Tejas sequence, rather than with the previous event responsible for the Seaway.
Fauna.
The Western Interior Seaway was a shallow sea, filled with abundant marine life. Interior Seaway denizens included predatory marine reptiles such as plesiosaurs, and mosasaurs that grew up to long. Other marine life included sharks such as "Squalicorax, Cretoxyrhina," and the giant shellfish-eating "Ptychodus mortoni" (believed to be  long); and advanced bony fish including "Pachyrhizodus", "Enchodus", and the massive long "Xiphactinus", larger than any modern bony fish. Other sea life included invertebrates such as mollusks, ammonites, squid-like belemnites, and plankton including coccolithophores that secreted the chalky platelets that give the Cretaceous its name, foraminiferans and radiolarians.
The Western Interior Seaway was home to early birds, including the flightless "Hesperornis" that had stout legs for swimming through water and tiny wings used for marine steering rather than flight; and the tern-like "Ichthyornis", an early avian with a toothy beak. "Ichthyornis" shared the sky with large pterosaurs such as "Nyctosaurus" and "Pteranodon". "Pteranodon" fossils are very common; it was probably a major component of the surface ecosystem, though it was found in only the southern reaches of the Seaway.
On the bottom of pervasive calcareous ooze, the adapted "Inoceramus" clams left abundant fossils in the Kiowa, Greenhorn, Niobrara, Mancos, and Pierre formations. There is great variety in the shells and the many distinct species have been dated and can be used to identify specific beds in those rock formations of the seaway. Many species can easily fit in the palm of the hand, while some like "Inoceramus (Haploscapha) grandis" could be well over a meter in diameter. In many cases, the shells retain a shiny or pearly luster. Most species' shells are flat, or nearly so, so as to float as a raft on the bottom mud; but a few, like "Inoceramus deformis" or immature "Inoceramus cuvieri" had deep curvature to float more like a boat. The shells of the genus are known for being composed of prismatic calcitic crystals that grew perpendicular to the surface. Given that the "Inoceramus" shells would be the only hard surface available in the seaway, the larger shells are often congested with oysters.