During the Late Cretaceous, a shallow seaway that extended from the Arctic Ocean through to the Gulf of Mexico bisected North America. This seaway was known as the Western Interior Seaway.
By the Late Cretaceous, Laurasia and Gondwana were almost completely fragmented and sea level was elevated worldwide.
Close up: North America, Western Interior Seaway.
The climate was warm and subtropical with seasonal rains.
Alberta, which lay around the western shore of the seaway, was criss-crossed by rivers, estuaries, swamps and deltas that all ultimately drained eastward into the shallow sea (Eberth, 1997).
The region was slowly sinking due to tectonic forces adding new land masses onto the western margin of British Columbia and uplifting of the Rocky Mountains. As a result, vast amounts of sediment (and fossils) accumulated in this setting (Eberth, 1997).
Over the course of many tens of millions of years, the Western Interior Seaway repeatedly expanded and contracted, leaving a record of advancing and retreating shorelines in the form of layers of alternating marine and non-marine sediments and fossils (Eberth, 1997).