undifferentiated rock units

Paleontology and geology

Through the Cretaceous, subduction of oceanic crust in the west continued to uplift the Rocky Mountains, and erosion continued to strip away sediments. In the Early Cretaceous, enough sediment had eroded off the mountains to fill in the shallow seas of the Jurassic, and swamps and forests covered the low-lying valleys. By the Middle Cretaceous, the accumulated sediments weighed down the crust, and the Western Interior Seaway spread over much of Alberta. But by the end of the Cretaceous, the uplift of the mountains and build-up of sediments caused the seas to drain away again. Ammonite and plesiosaur fossils have been found in deposits from the Seaway, but the sandstones and shales that formed in large rivers and coastal areas preserve Alberta’s most spectacular fossils. Sharks, rays, and many different kinds of bony fish swam in the waters. Frogs, salamanders, lizards, turtles, and crocodiles could be found along the edges. Pterosaurs and early birds flew in the skies, and early mammals lived in forests of sycamore, bald cypress, magnolia, and Metasequoia. Insects like beetles and flies, as well as spiders are preserved in fossilized tree sap (amber). The dinosaurs included ceratopsians, pachycephalosaurs, hadrosaurs, ankylosaurs, dromaeosaurs, and tyrannosaurids.