The Western Interior Seaway, stretched from the Arctic Ocean to the Gulf of Mexico. During the Cretaceous the Seaway was very dynamic, it's levels and boundaries shifting constantly, depositing cyclical sequences of sediments that geologists call cyclothems.

The Appalachian Mountains to the East rose during the Triassic Period and were by this time, already old and worn down by erosion. They contributed little to the influx of sediments to the Seaway.

The situation on the western side of the continent was very different. The Rocky Mountains were newly formed. They towered high above the land and were accompanied by very violent volcanic activity. Large volumes of sediments were carried by streams, rivers and wind to the Seaway.