wo tectonic plates known to us as the Farallon tectonic plate and the North American Plate. During the Cretaceous time, the Farallon plate subducted under the North American Plate. Subduction is the process were one tetonic plate, in this case the Farallon Plate, moves under another tetonic plate, the North American. The rate of this process is average of 2 to 8 centimeter per year to give you an idea of how long it would have taken to form the seaway. Basically, as one plate slid underneath, it pulled down an area or depression which could be filled with water, which we had high sea levels during this period.
http://almonsees.hubpages.com/hub/NorthAmericaInlandSea#