In the Western Interior Basin center, the Bridge Creek Limestone Member of the Greenhorn Formation is of late Cenomanian to early Turonian age and consists of alternating limestone and marl beds. Here the member has been divided into six ammonite biozones. The Bridge Creek Limestone represents transgression, with the upper half representing maximum Greenhorn high-stand. As transgression proceeded, ocean water from the Tethys Sea progressed north, west and east into the seaway. Many central and western stratigraphic sections have been extensively studied, but much less is known about the eastern margin of the Greenhorn Sea. Recently we obtained core samples from the Iowa Geologic Survey/United States Geologic Survey, cored in northwestern Iowa, near Hawarden. We are using planktonic foraminiferal porosity to interpret the sea level history at this eastern site. Planktonic foraminiferal porosity has been shown to be an effective proxy for water mass density. Many species of Modern planktonic foraminifera construct shells of greater porosity when grown in warmer/less saline, less dense water; whereas, they construct less porous shells when grown in cooler/more saline, denser water. Previous work in Pueblo, Colorado and Hot Springs, South Dakota has shown that during the late Cenomanian and early Turonian, sea level rise brought in warmer Tethyan water and was recorded by an increase in porosity; whereas, a relative sea level fall was recorded by a decrease in porosity. Our eastern record is compared to these central seaway sites.