Beginning in the Early Cretaceous, the Cretaceous Seaway spread south from what is now the Arctic Ocean and connected with a short northward extension from the Gulf of Mexico. This marine transgression of the ocean onto what was formerly land, was completed by the late Albian (~100 MA) thereby dividing North America in half. On the eastern side of the Seaway, sediments that would become the Dakota Formation were deposited as coastal and nearshore marine sands and silts. As the seaway continued to deepen and widen, this eastern shoreline moved progressively eastward throughout the Cenomanian. Meanwhile, on the western side of the seaway, sediments were carried eastwards and northeastwards by rivers from mountains located along what is the Nevada-Utah border.