The rocks exposed in Chaco Canyon record an interval in the Earth's history from approximately 85 to 75 million years ago during the Late Cretaceous Period. During this time, the region was situated at the edge of a shifting coastline of an ancient inland sea.
In the area of the Southern Rocky Mountains, mountain building activities produced a broad area of subsidence known as the Western Interior Basin, which flooded and created a vast epicontinental sea known as the Western Interior Seaway. By Late Cretaceous time, the Western Interior Seaway was hundreds of miles wide and extended from the Arctic Sea to the Gulf of Mexico dividing North America into two separate land masses. The shorelines of this epicontinental seaway were oriented generally north-south and repeatedly shifted position to the east or west in response to continuing changes in global sea level. This sea level change resulted in alternating marine and non-marine deposition of sediments as the sea transgressed (grew) and regressed (receded) over time