The Interior Plains were formed primarily through sedimentary deposition.

Over millions of years, sediments from eroding mountains and rivers accumulated in this area, creating a vast, flat landscape. This process was influenced by ancient seas that covered parts of the plains, leading to the formation of sedimentary rock layers. As tectonic activities and glaciation shaped the region, these sediments were compacted and altered, resulting in the fertile soils and flat terrains characteristic of the Interior Plains today.