These web pages will help familiarize you with the formation of Lake Agassiz or as it is called today, the Red River Valley. The photos here can be clicked on to reveal a larger version. I am not by any sense a geological expert, this web page is a term paper for Geology 304 a course available every fall a North Dakota State University.
As you can see by the first photo the Red River Valley is extremely flat, this is because the region was at one time a large glacial lake. Lake Agassiz has long since drained and we are left today with a lake-bed wherein lies some of the most fertile soil in the world. As the weather warmed, the glaciers receded allowing plant and animal life to flourish leading to the deep rich deposits on the floor of the lake that we see today.