What is this earthen wall?

What is this earthen wall?. . Captain DeLozier Davidson and his soldiers from Fort Randall constructed this military fort in 1859. It served as a supply post for Fort Wadsworth, now called Fort Sisseton. Soldiers manned the fort, also called a breastwork, until December 1864 when it was established as one of 20 American Indian scout camps that served as warning outposts for Fort Wadsworth., It was reported that a building about 16 feet wide and approximately 35 feet long stood at the center of the enclosure. Later, a cabin used as the area's first Norwegian school was moved to the fort site., The "Legend of Tetonkaha" is a poem about a young American Indian who lost her life trying to save her soldier friend at the breastwork from attack by her tribe., Oral history relates that a battle was fought at Oakwood Lakes between Dakota Sioux warriors and white soldiers shortly following the New Ulm Uprising in 1862, lending some historic basis to the "Legend of Tetonkaha.", Erected in 2008 in memory of Cal Halvorson (1942–2006) , Hunter and fisherman. Husband, father, grandfather of Norma, Russell, Rachel, Nate and Elizabeth. , By the Schultz and Larsen Families