Ridged fields - an agricultural practice to insulate crops from the cold (in Wisconsin covered by snow)?

Gallagher, J.P. 1992 Prehistoric Field Systems in the Upper Midwest. In Late Prehistoric Agriculture, edited by W. I. Woods, pp. 99-135 Studies in Illinois Archaeology No. 8, IHPA. Fig 4.1, p.97

Image courtesy of Illinois Historic Preservation Agency.