 Primarily formed after 1102 mya, the oldest formations are near Duluth, and the youngest to the northeast near Tofte.Insulated by the overlying roof rock, upwelling magma cooled slowly, and the mafic rock into which it cooled therefore is coarse-grained.These intrusions formed a sill some 16 km thick, primarily of gabbro, but with significant amounts of anorthosite and other related granitic rocks.