 Those two Middle Precambrian ranges are thought to have once been joined, but intruding magma of the Duluth Complex baked and engulfed the center of the mountain chain, separating it into the two ranges present today, as shown in the image at the top of this page.To the east, the complex abuts and intrudes into the Rove Formation, an older structure of sedimentary rocks.Gabbro and diabase structures of the Duluth Complex trend generally from southwest to northeast, and the differential erosion has left a series of ridges comprising these harder mafic rocks rising from the softer sedimentary rocks of the Rove Formation.