These are former sedimentary and volcanic strata that have been transformed by heat and pressure, possibly during the intense upheavals when the dry land was formed on Day 3 of Creation Week.
Among these metamorphosed volcanic strata are amphibolites, belonging to the Brahma Schist.
Deep inside the Inner Gorge of Grand Canyon, northern Arizona, are the crystalline basement rocks that probably date back even to the Creation Week itself.
Clearly visible in the canyon walls are the light-colored granites, such as the Zoroaster Granite, which are stark against the darker, folded strata of the Vishnu Schist and the other metamorphic rock units of the Granite Gorge Metamorphic Suite (see lowest purple and green shading in diagram).
Minnesota, however, has lots of rockhounding opportunities beyond Lake Superior Agates.