Have you ever admired the cliffs along the North Shore of Lake Superior, fished off a smooth outcrop in the Boundary Waters Canoe Area, overlooked one of the immense iron mines on the Mesabi range, or noticed the abundant knobby outcrops within the Minnesota River Valley? If so, you were looking through windows of younger material into the very old bed- rock, commonly referred to as "ledge-rock", that underlies all of Minnesota. Minnesota is situated at the southern edge of the Canadian Shield-the nucleus of the continent of North America that formed during Precambrian time. This period of time encompasses about 85% of earth's history. Geologists consider Precambrian time to have begun with the formation of planet earth about 4,550 million years (m.y.) ago and to have ended about 570 m.y. ago, when organisms with hard parts, such as shells, first appeared. The rocks formed in Minnesota during this enormous span of time record a complicated geologic history that involved volcanoes, ocean islands, mountain chains, and unstable geologic conditions that were very different from the Minnesota of today. Precambrian Minnesota resembled modem-day Indonesia for a while; later, it resembled modem-day California, and still later it resembled parts of the Middle East and eastern Africa.