Late Precambrian records in Minnesota again show volcanic and sedimentary activity. The Penokean mountains eroded, producing deposits of quartz sand grains now found in the thick Sioux Quartzite of southwestern Minnesota, dating from about 1,600 million years ago. Later quartzite formations can be seen as Nopeming Quartzite in the Duluth area, and the Puckwunge formation, near Grand Portage. Beginning about 1,100 million years ago, intense volcanic activity took place for the following 20 million years along a great continental rift from eastern Lake Superior to Kansas. Lava that reached the earth's surface cooled in huge overlapping flows, found now along the north shore of Lake Superior mostly as dark-colored basalt.
Lake Superior agates formed in the cavities left by gas escaping the cooling lava.
Sugarloaf Point SNA, on the North Shore, protects a prime example of volcanic formations dating from this period. After the rifting and outpouring of lava suddenly stopped, the rift zone continued to sink. Flowing water deposited thousands of feet of sandstone along this zone extending from Lake Superior to the Hinkley-Kettle River area, onward to the Twin Cities. The Post-Precambrian period featured advancing and retreating seas, development of plant and animal life forms, and lots of sediment. The best evidence of this period lies in southeastern Minnesota, where fossils of early marine plant and animal life abound in sandstone, limestone, and shale formations.