Pictured Rocks National Lakeshore - Geology

Geological history recorded in the sedimentary rocks and surficial deposits of Pictured Rocks National Lakeshore (PRNL) is limited to two widely separated intervals of geologic time, the Late Precambrian, Cambrian, and Early Ordovician Periods (500-800 million years before present) and the Late Quaternary Period (2 million years before present to the present). During the Cambrian and Early Ordovician periods, sediments were deposited in the shallow seas and near-shore deltas that covered what is now northern Michigan. These deposits became the sandstone units that are exposed within PRNL. Except for their exposure near Lake Superior, these units are presently covered by a veneer of Quaternary glacial drift.

Bedrock is best observed in the western one-third of the lakeshore where cliffs rise up to 30 m (180") from Lake Superior. These extend along the lake about 27 km (17 m) from Munising to Beaver Basin. For a short distance inland from the escarpment, bedrock is occasionally exposed.

The Jacobsville Formation, of Late Precambrian age, is the oldest formation exposed in the lakeshore. It is a fluvial/lacustrine, feldspar-rich quartz sandstone, deep red in color with white mottling. Only the top few feet of the formation rise above lake level within PRNL (e.g., vicinity of Au Sable Point). This formation was quarried for building stone in the late 19th century. The western side of Grand Island, just west of PRNL within the Hiawatha National Forest, features spectacular Jacobsville cliffs.

The Mid to Late Cambrian, light grey to white Munising Formation lies unconformably above the Jacobsville. The Munising Formation probably represents a complex shoreline/shallow water environment that was influenced by fluvial, wave, tidal and aeolian processes. The Munising is divided into three members: the basal conglomerate, the hard Chapel Rock sandstone, characterized by large, sweeping cross beds and the crumbly Miners Castle sandstone.

Capping the easily eroded Miners Castle Member of the Munising Formation in the western half of the Pictured Rocks, is the resistant Early Ordovician Au Train Formation. The Au Train Formation is a light brown to white dolomitic sandstone that lies above the distinctive caprock above the lip of Munising and Bridalveil Falls.

Fossils are completely absent from the Jacobsville Formation and uncommon in the Munising Formation; fragments of trilobites have been found in the Miners Castle member. The Au Train contains Middle Ordovician cephalopod and gastropod fossils.

Structurally, PRNL lies along the northern edge of the Michigan Basin, thus sedimentary bedding dips gently toward the south. Strata also rise very slightly eastward in the vicinity of PRNL so that the Jacobsville, which is below lake level at Miners Castle, is well exposed to the east of Hurricane River Campground, at Au Sable Point, and in the gorge at Sable Falls.