Pictured Rocks National
Lakeshore is a long, wonderfully scenic stretch of rocky cliffs along the
southern shore of Lake Superior near the town of Munising (see
map). The outcrops expose a succession of Proterozoic, Cambrian, and
Lower Ordovician sedimentary rocks. The rocks are most easily examined
from atop the cliffs and by boat.
The most distinctive
overlook at Pictured Rocks is Miner's Castle. The rocks here are
horizontally-bedded quartzose sandstones deposited in an ancient near-shore
marine to beach environment. This is the type locality of the Miner's
Castle Member of the upper Munising Formation (upper Middle Cambrian;
often misinterpreted as Upper Cambrian).
Above & below: Miner's Castle, with
quartzose sandstones of the Miner's Castle Member, upper Munising Formation,
upper Middle Cambrian.
The oldest rock unit exposed
at the surface in the Pictured Rocks area is the Jacobsville Sandstone.
Only the uppermost portions of the Jacobsville outcrop above lake level in this
area. The most easily examined Jacobsville outcrops here are at Miner's
Beach, at the mouth of the Miner's River.
The cliffs at Pictured Rocks
are almost entirely composed of shallow-water marine sandstones of the Munising
Formation (upper Middle Cambrian). The lower contact with the
Jacobsville Sandstone is a significant unconformity. The Munising at
Pictured Rocks consists of a lower Chapel Rock Member and an upper Miner's
Castle Member. The cliffs have extensive multicolored stains, the
result of seeping groundwater and surficial water cascading down cliff
faces. The colors are a principally a mix of stains from weathering and
the presence of organics. The cliffs vary in morphology from irregularly
vertical rock faces to steeply sloping and undercut cliffs. In places,
erosive wave action has produced arches.
The Chapel Rock Member has a
well developed basal conglomerate unit with cross-bedded quartz-pebble
conglomerates and pebbly sandstones (see the 2 photos below).