CASTLE ROCK LAKESTACK
Castle Rock is a prominent column of
rock located north of the town of St. Ignace, at the southern tip of the
eastern end of Michigan's Upper Peninsula (UP) (see
map). It is an old lakeshore stack, formed by shoreline erosion
during the existence of post-glacial Lake Nipissing (= the name for the
combined Lake Superior-Lake Michigan-Lake Huron basins about 4000 years ago,
with a higher common lake level than Superior has at present). Most
lakestacks are offshore features, completely isolated by wave action.
Castle Rock Lakestack directly abuts a Lake Nipissing shoreline cliff,
indicating Castle Rock was not originally an offshore, isolated stack.
Many other paleo-lakestacks can be observed on Mackinac Island and in the St.
Ignace area.
Castle Rock paleo-lakestack.
Castle Rock paleo-lakestack.
Castle Rock itself is made
up of dolostone block breccias - the Mackinac Breccia (upper Lower
Devonian to lower Middle Devonian). At Castle Rock, the unit consists
principally of jumbled broken blocks of dolostone derived from the St. Ignace
Dolomite (upper Upper Silurian). The breccia represents deposition from
many collapse events in paleocaverns of dissolved-out Salina Group evaporites
(Upper Silurian). Post-Silurian collapse of dissolved-out Salina
evaporites was a widespread phenomenon throughout the Michigan-Ohio region.
Castle Rock paleo-lakestack - looking
down from atop the pillar.
For more info. on lakestacks
and other shoreline features in this area, see:
Dorr & Eschman (1970) - Geology
of Michigan, pp. 208-213.
For more info. on the
Mackinac Breccia, see:
Landes (1959) - The Mackinac
Breccia. pp. 19-24 in Geology of Mackinac Island and
Lower and Middle Devonian South of the Straits of Mackinac. Michigan
Basin Geological Society.