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.



Home page