Castle Rock is a bluff of exposed St. Peter Sandstone from the Middle Ordovician period.

Newly exposed sections are white quartz, while older areas have browner tints from the formation of limonite.

The bluff is along the axis of the Sandwich Fault Zone, separating Ordovician exposures from those of the Cambrian period.

[1] Castle Rock State Park was one of eleven state parks slated to close indefinitely on November 1, 2008, due to budget cuts by former Illinois governor Rod Blagojevich.

[4] In March 2009 Quinn announced he is committed to reopening the state historic sites by June 30, 2009.