10/27/2026
This month’s 𝑊𝑜𝑜𝑑𝑒𝑑 𝑊𝑎𝑦 features Cooper’s Woods Preserve on South Bass Island in Ottawa County.
Cooper’s Woods protects several small, shallow caves that formed in the Silurian Bass Islands Dolomite. Underlying the Bass Islands Dolomite is the Salina Group, which is composed of multiple anhydrite beds. When exposed to water, anhydrite swells as it is converted to gypsum, increasing the volume by at least 33%. This swelling forced the surrounding rock layers to bulge and separate, leaving behind voids when the gypsum was eventually dissolved. With nothing left to support the displaced rock, these layers collapsed and created the caves seen in the area.
Please do not enter any caves so these sensitive features can be preserved for the future.