Alum Rock Park mineral-stained rock outcrops

Alum Rock Park was named for natural alum and other mineral deposits that leach from springs and seeps within the park; iron- and alum-stained rock ledges are visible along several trails. In the late 19th and early 20th centuries the mineral springs supported a small health-resort industry and a bathhouse whose remnants remain in the park. The park exposes local bedrock of the Franciscan Complex and offers distinct, rust-colored staining where groundwater interacts with iron-bearing rocks.