Nearby mineral springs are evidence of recent local volcanic activity.
The Soda Springs lie on a gravel bar in the San Joaquin River just north
of the Postpile. Here, gases driven upward from hot areas deep in the
earth combine with ground water to produce cold and highly carbonated
mineralized springs. The iron present in Soda Springs water oxidizes on
exposure to the atmosphere and stains river gravel a reddish brown.
[Illustration: uncaptioned]