Ironstone Concretions—teaching student specimens—set of 20 concretions from ancient soil near Torrey Pines State Park in Del Mar, San Diego County, California.  In the San Diego area, these dark brown to reddish brown ironstone concretions (commonly called "blueberries") formed in ancient soils on one of several ancient beach ridges that stood as topographic highs during an Ice Age interval from about 700,000 to 900,000 years ago (early to middle Pleistocene age).  These ancient soils probably formed during Ice Age glacial advances when San Diego’s climate was colder and wetter—more like Oregon and Washington today.  In this environment iron oxides, primarily in the form of the mineral hematite, precipitated out as a result of evaporating acidic groundwater moving downward through the soils.

The Mars Exploration Rover’s Spirit and Opportunity recently discovered similar ironstone concretions on Mars, and so a wetter climate once existed there too.  So, the ironstone concretions from San Diego County, California can be considered a Mars "blueberry" analogue.

Students should recognize concretions in general; they can range from smaller than a pea to over 6 feet in diameter. The size of these ironstone concretions, however, mostly vary from slightly smaller to slightly larger than a pea in diameter. These are babies, and textbook examples. For use in a classroom, these would fit in a large tightly corked test tube or screw-top culture tube if you wanted to pass them around.

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