Have you ever seen imaginary objects in cloud shapes? That's what's so creative about our human minds: We can look at them and imagine we see animals, a pirate ship or Elvis.
It works when you look down, too. You might see round, ovate or bumpy objects protruding like big goose-bumps from their encasing rocks. These are concretions. To some, concretions resemble fossilized eggs, marbles, cannonballs — or Elvis.
Concretions grow outward like Godzilla, enlarging through time (thousands of years) to consume places like Tokyo. Just kidding. Concretions form in rocks permeable enough to allow slow percolation of ground water carrying dissolved minerals (think "hard water"). They require a "seed" such as a fossil shell or similar object on which to grow. The microgeochemical environment around the seed results from tissue decay or has a chemical composition similar to the dissolved material in the slowly percolating water. Often the seed is dissolved during formation of the concretion, so few contain fossils as buried treasure at their centers.
Concretions exhibit a wide variety of compositions, the most common being calcium carbonate (calcite). Calcite, when scratched and doused with vinegar, releases carbon dioxide bubbles. Don't worry about contributing too much to global warming: soda pop burps produce more carbon dioxide.
Silica (solidified amorphous quartz) forms harder concretions. Bulbous silica concretions, mixed with orange-weathering iron minerals, are found in the Agate Desert northeast of Medford. Black silica flints found in the white Chalk Cliffs of England were concretions used by Neolithic peoples for tools and later, during the "Enlightenment," for flintlock guns.
The more permeable the surrounding rock, the larger the concretion will be. A place to see dark-gray, cannonball-sized calcite concretions in permeable sandstone is Eagle Mill road opposite Ashland's water treatment plant. Orange to dark-gray, egg-sized concretions are found in less permeable dark-gray shale at low lake levels near the cemetery at Emigrant Lake.
Septarian concretions are characterized by radial cracks that form in concretions when they dehydrate. The cracks are filled with crystalline cement, often calcite. One imaginative citizen brought in such a concretion firmly convinced its convoluted surface represented a petrified brain. I suggested petrified Jell-O. He wasn't amused.
Concretions are out of this world! Scientists studying images from the Mars Opportunity Rover called them "blueberries" (for their resemblance to blueberries in muffins). In places like Zion Park, Utah, hard concretions weather out of softer rock to accumulate in depressions. Picture, if you will, long-vanished giants playing a game of marbles with those concretions. Isn't imagination a wonderful thing?
Jad D'Allura is emeritus professor of the former Southern Oregon University Geology Department. Reach him at email@example.com.