Fossil-Bearing Concretions From Fossil Insect Canyon

Middle Miocene Barstow Formation--17 Million Years Old

Mojave Desert, California

Here are several freshly dug world-famous calcareous concretions from microlaminated green shales exposed in Fossil Insect Canyon, Mojave Desert, California. The calcium carbonate (limestone) concretions yield up a genuine bonanza of fantastically preserved three-dimensional insects, spiders, water mites, fairy shrimp, copepods (a tiny crustacean, of which Cyclops is the best-known freshwater type), ostracods (a minute bivalve crustacean), cladocerans (a small crustacean often called a "water flea"), and diatoms (microscopic single-celled plant). In order to secure the fossils, the concretions must be dissolved in a dilute solution of acid; many collectors prefer to use a gentle organic variety, such as formic acid, or even acetic (glacial acetic--not the kind found in vinegar)--although excellent results can also be obtained with muriatic acid, commercially available, highly potent hydrochloric acid. The mineralized fossil specimens, impervious to such acids, preserved as they are by chalcedony quartz (silicon dioxide) and celestite (strontium sulfate), primarily, can then be recovered from the insoluble residues remaining from the chemical reaction.

Several potentially fossiliferous calcareous concretions have weathered out naturally from the greenish shales of the middle Miocene Barstow Formation, exposed in Fossil Insect Canyon, Mojave Desert, California. Not every such concretion collected from the Barstow outcrops yields fossil arthropods; in general, they're anywhere from 20 to 75 perfect fossiliferous, depending on which of the numerous concretions beds one explores.

Here is a chunk of microlaminated, varved, shale from the middle Miocene Barstow Formation of Fossil Insect Canyon, Mojave Desert, California with a special story to tell. Still embedded in place (geologists refer to this naturally occurring situation as in situ) lies a potentially fossil-bearing calcareous concretion 12 millimeter in diameter (one-half inch). One can observe how the sediments on all sides of the concretion have been distorted, pushed, out of their natural planes of sedimentary deposition to conform to the concretion preserved within them. This provides excellent evidence that the concretions originally grew within sediments that had already been deposited, only later expanding and warping the soft layers around them.

Fossil-bearing calcium carbonate (limestone) concretions come in a wide variety of shapes and sizes within the middle Miocene Barstow Formation of Fossil Insect Canyon. From left to right: (1) A spherical type (30 millimeters in diameter--an inch an a quarter); (2) A mushroom-like variety (35 millimeter diameter--about an inch and three-eights); and (3) what some folks call a 45 caliber slug-style Barstow conretionary creation (30 millimeters long--around an inch and a quarter).

This is what many "Barstow bug" enthusiasts refer to as a "mammillary"-style concretion. It's 45 millimeters in diameter--an inch and three-quarters--from the Middle Member of the middle Miocene Barstow Formation, Fossil Insect Canyon, Mojave Desert, California.

Here are three Barstow concretions lots of visitors to Fossil Insect Canyon call "derby hat stones." All varieties of Barstow Formation concretions typically range from 8 millimeters (about a quarter inch) to roughly 75 millimeters (three inches) in diameter. From left to right, in diameter: 47 millimeters (almost two inches); 48 millimeters (almost two inches); and 40 millimeters (one and a half inches).

Three oblong-style concretions from the middle Miocene Barstow Formation of Fossil Insect Canyon, Mojave Desert, California--all three 50 millimeters long (two inches).

Here are two of the largest Barstow concretions I've observed in Fossil Insect Canyon--each is almost two and a half inches in diameter--that is, 60 millimeters. At left is a hemispherical variety--at right, a disc. I have yet to dissolve them in a diluted acid bath, though, wishing instead to preserve them as examples of nature's artwork.

Three additional potentially fossiliferous concretions from the middle Miocene Barstow Formation exposed in Fossil Insect Canyon. At left is an elliptical specimen, 45 millimeters long (one and three quarters inch, roughly); center, a sphere--35 millimeters in diameter (just under one and a half inch); at right, a modified sphere, with a partial "mushroom button"-- 40 millimeters in diameter (slightly over an inch and a half).

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