Pierre Shale
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The Pierre Shale is a geologic formation or series in the Upper Cretaceous which occurs east of the Rocky Mountains in the Great Plains, from Pembina Valley in Canada to New Mexico.
A broken concretion with fossils inside; Late Cretaceous Pierre Shale near Ekalaka, Montana
Baculites from the Pierre Shale showing sutures and remnant aragonite; western South Dakota, Late Cretaceous.
Image: Hesperornis BW (white background)
Image: Ichthyornis restoration
Aragonite
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Aragonite is a carbonate mineral and one of the three most common naturally occurring crystal forms of calcium carbonate, the others being calcite and vaterite. It is formed by biological and physical processes, including precipitation from marine and freshwater environments.
Aragonite from Los Molinillos, Cuenca, Spain, sample width about 4 cm
Aragonite crystals from Cuenca, Castile-La Mancha, Spain
Aragonite crystal cluster from Spain