A Breccia is a rock made up of angular rock fragments mixed with finer sediment. The one above was deposited about 450 million years ago (Ordovician) in a shallow sea in what is now the Ozark highlands, north of Mountain View, Arkansas.
Breccias can form by a variety of processes such as landslides, volcanic eruptions, storm events, cave and sinkhole collapses and others. This one likely formed during a storm. Turbulent ocean waves and currents washed fragments of marine organisms (dark gray) into calmer water where marine mud (light gray) had recently accumulated. The turbulence ripped some of the mud up and mixed it with the organic material.
Breccias are just one of many clues, recorded in rocks, that help geologists understand how the earth has changed through time: a story that itself constantly evolves as new rock is unearthed and studied.
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