conglomerate derived from the Franciscan-Knoxville sequence - teaching hand specimen
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The Mesozoic Franciscan-Knoxville sequence is a series of mostly metamorphic rocks characterized by serpentine, various cherts, shale and graywacke from which this conglomerate is derived. This accounts for the overall greenish hue of the clasts. "Clast" is derived from the Greek klastos, for broken, and refers to the rock fragments that make up sedimentary rocks.
This specimen was collected from the bed of Perkins Creek under the assumption that any conglomerate that had gotten that far would withstand student examination in a classroom. The source formation is uncertain.
The clasts in this conglomerate, originally loose pebbles in a stream channel, were stranded when the channel changed position, became buried, lithified into a conglomerate which was then uncovered, weathered and eroded. This chunk of conglomerate was then carried down the Perkins Creek drainage as a clast of conglomerate that is composed of smaller clasts, much like the flea having fleas. There is amazing variety in the rock types that make up this conglomerate. Many of the clasts are chert. Their angular nature indicates they were not carried very far before being trapped, buried and lithified to form this conglomerate.
This specimen is cut on one side and not polished.
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