rock, water, solid, fluid….

November 28, 2010 § 2 Comments

Sandstone formation at Devil's Punchbowl, Pearblossom

Recently,  I accompanied Altair Maine’s high school Geology class to the Devil’s Punchbowl formation in Pearblossom.  At Devil’s Punchbowl, the San Andreas fault forks into two branches, one of which goes right through the Punchbowl. Rock formations one side of the bowl angle toward the east. But on the other side of the fault, they angle toward the west. A tiny stream winds through the bottom of the bowl.

Mr. Maine showed us how the make up of the boulders around us reveals patterns caused by water flow ages ago. Layers of fine sediments were laid down during a period of peaceful steady water flow. Layers of coarse material such as large rocks, were deposited by distant flood events. All of the huge boulders around us revealed periods of peaceful water flow alternating with flood events.

Pattern of deposition of sediment in streambed at Hahamongna

One could even read the direction that water once flowed. When there are rocks in a stream, a hollow is eroded upstream from the obstacle, while a characteristic trailing bump forms downstream from it. With a practiced eye, one finds the same patterns embedded in sedimentary rock.

Over the past thirteen million years, these sediments once deposited by stream flow formed under pressure into rock. Thanks to relatively recent seismic events, all these layers of natural history were revealed like an open book to our tour group. I left with a sense of wonder at the relationship between things we think of as solid, and the things we think of as fluid.

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