Sibley rock display: Cretaceous sandstone

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Sibley's rock collection: Cretaceous sandstone

Cretaceous sandstone:
ripped from ancient rocks by volcanic eruptions
and now occurring as “xenoliths,” or foreign blocks with the lava

This rock is displayed in “Sibley's Geological Treasures” collection on the roofed patio of the visitor center in the Robert Sibley Volcanic Regional Preserve. The above caption is the same that is given on-site (June 11, 2012).

Sandstone is a sedimentary rock composed of quartz and feldspar grains. These sand-sized mineral grains are typically held within a silt or clay matrix.

Cretaceous sandstone formed during the Cretaceous period between 145 to 65 million years (Ma) ago. Its inclusion as xenolith in lava happened during times of volcanic activity; for example, about 10 Ma ago at Sibley's.