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.
