Stop 10

Retrace your steps to the Antarctica rock and continue left to the corner of Building 1, turn right at the corner of Building 1 and continue walking until you are almost to the courtyard of Building 2. On your right is a large specimen of blueschist, a metamorphic rock collected in Marin County, located north of San Francisco. The minerals that make up the rock, including small garnets, and the folded structure of the rock show evidence of immense pressure when the rock was being formed about 160 million years ago. A polished slice of the same rock at its base shows off the highly folded structure. Its name reflects the abundance of a blue mineral called glaucophane; its glittery look is from muscovite mica. Laboratory experiments tell scientists the temperatures and pressures under which these minerals are created, and, in turn, those minerals help scientists reconstruct the geologic setting in which the rock was formed. The presence of blueschist in Marin County is a clue to the tectonic history. Before there ever was a San Andreas Fault, California was the site of a subduction zone, where one of the Earth’s major tectonic plates was being subducted, or consumed underneath the North American Plate.

To the right of the blueschist is a most unusual tree, the Dawn Redwood (Metasequoia glyptostroboides). This is a deciduous conifer that was common in Asia and North America until a few million years ago. This species of tree was believed to be long extinct by western scientists until a small grove was discovered in China in 1946. Since that time, seedlings have been planted in various parts of the world, and we feel privileged to have one of the trees on the USGS campus. Unlike the two California species of redwood, the needles of this Dawn Redwood turn brown in autumn and fall from the tree, to be replaced with new foliage each spring.

To the left of the blueschist are two Giant Sequoias (Sequoia gigantea) or mountain redwoods. They are planted in soil brought here from their native environment on the western slope of the Sierra Nevada. The native soil is crucial to their growth and health because Giant Sequoias prefer glaciated soil.

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