Sibley rock display: conglomerate from Orinda Formation

Custom Search

Sibley's rock collection: conglomerate from Orinda Formation

cemented river gravel from Orinda Formation

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).

Conglomerate is a sedimentary rock composed of water-worn rock or pebbles that are held within a finer-grained matrix of siliceous (silica-containing or silica-resembling) or argillaceous (clayey) substances.

In contrast to breccias, which contain angular fragments in their matrix, conglomerates consist of matrix-embedded rounded fragments.