Graywacke forms in the seas near fast-rising mountains. The streams and rivers from these mountains yield fresh, coarse sediment that doesn't fully weather into proper surface minerals. It tumbles from river deltas down slope to the deep seafloor in gentle avalanches and forms bodies of rock called turbidites.
This graywacke is from a turbidite sequence in the heart of the Great Valley Sequence in western California, roughly 100 million years old. It contains sharp quartz grains, hornblende and other dark minerals, lithics and small blobs of claystone. Clay minerals hold it together in a strong matrix.
For more photos see the Sedimentary Rocks Gallery.