Tuffs and lava

The sequence of rocks at this stop tells a dramatic story. The dark basalt at the bottom of the sequence is overlain by light-colored tuff, the layers of ash that settled on the hot lava following an eruption. Like the Orinda rocks we saw earlier, the tuffs here are baked red by the heat of the lava. The characteristic red color may be due to the presence of water or steam at the contact horizon. If you look carefully just above the transition from basalt to tuff, you should note a thin black layer. This likely represents a secondary lava flow that occurred as the ash was still settling. Above that are even thinner dark bands: these could represent additional lava flows, or they could be creek bed deposits produced after the eruption.