Orinda Formation

Not all of the rocks in Sibley are volcanic in nature. The exposure here shows layers of cobblestone, sandstone, and siltstone that would have been deposited by rivers running down an alluvial plain towards the paleo-bay to the east. We know the direction of water flow because the individual cobbles in these rocks match the geology of the Franciscan Complex found in San Francisco and the Marin Headlands. Franciscan rocks formed during the subduction of the Farallon Plate under California in the Mesozoic Era (Age of the Dinosaurs). These rocks were uplifted into coastal mountains in the Miocene, and some of the smaller eroded fragments were carried by the river to this spot. The coarseness of the cobbles demonstrates that the ancient river flow was energetic, a common feature in modern alluvial fans with big, fast rivers. These alluvial sedimentary deposits are part of the Orinda Formation, the layer that underlies the Moraga. The environment of the Orinda persisted through the time of Round Top’s eruptions. Note the baked, red color at the top of the layer—this was caused by contact with the hot Round Top lava, and it is something we can study on future stops.