Mudstone

This is a beautiful, quiet stop. Pause in front of the stone labyrinth and face the interpretive sign. Look up and to your left and notice that the the rock is the same as the basalt that you saw in the quarry pit. Now look up and to your right, and try to find the alluvial deposits of the Orinda Formation. Note the orientation of the layers: they were originally horizontal, but they have been tilted dramatically eastward. The Orinda rocks here are finer-grained than at the last stop, and they are more like a silty mudstone. This could either reflect a change in flow energy (for example, if the sediments were deposited at the edge of a channel rather than at the middle) or they could indicate a different sediment source. When the Moraga volcanics were active, the lava flows buried the alluvial deposits, and after the flows had cooled, they would be covered over by new, river-borne sediments. This cycle of inter-bedding, along with tectonic uplift and transpression, created the complex assemblage of rocks that we see today in this area. An important thing to remember when studying sedimentary rocks is that the vertical thickness of a rock layer does not necessarily correlate with how much time it represents. For example, do you think the basalts and mudstones were laid down at similar rates?