Siesta syncline

Opposite you is the beautiful Siesta Valley, bordered by the low hills in front of you. The valley is the central axis of the Siesta Syncline, and all the rocks in Sibley tilt down toward the valley bottom. The layers that underlie the far hills, on the other side of the valley, tilt down toward us. The rocks in the valley belong to the Siesta Formation; these valley sediments overlie the Moraga volcanics that were produced by Round Top and that we have examined throughout Sibley. The Siesta Formation is characterized by relatively fine sediments that were deposited in a large lake basin about 9.0–8.5 million years ago. When the lava flowed from Round Top, the blocking of creeks and small rivers created ponds and small lakes. During subsequent periods, tectonic movements within the East Bay fault system created the large lacustrine ecosystem that produced the Siesta Formation deposits.

Congratulations on the completion of this tour of Sibley!