Nonconformity, Kehoe Beach, Point Reyes National Seashore, California


ID 2996

The Middle Miocene Laird Sandstone rests nonconformably upon Cretaceous granitic rocks, which are exposed in the foreground during a low tide. A nonconformity forms the boundary between the older granitic rocks and the younger layered sedimentary rocks. 

The granitic rocks originally crystallized beneath the earth's surface in a magma chamber, before being uplifted and eroded. In contrast, the basal part of the Laird Sandstone was reworked by waves and deposited in a shoreline setting. The Laird Sandstone contains numerous fossil fragments and rare entire specimens. The fossils include barnacles, bryozoa, "sand dollar" echinoderms, and molluscan shell fragments. 

The Laird Sandstone is transitional into the overlying Monterey Formation (background). The Laird-Monterey section records progressive deepening after transgression of the middle Miocene seas.


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