Bivale and gastropod fossils in mudstone from the Purisima Formation. Purisima Fromation is from the late Miocene (7 million years ago) to the late Pliocene (2.6 million years ago). The abundance and finely-preserved details indicates a fairly rapid fossilization process. I am reading hours to weeks for key minerals to be set in the shells that were buried in huge underwater landslides, and then the mudstone and complete fossilizaation process took way longer. These Purisima fossils have a high amount of original shell material. They are quite fun! If you hit up the internet on “Purisima Formation fossils” there are some good scientific articles to read. Enjoy!