Coming out of the shell

This is not exactly what it looks like! It's not a shell - well, it was a shell, but not for a long time..... This fossil was created when a shell was first filled with fine mud; then the shell was buried completely; then the calcium of the shell was slowly replaced and lithified (turned into solid rock) by a slow process of chemical replacement in the strata (the mud). Millions of years later, when the mudstone was eroded by the sea (on the south coast of England) this fossil was revealed. I picked it up on a geology field trip about 37 years ago. It now adorns our study window-sill - along with a pile of other rock samples!