It’s not difficult to find fossils in Iwamura. The soil is mostly red clay. Under a metre or two of that there’s a chalky layer, and wherever it’s exposed you can find fossils. I know a river not far from here that has a fossil layer about 17 million years old. Maybe the layer in Iwamura is a similar age.
We’re at an altitude of about 550 metres. The land must have risen quickly, since it looks like the rock has had very little time to compact. You can easily break it open with your hands. Get it wet and it crumbles. Apparently it’s rich in minerals and in places is quarried as a fertiliser.
I wonder what allowed the fossilised shells to persist while all the others were crushed into rock around them.