Fossilized Shells Preserved in Rock
Along a seaside road on Naru Island, an outcropping of sharp rocks juts up from the ground at roughly 45 degrees. Even to the untrained eye there are clearly defined layers of different types of rock. But it is what lies half-buried within these rocks that is most interesting and is the reason this site is recognized as a natural monument by Goto City.
Shells Trapped in Rock
On many of the flat surfaces, both vertical and horizontal, you can see shells, the same kinds you might expect to find at a beach. There are spiral shells and flat (bivalve) shells, many small ones of both types and a few larger bivalves. Finding fossilized shells buried within such rocks is unusual to begin with, but these shells are the remains of freshwater shellfish.
The evidence of these fossilized shellfish means that the layers of sand and mud that formed this rock stratum were at some point in the past under a body of fresh water. Similar shell fossils have been found in nearby sites, not only in the Goto Islands, but also in parts of northern Nagasaki Prefecture.
As a result, scientists suggest that the strata of dirt and sand that formed this land were once at the bottom of a shallow freshwater lake or river that stretched for approximately 100 kilometers.