What is most interesting here is not simply this outcropping of rocks, but what lies half-buried within them, something significant enough to make Goto City designate these rocks a natural monument.
On many of the flat surfaces here you can see shells—spiral shells and flat (bivalve) shells. Fossilized shells buried within rocks are unusual to begin with, but these particular shells are from freshwater shellfish. That means the sand and mud layers that created these rocks were at some point under a body of fresh water when Japan was separated from the Asian mainland tens of millions of years ago.
Similar shell fossils have been found in nearby sites in other parts of Nagasaki Prefecture, leading scholars to believe that the strata of dirt and sand that formed this land were once at the bottom of a shallow freshwater lake or river on the continent. That makes this site of particular interest not only to geologists, but also to travelers who might want to pause a moment to look at these shells from far away in both time and place, and ponder how they got here.