The sedimentary layer of the Earth’s crust that formed these islands, a geologic stratum called the Goto Group, was composed of highly compressed sand, mud, and other materials. At the time it was being formed, it was under a shallow body of water. Eons of pressure from accumulating layers of mud and sand pressed these ripple imprints into this pattern.
The rocks we see here were originally part of that horizontal stratum, but deformations in the Earth’s crust bent this multilayered stratum downwards. Over time, the top layer was stripped off by wind or water erosion, exposing the ripple patterns we see here.
The site was declared a Natural Monument by Nagasaki Prefecture in 1959, but now that the surface has been exposed to the elements for many years, the ripple pattern has begun to fade.