These are the remains of building pillars, likely made out of chestnut wood.

Chestnut wood is a valuable resource for building-- it hardens when it dries, it’s durable, and takes a long time to rot. You can also grow chestnut trees rather quickly, and you can enjoy the nuts while it grows. These reasons are why you can find many chestnut forests around the Jomon archaeological sites.

So what was the building with the standing pillars used for back then? Honestly, we have no idea. The Ōyu Stone Circles also has remains of their own “Pillar-Supported” building, so let’s think about these two mysteries together as a pair.