[SunkenCiv]

Mine was in reference to the idea that logs sink.

[Nikas777]

Logs do sink.

[ApplegateRanch]

It was wetter, until the logs soaked up the water, and sank, making the area drier...until the logs dried out, refilling the lake and starting the cycle all over again. This went on for centuries...perhaps millennia...until the logs finally rotted to the point that they no longer functioned.

This is why logging is an environmental disaster that leads to global desertificational flooding; and why saw mills are always associated with ponds full of both floating and sunken logs.

You need a carefully controlled mix of both dry and waterlogged logs to maintain a mill pond, without it either flooding or drying out.

Obviously these Paleolithics hadn't yet discovered that fact.