Around 36 million years ago, during the Oligocene, this portion of Mississippi was a flood plain, with a large river running through stands of ancient Sequoia, Oak, Fir, Maple, and some other species of trees that are now extinct. At some point, the river flooded in a flash storm, and many of the trees were knocked down, tumbled downstream, and became packed together into a large logjam. This was then quickly buried by sand and clay. Over the next few million years, the wood in the buried logs was gradually replaced by minerals from the groundwater, turning them into stone. The overlying sediments were then slowly worn away by erosion, exposing the petrified wood at the surface.
The petrified logs were known at least as far back as 1854 (and both Union and Confederate soldiers camped here at separate times during the Civil War). In 1888, Frank H Knowlton of the US Geological Survey authored some papers describing several new species of fossilized trees. For almost 100 years the area was unprotected and was believed to have been abandoned, and freelance rock hunters came in and removed hundreds of tons of petrified wood.
In 1962 the land was purchased by the local Shabilion family, which still owns it. They put up a fence to protect the logs, and opened it to the public as a park a year later. In 1966 it was registered as a US National Natural Landmark.
Some photos from a visit.