Specimen Ridge forms the south side of the Lamar River valley and is the location of layers of mudflow conglomerates, tuffaceous sandstones, and lacustrine mudstones from repeated volcanic eruptions that have preserved a succession of petrified forests. The area of the fossilized forests covers 40 square miles, larger than any other region of petrified trees in the United States.

Many of the trees are in upright positions and have long been interpreted to have been buried in place. However, Creationists have published an alternative explanation that calls upon a single catastrophic volcanic eruption that produced a large mudflow, up-rooting trees and transporting them to the deposition area. However, their explanation includes no evidence, presenting only a just-suppose story that does not address the specific conditions found in the rocks.

A scientific review of the evidence provides an argument for a suite of related causes that buried in-place a succession of forests. Trees have root zones in the fine-grained tuffaceous sandstones; rocks that have no evidence of being from a current-dominated depositional setting. The conglomerates overlie the sandstones and have flow-structures that indicate they buried in-place trees, and the sections of these trees in the conglomerates have abraded surfaces. Some trees are surrounded by lake sediments that indicate settings of rising water from rivers damed by volcanic deposits.