A Miocene Forest Floor
The Mae Moh lignite mine in northern Thailand is the largest open-pit coal mine in Southeast Asia. It sits within the Mae Moh Basin, an intermontane depression formed during the Tertiary period. The basin gradually filled with sediment and dense vegetation, creating the conditions for the formation of lignite, or "brown coal." The main coal-bearing layer, the Na Khaem Formation, contains multiple lignite seams that are 250 to 400 meters thick. These seams of low-grade coal are the compressed remains of a vast freshwater swamp that existed here approximately 15 million years ago, during the Miocene epoch.
Within these coal layers, miners have discovered small nodules of amber—fossilized tree resin. This amber is a microscopic record. Trapped inside the hardened resin are exquisitely preserved pollen grains from the ancient forest. Analysis of this pollen provides direct evidence that the ancestors of modern Southeast Asian tropical rainforest trees were already established here. The presence of specific pollen types shows a wet, tropical, evergreen or semi-evergreen forest ecosystem remarkably similar to the one that exists in the region today. This discovery confirms that this type of complex forest biome predates the evolution of early hominins.