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.
The Fossil Record
The mine is a significant source of paleontological information beyond the pollen record. The same geological processes that formed the coal also preserved a wide assortment of other fossils. In 2003, a shell bed measuring up to 12 meters thick was discovered between two lignite layers. This deposit, consisting mainly of freshwater snails from the genus Bellamya, is believed to be the thickest freshwater shell deposit in the world. In addition to snails, fossils of fish, crabs, crocodile bones, and turtle shells have been unearthed, showing the Miocene aquatic and terrestrial life.
The mine itself is operated by the Electricity Generating Authority of Thailand (EGAT) and covers an area of about 30 square kilometers. Lignite was first discovered in the district in 1917, and large-scale mining operations began in the 1950s. The extracted coal, amounting to about 40,000 tonnes per day, primarily fuels the adjacent Mae Moh Power Plant, a significant source of electricity for the country. The entire mining and power generation complex is a massive industrial operation, scheduled to be phased out by 2050 as Thailand transitions to other energy sources.