How is sub-bituminous formed?

Sub-bituminous coal is formed from the accumulation of plant material in swampy environments, similar to other types of coal. Over millions of years, this plant matter is buried under layers of sediment, where heat and pressure cause it to undergo chemical changes. As the organic material matures, it loses moisture and volatile compounds, resulting in a higher carbon content compared to lignite but lower than bituminous coal. The process typically occurs over geological timescales, involving significant geological and environmental changes.