The only common sedimentary rock which doesn't fit easily into either the clastic or chemical category is Coal. Coal is formed of partially decomposed plant material. As such, it is not clastic (particles of pre-existing rock) or chemical (caused by chemical precipitation). Coal is formed in swamps where the rate of decomposition is significantly slower than the rate of accumulation of dead plant material. The Everglades in Florida are an example of a modern coal swamp environment. As organic material accumulates it undergoes the process of Coalification which is controlled by the amount of pressure and temperature. Coal is classified by its chemical and physical characteristics into three major types, Lignite (the lowest grade), Bituminous Coal, and Anthracite (the highest grade).