Study the assortment of compressions available. Notice distortion in original morphology is introduced by flattening. Peat is an accumulation of virtually unaltered plant material, while anthracite is nearly pure carbon with little trace of the original plant material. All materials can be burned for fuel, but the energy content per weight increases with degree of metamorphism and the proportion of impurities generally decreases. Study thin sections of coal at the microscope.
Impressions are essentially compressions sans organic material.
If the sediment is very fine-grained, impressions may faithfully replicate remarkable details of original external form, regardless of subsequent consolidation of the sediment. Study the specimens of impressions, noting the several categories of plant parts represented. Impressions may also occur if, when layers of rock are split apart, the organic material adheres to only one side of the rock. In this type of sediment, relatively coarse sand grains are mixed with silt and clay. This type of sediment is common in river and flood plain environments so is important for terrestrial plant preservation.