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Impression Fossils
As the name implies, impression fossils form when a leaf or plant leaves behind an impression in rock. This occurs when sediment covers a plant and the water in the tissue dries up. The plant decomposes over time and only the impression remains. Despite this, however, impression fossils can feature a great deal of detail. In some cases small hair-like fibers are visible and more detail can be revealed with a light microscope or electron microscope.
Compression Fossils
As the name suggests, compression fossils result when a plant is trapped in layers of sediment and put under pressure and occur in shale, clay and even volcanic ash. One distinction between impression and compression fossils is that the latter has a layer of carbonaceous material coating it. Coal is a good example of a compression fossil, and in some cases, fossil plant material can be found in the layers. Other compression fossils will reveal more detail in the plant and occasionally organelles can be seen under a microscope.
Cast fossils
Cast fossils, also called mold fossils, have a three dimensional structure. They occur when a plant is trapped between layers of sediment and decomposes, but leaves behind a hollow impression that acts like a plaster mold. If sediment seeps into the empty space and hardens into rock, it creates a cast fossil. These are particularly valuable for understanding plant structures, especially features like branches and stems. Additionally, cast fossils reveal the actual external structure of the plant.
Permineralization
Permineralization is not common but many people are familiar with it because of the Petrified Forest National Park where the process has left fossilized logs in abundance. In this case, a dissolved mineral solution permeates the plant, and as the tissue dissolves it leaves behind a solid formation that shows great detail. Carbonates, silicates and occasionally oxides can do this. Some amount of organic material may be left behind as well allowing a study of the plant at the cellular level.
Unaltered Plant Material Fossil
On very rare occasions, some plant material can be left relatively unchanged under very dry or cold formation conditions. Calcium carbonate is important in this process and spores, pollen and amber that contain it may be fossilized with unaltered material. In amber, which is fossilized tree sap, grains of pollen, spores and even insects can be trapped. When this happens it is possible to extract DNA from the sample.