During the Carboniferous Age, when amphibians had reached the land, there was much variation in their development. Conditions were ideal for their preservation. The Upper Carboniferous, meaning 'coal-bearing', was the time of coal formation. Great forests and swamps flourished in the northern hemisphere, in North America as well as in Europe. Coal is mostly carbon, but at first it consisted of dying trees, leaves, ferns and other vegetation which became buried in the mud. Next it turned into peat, then soft brown coal, and finally into black coal. This takes a long time during which many living things are buried. The reason why coal-bearing rock comes in layers is due to earth movement. During the Upper Carboniferous the land sank so that the swamps became flooded by the sea. The land then rose again, so that fresh forests formed. In between two coal layers, or seams, lies a rock deposit containing sea creatures.
Masses of plant remains have been uncovered during coal mining, beautifully preserved as impressions in the hardened mud, even on the coal itself. There are fern prints, bark patterns, marks where an insect has crawled over the mud, tracks of amphibians, and skeletons of fish and salamanders. Some branches, trees, roots and cones are so well preserved that even the minute cell structure of the original wood can be seen clearly under the microscope.
