310 million years ago, during the period of earth history known to
geologists as the Carboniferous, County Durham lay almost astride the
equator. The continental shield area, near the margin of which Northern
England lies, has moved progressively northwards to its present
position over this period of time. During the Carboniferous, the area
destined to become northern England was covered in low-lying,
equatorial swamps, crossed by large rivers and covered by forests of
primitive trees. These were not like modern trees, but were the
ancestors of the plants today represented by club-mosses, early
conifers, horse-tails and ferns. As this luxuriant vegetation grew,
thick accumulations of peat built up under the forest trees which, when
buried by mud, silt or sand, became compacted to form the coal seams of
Durham Coalfield. The name Carboniferous is used for this period of
geological time because during this interval most of the coal seams of
Europe were formed. The name Coal Measures is used for the group of
rocks that contain most of the workable coal seams in Britain.
Fossil trees are quite frequently found in Coal Measures rocks, but this tree is different. It is a petrifaction: the actual woody tissue of the tree is mineralised with calcite and ankerite (calcium, magnesium, iron carbonate) so that the details of the internal structure of the trunk are preserved, in places including the tiny individual cells. The internal structure of the tree trunks show up well when they are cut into individual cells. The individual structure of the tree trunks show up well when they are cut with a rock-saw. The wood was first mineralised with calcite when the empty dead cells were filled with mineral, leaving the organic cell walls intact. Subsequently the mineralised wood underwent recrystallisation that destroyed much of the original plan structure and produced a root-like pattern composed of radiating crystals. It is circular patches of radiating crystals of calcite that look so attractive in the polished cross-sections.
The fossil trees have been examined by Professor Andrew Scott of London University, who reports that they are examples of an ancestor of modern fir trees (conifers). Logs of this type with similar carbonate mineralisation, have been found elsewhere in the world, but not previously in Britain. They are called Cordaites, a name used for extinct seed-bearing plants related to the conifers, Cordaites was a tall tree, up to 30 meters high, that had narrow strap-like leaves about one meter long. Although the long stems have not previously been found in Britain, leaves and branches are common fossils in the Coal Measures. The occurrence of mineralised tree trunks at Priors Close and not at other Coal Measures in Britain needs explanation.