Explanation and Evidence
sort of catastrophe must have happened to a grove of large Cordaites
trees some 310 million years ago. They were uprooted, stripped of
foliage and branches, probably by a river in flood that carried the
trunks downstream and deposited them like flotsam in a lake delta. This
type of river activity is common in Coal Measures, but it usually
doesn't produce petrified trees. Examinations of structures within the
sandstone about the High Main Coal, confirms that the fossil trees were
carried by a river into a sandy delta.
One factor at Priors Close that is different from most common Coal Measures sections in the presence of small mineralised faults or mineral veins. These faults are common in the Coal Measures of Great Lumley area. Towards the end of the life of the Lumley Sixth Pit, a little to the north of Priors Close, an underground roadway was driven south below Lumley Thicks and passed through a strong fault which dropped the strata 24 meters wide composed of massive white barytes (barium sulphate). Further south a second fault contained a barytes vie 0.5 meters wide. The east-west and northwest faults encountered at Priors Close surface mining are aorta of the same suite as those at Lumley Pit. These faults are also mineralised and carry calcite (calcium carbonate), marcasite (from sulphide), galena (lead sulphide) and sphalerite (zinc sulphide). They are only narrow and impersistent veins and the dominant mineral is calcite and not barratries as was found at Lumley Sixth Pit.
The mineral matter that preserved the fossil trees is 70% calcite. The petrifaction process took place rapidly when the tree trunks were buried in the delta sands by immersion in the concentrated solution of calcium carbonate that replaced the woody tissue with limestone. It might be coincidence that the petrified trees have only been found near to one of the small mineralised faults at Prior Close, though the close association of the trees and this fault makes some connection seem likely. It may be that at least part of the calcium carbonate in solution that produced the petrified trees issued from the fissure.