The woody internal structure was clearly evident, thus the samples were not the remains of roots that had grown into this weathered rock from trees on the present land surface.When sampled, the fossil wood readily splintered, diagnostic of it still being ‘woody’ in spite of its impregnation with iron minerals during fossilisation.Evolutionary geologists consider that the top three metres (10 feet) of the Marlstone Rock Bed represent the whole of the Tenuicostatum Zone, the basal zone of the Toarcian Stage, Amongst the remaining quarries still ‘working’ the top of the Marlstone Rock Bed are the Hornton Quarries at Edge Hill near the village of Ratley, on the north-western edge of the Edge Hill plateau, some 10½ km (6½ miles) north-west of the town of Banbury (Figures 2 and 3).Building stone, known as ‘Hornton Stone’, has been quarried there since medieval times.Carbon 14 is continually being created in the Earth's atmosphere by the interaction of nitrogen and gamma rays from outer space.Since atmospheric carbon 14 arises at about the same rate that the atom decays, the Earth's levels of carbon 14 have remained constant.
All the fossil wood samples were from short broken lengths of what were probably branches of trees fossilised . By comparing this light output with that produced by known doses of radiation, the amount of radiation absorbed by the material may be found. Most mineral materials, including the constituents of pottery, have the property of thermoluminescence (TL), where part of the energy from radioactive decay in and around the mineral is stored (in the form of trapped electrons) and later released as light upon strong heating (as the electrons are detrapped and combine with lattice ions).Figure 3(b) Closer view of the quarry face of the south wall showing the oxidized limestone of the top of the Marlstone Rock Bed which is quarried as ‘Hornton Brown’ building stone. During two visits to the Hornton Quarries, it was established that fossil wood occurs alongside ammonite and belemnite index fossils (see aside below) in the ‘Hornton Stone’, the oxidized silty top of the Marlstone Rock Bed.The ammonite recovered in the quarries is Many such belemnite fossils had been found during quarrying operations (Figure 6).