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The ‘date’ relative to other index fossils and rock layers is, of course, determined by the species’ position in the evolutionary ‘tree of life’.Both are fossils of squid-like creatures, common to abundant in so-called Mesozoic rocks.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).being oxidized to limonite (hydrous iron oxides), and also in a few areas the sand content is higher.
Those fossils that seem to work well for identifying and ‘dating’ rock strata are called ‘index’ fossils.
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.
The radiocarbon (C ‘ages’ ranging from 20,700 ± 1,200 to 28,820 ± 350 years BP (Before Present).
For sample UK-HB-1, collected from on top of the belemnite index fossil (Figure 5), the results from the two laboratories are reasonably close to one another within the error margins, and when averaged yield a years for the index fossils found with the fossil wood, and thus for the host rock.