Excavations at Tintagel churchyard in 1991 uncovered an unusual olive brown silty basal layer beneath the main mound (C). Since this layer was clearly exotic to the excavation area, analyses were carried out to locate the likely geological origin.
The silts show strong textural and mineralogical similarities to shales at Hole Beach around 1 km to the south. Proof that this is the actual source is not possible, but exhaustive searching nearer the churchyard has failed to find any similar deposits. There is no obvious reason why people would transport this material to the churchyard as it would be too weak for load-bearing or weather-resistant construction.
However, if it was used in any way, the layer found at the excavation could represent the breakdown product of discarded rock waste.