A Possible Failure Mechanism for the AD 1575 'Wonder Landslide'

The Wonder Landslide is located on the eastern slopes of Marcle Hill in Herefordshire. Marcle Hill forms part of the Woolhope inlier, a much-faulted asymmetrical pericline of predominantly Silurian calcareous siltstones and nodular limestones. The landslide is recorded as occurring on February 17th 1575 AD. Based on the results of a field investigation and back analyses of the failure it appears that the landslide had a translational block-glide form that failed along a shear surface in Fuller's Earth clay at considerably less than peak strength and at a depth of approximately 5 metres. It is suggested that the reduction of shear strength within the clay, to near residual levels, was a result of the long term extrusion of the clay from beneath a limestone cap-rock and denudational unloading. High water levels were also required for the failure to occur and these were probably a consequence of exceptionally heavy and prolonged rainfall associated with the climatic deterioration during the Little Ice Age (approximately 14th to 18th Centuries AD). The total volume of material displaced in the landslide appears to have been in excess of 60,000m 3.