SummaryGeological investigations and borehole drilling on the southern margin of the Bovey basin around Mainbow ball clay mine and Ringslade open pit have shown that Devonian slate up to 30 m thick is superimposed on Tertiary sediments. A drag fold beneath the slate indicates that the slate has moved eastwards over the Tertiary material. As the slate is relatively intact and not disaggregated, hillcreep cannot be responsible for the superimposition. A large landslip involving a rotational shear cannot be responsible, as the movement plane dips into the hill over too large an area. The southern margin of the Bovey basin is therefore interpreted as a thrust fault of Tertiary age, with thrust plane dipping 10° to 30° S.
It is suggested that this is a rejuvenation of the Variscan thrust recognized in the Bickington area and further west in the Holne area and that the movement on the Tertiary thrust is related to the dextral wrench movement on the Sticklepath fault.
A shear interferometer for the measurement of atmospheric seeing