I would like to thank Mr Short for his kind remarks and for the historical information relating to ‘bell-pits’ and the interesting troglodyte dwellings at Doncaster which had also been mentioned. The deepest ‘bell-pit’ workings which the writer has seen were in the well-known Tankersley Ironstone overlying the Flockton Thick seam (Anthraconaia modiolaris Zone) of the Yorkshire coalfield. Shafts just over 40 ft deep are relatively common and a figure of 70 ft has recently been quoted in the press. Although the measures above the Flockton Thick are reasonably competent it is unlikely from the point of view of stability that this latter figure refers to ‘bell-pits’ in the true sense.
Geophysical techniques in the initial exploratory phase of a site investigation programme may give useful information. Unfortunately present methods do not lend themselves to the detection of ‘room and pillar’ workings, which pose the most uncompromising problems. It seems logical, however, to extend the use of geophysical methods to include later phases of site investigations when more detailed information is required and very close traverses may be warranted. Attention should be drawn to the fact that in recent years instrumentation has made great advances in many fields employing geophysical methods. For some unknown reason this does not include the important field of land-based site investigations.
R. K. Taylor
Engineering Geology Laboratory, Department of Geology, University Science Laboratories, South Road, Durham.