The Ingletonian Series comprise an alternating succession of ancient metamorphosed sediments, mainly slates and grits, which outcrop as a small inlier just to the north of Ingleton in west Yorkshire (Rayner & Hemingway 1974). They are tightly folded into a series of almost vertical isoclinal folds and unconformably overlain by the Carboniferous Limestone (Fig. 1). The unconformity between the Ingletonian and the Carboniferous Limestone is one of the classic unconformities in geology and one which has often been used to illustrate the essential nature of an unconformity. The Ingletonian is equally important as a source of high quality roadstone. The purpose of this Photographic Feature is to bring together these two aspects of geology: the classical and the engineering.

The fact that the Ingletonian rocks are unfossiliferous together with stratigraphical and structural considerations, led to the Series being regarded as Precambrian in age and they were officially considered so by the Geological Survey (Edwards & Trotter 1954). In the 1970s, however, radiometric age determinations were made on seven samples of Ingletonian slate from Horton in Ribblesdale, 11 km east of Ingleton (O'Nions, Oxburgh, Hawkesworth & Macintyre 1973). The Rb-Sr isotopic age determinations yielded an age of 505 Ma which was interpreted as being the time when the folding and development of the slatey cleavage took place. It was considered that the interval between this event and the time of sedimentation could not be more than 50 Ma, giving the age of the deposit as 555 Ma and hence putting the

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