At Horton in Ribblesdale, England, excavation has exposed an erosional unconformity between Ingletonian slates and grits and overlying Ashgillian quartzites. Seven Ingletonian slates define a Rb–Sr isochron with an age of 505 ± 7 m.y. (λ = 1·39 × 10−11yr−1) This age is interpreted as the time of dewatering, folding and development of the slaty elavage. The initial 87Sr/86Sr ratio of 0·7071 is so low that the Ingletonian deposition was probably not earlier than Cambrian and could be early Ordovician. K–Ar ages on the same samples average 433 m.y. with a spread from 417 to 451 m.y. and probably relate to a second folding of the Ingletonian near the end of the Silurian.