The ODP Leg 171B drilled a transect of four sites at Blake Nose in the NW Atlantic providing an excellent record of the K-T boundary. At the deepest, Site 1049, the boundary is marked by a 9-17 cm thick layer formed mostly of green spherules composed of Fe-rich smectite resulting from the diagenetic alteration of tektites and impact glasses. Minor amounts of authigenic zeolites and palygorskite also occur. This association represents a notable break in the clay mineral composition of Cretaceous and Tertiary sequences. The clay mineral assemblages of the Cretaceous and Tertiary sediments are dominated by inherited clays. Aluminium-rich smectite of pedogenic origin is abundant in both Cretaceous and Tertiary sediments, indicating a relatively warm and hydrolysing climate across the K-T boundary. At Hole 1049A, where the oldest sediments of the interval were analysed, an increase in kaolinite and smectite down core suggests tectonic rejuvenation and distal transport of illite and kaolinite, probably accompanied by more hydrolysing conditions during the late Maastrichtian.