Abstract: The sedimentary rocks of the Taconic Cow Head klippe contain three clay-mineral suites of progressively younger stratigraphic occurrence. An illite-14A chlorite suite is the oldest, occurring in the Middle Cambrian to early Lower Ordovician part of the 310-m Cow Head Breccia. These earliest clays were transported from the stable craton and shelf, slowly accumulating during 70 × 106 yr on the continental slope in limestone breccia, green and gray shale, and argillaceous limestone. The illite and 14A chlorite are judged to be largely detrital. An illite-expandable chlorite suite is in early to late Lower Ordovician limestone breccia, green and gray shale, and argillaceous limestone of the Cow Head Breccia. A corrensite-illite-smectite suite of late Lower to Middle Ordovician age occurs in the Cow Head Breccia and throughout the overlying 200-m ‘Red Shale’ and the more than 400-m ‘Green Sandstone’ flysch sequence of volcanogenic sandstone and gray shale. Beginning in the early Lower Ordovician, increasing amounts of Mg2+-rich volcanic detritus were rapidly transported westward from a developing volcanic island arc in central Newfoundland. During burial metamorphism, volcanic materials and their alteration products reacted to form the illite-smectite with 5–10% expandable layers plus corrensite or expandable chlorite found in the younger two clay-mineral suites.