The Upper Tertiary geosynclinal beds of the East Coast region of the North Island of New Zealand rest on an incompetent substratum of Lower Tertiary marls and bentonitic rocks and show, in the structures described, evidence of décollement. A Lower Tertiary outcrop of 3½ square miles represents an intrusion through some 10,000 ft of the Upper Tertiary succession and lies close to a sharp structural subsidence of similar area. The processes of formation of these structures are considered and it is concluded that at least 8 cubic miles of Lower Tertiary material were transferred from beneath the subsidence, which coUapsed in consequence, to be extruded near by to surface through the overlying beds.