Volcanic rocks which have piled up more or less continuously since the middle Tertiary virtually compose the Iceland landmass. The rocks can be divided into four major formations: Tertiary Plateau Basalt formation; lower Pleistocene Grey Basalt formation; upper Pleistocene Palagonite formation; and Weichselian till, Holocene sediments, and volcanic rocks. Basaltic lava flows make up the largest percent of the Tertiary Plateau Basalt formation. The Pleistocene formations contain a greater variety of rock facies and notably more subglacial hyaloclastic sediments, fluvial and marine sediments, and tillites. The rocks contain records of sudden climatic changes--e.g., about 3 m.y. ago (at the beginning of the Pleistocene). Holocene volcanism in Iceland has produced mo tly basaltic lavas, but also intermediate and acidic rocks. Volcanic fissures and faults charaterize the Neovolcanic zone, which is believed to be related to the position of Iceland on the Mid-Atlantic Ridge.