of plagioclase and clinopyroxene regionally show significant evidence of reaction with the associated magma, including sieve-textured cores and random, resorbed and embayed outer margins wherever they are located. Lava tubes are widespread in the Northern Cordilleran Volcanic Province, and are typically basaltic in composition. At Level Mountain, lava tubes reach diameters of to . These owed their origin to highly fluid lavas with temperatures of at least . In Nisga'a Memorial Lava Beds Provincial Park of northwestern British Columbia, lava tubes are present that were formed during one of Canada's most recent volcanic eruptions in the 18th century. Lava Fork at the British Columbia-Alaska border is influenced by lava flows from a recent volcanic eruption that later collapsed into underlying lava tubes after the lava solidified. Sections of these collapsed lava tubes now form volcanic pits. Extensive areas of nearly flat-lying lava flows throughout the Northern Cordilleran Volcanic Province can cover areas of at least and are generally composed of highly-fluid basaltic lava. However, lava plains that pre-date the last glacial period have been eroded and overridden by glacial ice, affording a less distinctive form to these older landforms. For example, lava beds at least a million years old in central Yukon contain unconsolidated glacial deposits that were deposited when glacial ice rode on top of the lava flows comprising the lava beds. Subvolcanic intrusions in the Northern Cordilleran Volcanic Province are exposed in areas of high relief. This includes volcanic plugs found at the Mount Edziza volcanic complex, Level Mountain, Hoodoo Mountain and in the Atlin and Maitland areas. Volcanic plugs in the Atlin and Maitland areas consist of olivine nephelinite and basanite magmas. Minor plugs made of gabbroic and granitic magma are associated with volcanic stratigraphy at the Mount Edziza volcanic complex and Level Mountain. Terranes and boundaries The bedrock of the Northern Cordilleran Volcanic Province consists of four large terranes, known as Stikinia, Cache Creek, Yukon–Tanana and Cassiar. Stikinia is a sequence of late Paleozoic and Mesozoic aged volcanic, plutonic and sedimentary rocks interpreted to have been created in an island arc environment that were later placed along a pre-existing continental margin. The Cache Creek Terrane is believed to have formed widely in a pre-existing oceanic basin. It comprises late Paleozoic to Mesozoic aged oceanic melange and abyssal peridotites intruded by younger granitic intrusions. The Yukon–Tanana and Cassiar terranes consist of shifted sedimentary and metamorphic rocks that