The fourth phase of activity began one million years ago when the Cordilleran Ice Sheet retreated from the upper flanks of the adjacent lava plateau, creating Mount Edziza proper which is the most northerly of the four central volcanoes. It is a steep-sided stratovolcano and the largest and highest of the peaks that form the volcanic complex with an elevation of , overlapping the northern flank of Ice Peak. The stratovolcano is made up of a fine-grained volcanic rock called trachyte and is associated with several lava domes which were formed by trachyte lava flows and explosive eruptions. Its smooth northern and western flanks, only slightly channeled by erosion, curve up to a circular summit ridge which surrounds a central, ice-filled caldera in diameter. Many glaciers cover Mount Edziza proper, including the Tencho Glacier on its southern flank. Active cirques on the eastern flank have breached the caldera rim, exposing the remnants of numerous lava lakes which ponded in the caldera 900,000 years ago and rest on hydrothermally altered breccia of the main conduit. Piles of pillow lava and hyaloclastite, formed by subglacial eruptions, are found on the flanks of Mount Edziza and nearby Ice Peak, as well as on the surface of the surrounding shield volcano. Pillow Ridge on Edziza's northwest flank was formed when basaltic lava erupted beneath the regional Cordilleran Ice Sheet when it was close to its greatest thickness.