Volcanism in the central segment began at least 4.0 million years ago at the deeply dissected Mount Cayley massif. This eruptive period, lasting until 0.6 million years ago, produced dacite lava flows and pyroclastic breccia. A central plug dome forming the summit spires of Mount Cayley represents the youngest feature that formed during this eruptive period. Subsequent activity 0.3-0.2 million years ago began with the eruption of a dacite lava flow into the Shovelnose Creek valley. This resulted in the formation of two small lava domes. Mount Fee is a long and wide spine of rhyodacite situated on a mountain ridge east of the Squamish River. Like the Mount Cayley massif, it predates the appearance of the Cordilleran Ice Sheet. Other volcanoes in the central segment, such as Slag Hill, Ember Ridge, Cauldron Dome, Pali Dome and Ring Mountain, were formed when lava came into contact with the Cordilleran Ice Sheet. They are similar in structure to tuyas, displaying oversteepened ice-contact margins.