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If you climb the Tusk bring a good pair of binoculars and a copy of Mathews guide to "Garibaldi Geology." The scenery spread out around you is a chronicle of fire and ice, an entire landscape of lakes, mountains and valleys that were born and reshaped during the brief final chapter of our local geological history.
South of your perch atop the Tusk, across Garibaldi Lake and beyond the rounded summit of Mount Price, the pyramid-shaped cone of Mount Garibaldi rises above the surrounding granitic mountains. Twenty thousand years ago the older mountains didn't look much different than they do today but Mount Garibaldi, which now fills the southern horizon, didn't even exist. It was built during the waning stages of the Fraser Glaciation when the ice surface stood at about 1,300 metres above sea level. Composed of dacite, a viscous lava prone to explosive behavior, the summit cone of Garibaldi was built in several stages. The final eruption produced a spine of glowing semi-rigid lava that repeatedly collapsed onto the growing cone. And Mathews presents compelling evidence that the cone actually extended onto the ice surface.
When the regional ice sheet stood at more than 2,000 metres above sea level, the eruption of a satellitic vent on the north flank of Garibaldi, built the steep-sided, flat topped lava pile known as The Table. Mathews called it a "Tuya" after similar features he had mapped on the Tuya Plateau of northern B.C. The name stuck and became part the lexicon of technical geological terms. The Table, like all Tuyas, was formed by lavas that erupted into and were contained by a vertical pipe thawed into the ice. Having nowhere to go, successive flows piled up like a stack of giant hotcakes.
The final gasp of Garibaldi's long and varied life came sometime after the last ice had disappeared from the valleys. A massive effusion of lava issued from Opal Cone on its southern flank producing the Ring Creek lava flow, 17 km long and two km wide, which rumbled down Mamquam valley almost to Squamish.
Garibaldi Lake, which fills the foreground of your view from the Tusk, is an even more recent addition to the landscape than Mount Garibaldi. After the Cordilleran Ice Sheet had disappeared from the highlands, leaving only isolated alpine and valley glaciers, an eruption from Clinker Peak on the shoulder of Mount Price, sent massive lava flows into upper Culliton and Rubble Creeks. Garibaldi Lake is now ponded behind a lava dam that was formed by an arm of the Clinker Peak flow.