You are looking at what remains of the ancient land of Quesnellia [Quin-ell-ia].
The granite base and volcanic islands of Quesnellia, on the Pacific plate, were crushed against the westward-moving North American continental plate. A terrain is a belt of rocks that differ in age and type from those around them. This terrane, called Quesnellia, is named for a town in central British Columbia where the volcanic rocks are well exposed.
Granite cooled in the earth’s crust 206 to 144 million years ago, during the Jurassic period of the Mesozoic era. Undersea volcanoes erupted above the granite. Here at Swan Lake, the volcanic islands of Quesnellia have long since eroded but, in Simpson Peak, we can see the granite roots of a once-mighty volcano and the angular spikes of an ancient magma chamber.