Vancouver Island was formed many millions of years ago as a equatorial sub-continent, called Wrangellia that eventually traveled north and collided with the west coast of North America. This is why Vancouver Island consists of many different types of sedimentary layers of rock as opposed to the mainland's predominantly granite rock makeup.
Mount Arrowsmith is made mostly of Karmutsen Formation basalt and pillow lava from the Late Triassic age. Much of this area was heavily glaciated through several large and small ice ages, with only the tops of the highest peaks escaping the scouring action of the ice. This is what accounts for the sharp rugged peaks, deeply notched couloirs and cirques on the north and northwestern sides.
The hydrology of this massif is such that all of its water runoff ends up in either the Little Qualicum or Englishman Rivers and the snowpacks are valuable in aiding water levels for summer run salmon and trout.
The mountain and its surroundings are home to cougar, deer, wolves, bear and elk. On the high ridges the white tailed ptarmigan can be found blending in perfectly with its surroundings. Many of these animals are subspecies of their kind, and rare, endangered, and known only to Vancouver Island.
The rarest of mammals in Canada, the Vancouver Island marmot, used to have colonies on the Arrowsmith Massif, but their numbers have dwindled to near extinct levels. Still, as recently as the fall of 2000 a pair, probably transient, was seen on one of the popular high trails and often their whistles are reported on the more remote parts of the mountain. Hopefully the marmot recovery breeding program, taking place on Mt. Washington, will bring them back from the brink of extinction and they will call Arrowsmith home again.