The dry valleys display classical features of glacial erosion. Outlet glaciers from the plateau ice sheet spill over a protective basement rim of rock and descended through the valleys to the Ross Sea, sculpting the valleys as the ice moves. A broad, glacially-carved pass through the Olympus Mountains, Bull Pass connects McKelvey Valley to Wright Lower Valley. Besides the spectacular alpine views that the pass affords, Bull Pass is best known for the fantastically shaped granite boulders, some weighing several tons, that decorate the landscape. The magnificent rocks were carved by chemical and mechanical weathering to produce cavities known as tafoni.
Moisture breaks down the rock, gradually opening pits and cracks.
These are then enlarged by windblown sand and ice, eventually eroding huge boulders into fragile shells before they collapse completely into sand. Throughout Bull Pass, the ground is littered with other beautifully shaped rock fragments called ventifacts, meaning wind-faceted stones. Found in other arid regions of the earth, these smooth rocks are gradually worn down and polished by the sand and ice crystals carried by winds off the polar plateau. For scientists ventifacts are helpful indicators of glacial history. The development of facets on the stones can give a good idea of how long the debris has been exposed to wind, and, therefore, uncovered by glaciers.