Grand Teton National Park, Wyoming. Hiking in lower Cascade Canyon as early-morning clouds drift around the granite spires of the Teton Range in Grand Teton National Park. During the height of the Pleistocene Epoch which began more than 1.5 million years ago, the Teton Mountains and the Yellowstone Plateau to the north were smothered in deep glacial ice. The most recent glacial advance reached its maximum extent about 20,000 years ago and scooped out the basins for the lakes which lie at the base of the Teton Range today.