Elsewhere in the high country, the landscape of bare rock and talus left by former glaciers is replete with hanging valleys, waterfalls, serrated ridges (arêtes), cirques, and hundreds of alpine tarns. Some of the highest peaks retain permanent snowfields and even glaciers. Palisade Glacier, the largest in the Sierra, is located near the park's edge in the John Muir Wilderness.: These glaciers are not holdovers from the Ice Ages; rather, they were most likely formed during cold periods in the last 1,000 years. The park's glaciers are now melting rapidly due to increased temperatures, and may disappear completely within a few decades.: