Over time, the creeping forest has reclaimed most of the once giant sand dunes. NPS Photo/Emily Mesner. The Great Kobuk Sand Dunes are the largest active dunes in the Arctic. NPS Photo. Relic from the Past. Kobuk Valley’s sand dunes are a relic of the last Ice Age. 28,000 years ago, the Earth cooled and glaciers began to form high in the mountains surrounding the valley. Over time, the slow, grinding advance and retreat of the glaciers ground the rocks beneath them into a fine sand which was blown by the wind into the sheltered, ice free Kobuk Valley.