Kobuk Valley has 61 miles (98�km) of the Kobuk River and three regions of sand dunes. Created by glaciers, the Great Kobuk, the Little Kobuk, and the Hunt River Sand Dunes can reach 100 feet (30�m) high and 100��F (38��C), and they are the largest dunes in the arctic. Twice a year, half a million caribou migrate through the dunes and across river bluffs that contain ice age fossils.