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Buried under the snow of following years, the coarse-grained hoar frost compresses into lighter layers than the winter snow.As a result, alternating bands of lighter and darker ice can be seen in an ice core.Ice cores have been studied since the early 20th century, and several cores were drilled as a result of the International Geophysical Year (1957–1958).Depths of over 400 m were reached, a record which was extended in the 1960s to 2164 m at Byrd Station in Antarctica.Below this depth, electromechanical or thermal drills are used.The cutting apparatus of a drill is on the bottom end of a drill barrel, the tube that surrounds the core as the drill cuts downward.Cores are drilled with hand augers (for shallow holes) or powered drills; they can reach depths of over two miles (3.2 km), and contain ice up to 800,000 years old.
In polar areas, the sun is visible day and night during the local summer and invisible all winter.Ice is lost at the edges of the glacier to icebergs, or to summer melting, and the overall shape of the glacier does not change much with time.Impurities in the ice provide information on the environment from when they were deposited.The bubbles disappear and the ice becomes more transparent.The weight above makes deeper layers of ice thin and flow outwards.
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A design for ice core augers was patented in 1932 and they have changed little since.