<P> An ice core is a vertical column through a glacier, sampling the layers that formed through an annual cycle of snowfall and melt . As snow accumulates, each layer presses on lower layers, making them denser until they turn into firn . Firn is not dense enough to prevent air from escaping; but at a density of about 830 kg / m it turns to ice, and the air within is sealed into bubbles that capture the composition of the atmosphere at the time the ice formed . The depth at which this occurs varies with location, but in Greenland and the Antarctic it ranges from 64 m to 115 m . Because the rate of snowfall varies from site to site, the age of the firn when it turns to ice varies a great deal . At Summit Camp in Greenland, the depth is 77 m and the ice is 230 years old; at Dome C in Antarctica the depth is 95 m and the age 2500 years . As further layers build up, the pressure increases, and at about 1500 m the crystal structure of the ice changes from hexagonal to cubic, allowing air molecules to move into the cubic crystals and form a clathrate . The bubbles disappear and the ice becomes more transparent . </P> <P> Two or three feet of snow may turn into less than a foot of ice . The weight above makes deeper layers of ice thin and flow outwards . 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 . The outward flow can distort the layers, so it is desirable to drill deep ice cores at places where there is very little flow . These can be located using maps of the flow lines . </P> <P> Impurities in the ice provide information on the environment from when they were deposited . These include soot, ash, and other types of particle from forest fires and volcanoes; isotopes such as beryllium - 10 created by cosmic rays; micrometeorites; and pollen . The lowest layer of a glacier, called basal ice, is frequently formed of subglacial meltwater that has refrozen . It can be up to about 20 m thick, and though it has scientific value (for example, it may contain subglacial microbial populations), it often does not retain stratigraphic information . </P> <P> Cores are often drilled in areas such as Antarctica and central Greenland where the temperature is almost never warm enough to cause melting, but the summer sun can still alter the snow . In polar areas, the sun is visible day and night during the local summer and invisible all winter . It can make some snow sublimate, leaving the top inch or so less dense . When the sun approaches its lowest point in the sky, the temperature drops and hoar frost forms on the top layer . 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 . </P>

What part of the ice core is actually examined by scientists