Dating glacier layers
Radioactive elements, either of natural origin or created by nuclear testing, can be used to date the layers of ice in the cores.
Some volcanic events which were sufficiently powerful to have distributed material around the globe leave a signature in the ice which can be detected in many different cores, allowing synchronization of the time scales between two different locations.
Cores are recovered by drilling with hand augers (for shallow holes) or powered drills; the deepest cores recovered reach depths of over two miles, and can contain ice up to 800,000 years old.
Both the physical properties of the ice itself and material trapped in the ice can be used to reconstruct information about climate over the age range of the core.
He said that it didn't but that the weight of the ice actually depressed the land. At any rate, he talked about the seasonal layers in the ice.
These layers can be counted only so far down because the weight of the ice compresses the layers more and more the deeper they are in the ice.
Since the ice forms from the incremental buildup of annual layers of snow, lower layers are older than upper, and an ice core contains ice formed over a range of years.
Impurities in ice cores may depend on location; for example, coastal areas are more likely to include material of marine origin, such as sea salt ions.