The confidence in the chronology becomes less the lower in the ice sheet one goes, however. The amplitude of the annual oscillations slowly decreases relative to other factors, and historic markers are fewer and farther apart. Glaciologists estimate that uncertainties in identification of layers will probably limit the number of countable layers to less than about 8,500 (Hammer, et al., 1978).

　　Antarctic Ice Cores

　　The claims that layers of ice were formed 160,000 years ago or more come primarily from interpretation of ice cores in Antarctica (Jouzel, et al., 1987; Barnola, et al., l987). The Soviet Antarctic Expeditions at Vostok in East Antarctica recovered an ice core which was almost 7,000 feet long in a region where the total ice thickness is about 12,000 feet (Lorius, et al., 1979; Lorius, et al., 1985). Since the current precipitation rate is so much less than Greenland (on the order of one inch per year) the crude calculation of age, without corrections for compression and horizontal motion for the lowest layers is more than 100,000 years.

　　However, such estimates are critically based on the assumption that the accumulation rate has not varied greatly over the past. Unlike the Greenland ice cores, annual oscillations of 18O and other parameters cannot be traced deeply into the ice sheet on Antarctica. In Greenland, the high precipitation rates not only provide relatively thick annual layers for analysis, but the accumulating snow quickly seals off the ice beneath and protects the record from metamorphosis by pressure and temperature changes in the atmosphere. In Antarctica, by the time the ice has been buried deeply enough to no longer be influenced by the atmosphere, annual variations have been greatly dampened by diffusion (Epstein, et al., 1965; Johnsen, et al., 1972).

　　The technique used to estimate the age of an ice layer deep in the ice sheet is to measure its 18O content and compute the atmospheric temperature which is observed to produce such concentrations today (Jouzel and Merlivat, 1984). Through a second-known relation between temperature and precipitation rate, again observed in today's atmosphere, the accumulation rate for a given layer is calculated (Lorius, et al., 1985). Once the accumulation rate is calculated for each layer, the depth and age for each layer in the ice is calculated by integrating the annual accumulation downward from the surface.

　　There are several historical markers in Antarctica which can be used to cross check these calculations for the past few thousand years. But historical volcanic events are not known beyond a few thousand years in the past which provide any certainty to the calculation of age. This method would be reasonably reliable if precipitation rates had been similar in the past. However, some creationist models predict significant quantities of snow immediately after the Flood (Oard, 1990). Perhaps as much as 95% of the ice near the poles could have accumulated in the first 500 years or so after the Flood.