Features of the Firn on Upper Seward Glacier St. Elias Mountains, Canada

Particular attention is devoted to diagenetic changes within firn, leading to an increase in density and producing structures such as ice layers, lenses, and glands. Density increases irregularly downward in the Seward firn, with layers of exceptionally low density underlying impermeable horizontal ice masses. The mean rate of density increase with depth in the first 50 feet is 0.0054 per foot, being more rapid-0.01 per foot-in the first 20 than in the last 30 feet, where it is 0.0023 per foot. The rate of density increase in time varies with depth and with the season. Mean density of the 1948-1949 firn layer increased by 0.10 in ten weeks during the summer of 1949, but mean density of the 1946-1947 layer at a depth of 15-18 feet increased only 0.021 in a year. On the upper Seward, a geo-physically temperate glacier, the increase of firn density is caused about 25 per cent by meltwater freezing and about 75 per cent by compaction-settling within the zone of annual chilling, the upper 40-50 feet. Horizontal layers and lenses of ice in firn are formed by freezing of meltwater that has percolated downward and collected above or moved laterally along impermeable horizons. An estimated 20-25 per cent of the Seward firn is converted to solid ice by this process before passing beyond the depth of annual chilling. Ice glands-crude vertical columns of ice within the firn-are likewise attributed to the freezing of meltwater that has been concentrated along channels extending downward into the firn. The process is pictured as occurring in two stages: first the development of a water-rich condition without much freezing, followed by freezing of the entire mass as a unit, rather than as a process of incremental growth produced by alternate meltwater invasions and repeated freezings along a single channel. As ice layers and lenses are thickest and most numerous near ice glands and extend horizontally from them, a common origin is indicated. These bodies of ice form within the firn during spring when the winter's chilled layer is being ameliorated by the heat given up through freezing of the meltwater that has percolated downward from the surface.