Folds in firn
Folds in firn on glaciers of the Gilkey Trench, Alaska, have been observed during years of heavy snowfall and reduced ablation. The firn folds were formed by up-buckling and decollement resulting from intense lateral shortening within the underlying glacier ice. Trends of the firn fold axes were normal to the shortening direction (σ 1 ), and subparallel to foliations in the glacier ice and the ice-flow direction (σ 3 ). Extension crevasses normal to the fold trends were also common in the area of the firn folds. Peculiar parallelogram patterns formed by recrystallization of the lowermost centimetre of firn were exposed in the ablated cores of some folds. The patterns were consistently oriented with respect to the firn fold axes and englacial structures, and they seem to reflect the same stress field.