A Tentative Theory of Ogive Formation

Abstract

AbstractThe dark and light bands on glaciers known as ogives are only found beneath ice falls and avalanche fans. They are not to be confused with sedimentary layering, which may appear similar. Vareschi’s pollen studies are considered in relation to the present theory; his evidence is re-interpreted and shown to support the theory put forward.The Norwegian glacier Austerdalsbreen has a fine double set of ogives, one set on ice from Odinsbreen and the other on ice from Thorsbreen. These ogives are continuous from near the feet of these ice falls down to the end of the main glacier. The ice from the collecting ground of Jostedalsbreen which moves slowly towards the head of these ice falls is normally stratified as seen in the deep crevasses immediately above the ice falls. The high velocity of flow, 2,000 m. per year in the upper part of Odinsbre ice fall, causes the ice to stretch into a thin and heavily crevassed layer which exposes a very high proportion of surface per unit volume to the sun, the rain and the snow. In summer this leads to:(1) crystal changes, primarily of enlargement, (2) an infusion of dirt which blows on to the glacier from the neighbouring snow-free and vegetation-free land surfaces, and (3) water filling the bottom of some of the deeper crevasses, which may later freeze. On the other hand, the ice which passes down the ice falls in winter is largely protected by a mantle of snow; crystal changes then are slow, little dust collects, and less water pours into the crevasses which, instead, are filled with new snow. So the ice reaching the lower part of the ice falls and moving on to form the main glacier, Austerdalsbreen, has been subjected throughout its mass to seasonal differences. These differences seem to be more systematic in the deeper ice, and only when the chaotic surface layers are melted away, do they appear on the surface of Austerdalsbreen as well defined ogives. The greater proportion of blue, bubble-free ice with large crystals in the “summer” ice, is alone sufficient to distinguish it from the lighter-coloured “winter” ice with its more frequent bands of white bubbly ice having very small crystals. But as ablation continues, more and more dust is brought to the surface and this still further darkens the “summer” ice. In addition the darker ice melts more readily and tends to form troughs in the glacier surface which retard stream flow, hold the lingering snows, and trap further dust and dirt. The cracks between large crystals also hold dirt better than the smooth hard white surfaces which occur more frequently in the “winter” ice. Hence the ogives remain distinct throughout their journey down Austerdalsbreen. Near the snout the medial moraine is very abundant along the darker bands deriving from Thorsbreen, and this further supports our view that this darker ice was in the ice fall in summer when most debris falls from the rock walls on to the ice fall.