Snow, Ice, Avalanches, and Glaciers - Mountain Geography: Physical and Human Dimensions - page 196

FIGURE 4.37 Ogives in East Twin Glacier, Alaska. (Austin Post photo courtesy of the Geophysical In-
stitute GeoData Center, University of Alaska, Fairbanks.)
Another interesting type of ice structure is the ogive. Ogives are arcuate structures
in the ice bowing down-glacier (Fig. 4.37). Two types of ogives are recognized: band
ogives (alternate light and dark coloration of the ice) and wave ogives (alternate crest
and trough structures within the ice) (Benn and Evans 2010; Godsell et al. 2002). Ogives
usually form downslope from an icefall. They are thought to represent the annual flow
of ice through the icefall: the dark portion of the band from the summer, when debris
falls or is blown onto the glacier surface; and the light portion from the winter, when the
icefall is mainly snow-covered. Some of the earliest systematic observations on ogives
were made on Morsárjökull, southeast Iceland, and in the Jotunheimen, Norway (King
and Ives 1956; King and Lewis 1961).
One of the most conspicuous surface features of mountain glaciers is the linear accu-
mulation of rocky debris oriented in the direction of flow. Known as lateral and medial
moraines, these accumulations result from rocks that have fallen onto the ice, ablated
from the edges of the ice, and accumulated from the debris input of tributary glaciers
(Benn and Evans 2010). When a smaller ice stream joins a larger glacier, it usually car-
ries with it a load of rocky debris along its edges (lateral moraine) that becomes incor-
porated into the ice as a vertical partition between the two ice masses. The material
then becomes a medial moraine on the main glacier (Fig. 4.38). What we see is only the
surface expression of the rock debris, which extends into the ice, frequently all the way
to the bottom (except for material contributed by smaller ice streams that join at shal-
lower levels) (Fig. 4.39). The presence of moraines on the ice alters the mass balance,
since the rock material is dark in color and can absorb more of the sun's energy. On the
other hand, if the rocky burden is thick enough, it may serve as an insulative cover and