Snow, Ice, Avalanches, and Glaciers - Mountain Geography: Physical and Human Dimensions - page 197

inhibit local melting of the underlying ice. This leads to more rapid melting of the ice on
either side of the debris, leaving the moraine, often with an ice core, exposed as a high-
er ridge. As the ridge builds through differential melting, some of the rocky material
may slide or tumble onto the ice; in this way the moraine is widened and the underly-
ing ice is again exposed to melting. The moraines gradually widen toward the terminus,
eventually ending up as a jumble of rock debris covering the terminus of the glacier
(called an ablation moraine ) . If the glacier is retreating, the underlying ice may melt,
leaving the rocks lying about in heaps. On the other hand, isolated masses of ice may be
preserved indefinitely under the debris as ice-cored moraine. Such moraines can eas-
ily be confused with moraine-covered snow accumulations. Snow can accumulate along
the margins of glaciers and be buried by morainal material falling of the glacier front.
In time, the buried snow will metamorphose to ice. This ice can be distinguished from
glacier ice because it is composed of very small crystals, in contrast to the large crystals
of the latter. This understanding was pioneered by Gunnar Østrem (1962, 1964b) based
on research in Lappland and Baffin Island. The glacier terminus is essentially the end of
the journey for the larger rock material. The finer debris, however, can be transported
farther through the action of glacial melt streams and wind.
FIGURE 4.38 Moraines in the Barnard Glacier, Saint Elias Mountains, southeastern Alaska, 1984.
Those at the very edges of the glacier are lateral moraines. Those in between are all medial mo-
raines and represent the confluence of two lateral moraines somewhere up-ice, as can be seen
in several places in the photo. (Austin Post photo courtesy of the Geophysical Institute GeoData
Center, University of Alaska, Fairbanks.)