Snow, Ice, Avalanches, and Glaciers - Mountain Geography: Physical and Human Dimensions - page 187

FIGURE 4.32 Small valley glacier located in the North Cascades, Washington. The photo was taken
near the end of the summer (9 September 1966) and the firn line is evident midway up the glacier.
The very lightest-toned snow had fallen within the previous few days. (Austin Post, U.S. Geologic-
al Survey.)
Glacial Climatic Response and Mass Balance
Today's glaciers are only a vestige of what existed during the height of the Pleistocene
epoch; nevertheless, active mountain glaciers still occur in all latitudes. The Pleistocene
is the most recent of several “ice ages” affecting the Earth; it represents approximately
2.6 million years of major fluctuation in environmental conditions which “ended” about
12,000 years ago (Riccardi 2009). Early researchers found that four major ice advances
had occurred in the Pleistocene (Martini et al. 2001). More recent studies at finer tem-
poral resolution have identified many more advances and retreats during that epoch
(Benn and Evans 2010; Barry and Gan 2011). During glacial advances, continental gla-
ciers developed in the high latitudes, then advanced equator-ward into the middle lat-
itudes of the continents, while mountain glaciers advanced downslope from highlands
and spread into the surrounding lowlands. It is generally felt that each major glacial
advance coincided with a period of lower temperatures, but the exact requirements