Page:The Selkirk mountains (1912).djvu/174

150 CHAPTER VII.

GLACIERS OF THE SELKIRKS. (From a Monograph by A. O. Wheeler.)

Structure: In popular terms, a glacier is a river of ice draining a rock-basin above tilled to overflowing with ice and snow and technically known as névé or firn but popularly called a snow-field. The area of the Illecillewaet Snowtield is about 10 square miles, but there are many smaller in the Rocky Mountain system and some much larger, notably in the north, to wit, the Columbia Snowfield roughly estimated to cover 200 square miles at a mean altitude of 10,000 feet.

In these high regions, the heavy precipitation is always in the form of snow which keeps the great rock-basins full to over-flowing. The prodigious weight of the mass compacts the under-snows into clear crystal ice. To illustrate: if you take a handful of snow, squeeze it until it becomes a transparent crystal and then let it freeze, you have the counterpart of glacier-ice.

This overflow pushed slowly down the mountain by the enormous pressure from above constitutes the glacier—is the glacier. Its upper part is covered with snow, and its lower part, where the warmer air melts the surface-snow, is exposed ice. This is the "ice-fall" and. in contrast to the snow-covered part, is called "dry-glacier". Heat and cold, contraction and expansion, and the unevenness of the bed over which it moves downward, cause large cracks to occur across the ice-river. They are of all sizes and are called crevasses. Sometimes they cross each other, and the result is that when the pressure from above closes the cracks, great ice-pillars of fantastic shapes, called seracs. are formed. The flow of a glacier is just like that of a river: the centre of the stream moves faster than the sides or bottom: while on a curve the outside edge flows more rapidly than the inside. Again, at a narrow place, the flow is faster and the glacier is broken and cracked, resembling the broken water of a rapid.

Moraines are the piles of mud, rock, and loose boulders always along the sides, at the end (tongue, snout or forefoot), and sometimes in the middle of a glacier; and called respectively lateral, terminal and medial moraines. The materials of which they are formed have fallen from surrounding cliffs and, through the course of ages, have been carried by the glacier and deposited in their places in somewhat the same manner as a rapid stream or river deposits along its shores driftwood and detritus; only the flow of the glacier is very, very slow, a few feet in the year, and the process of piling up has taken ages. If the rate of the flow of the glacier over its rocky bed, caused by gravitation and pressure from above, is greater than the annual loss by melting during the summer, the glacier is advancing; if less, it is retreating.

In 1898 a supplementary minute-book was opened at Glacier House for the purpose of recording matters of general scientific interest. Among the first entries made is a general statement by the late Wm. S. Vaux, Jr., of the series of investigations carried on by himself, his brother and Miss Vaux in connection with the