The Spell of the Rockies

by Enos A. Mills

A glacier may move forward only a few feet in a year or it may move several feet in a day. It may be only a few hundred feet in length, or, as during the Ice Age, have an area of thousands of square miles. The Arapahoe Glacier moves slowly, as do all small glaciers and some large ones. One year's measured movement was 27.7 feet near the centre and 11.15 near the edge. This, too, is about the average for one year, and also an approximate movement for most small mountain glaciers. The centre of the glacier, meeting less resistance than the edges, commonly flows much more rapidly. The enormous Alaskan glaciers have a much more rapid flow, many moving forward five or more feet a day.

A glacier is the greatest of eroding agents. It wears away the surface over which it flows. It grinds mountains to dust, transports soil and boulders, scoops out lake-basins, gives flowing lines to landscapes. Beyond comprehension we are indebted to them for scenery and soil.

Glaciers, or ice rivers, make vast changes. Those in the Rocky Mountains overthrew cliffs, pinnacles, and rocky headlands. These in part were crushed and in part they became embedded in the front, bottom, and sides of the ice. This rock-set front tore into the sides and bottom of its channel—after it had made a channel!—with a terrible, rasping, crushing, and grinding effect, forced irresistibly forward by a pressure of untold millions of tons. Glaciers, large and small, the world over, have like characteristics and influences. To know one glacier will enable one to enjoy glaciers everywhere and to appreciate the stupendous influence they have had upon the surface of the earth.

They have planed down the surface and even reduced mountain-ridges to turtle outlines. In places the nose of the glacier was thrust with such enormous pressure against a mountainside that the ice was forced up the slope which it flowed across and then descended on the opposite side. Sustained by constant and measureless pressure, years of fearful and incessant application of this weighty, flowing, planing, ploughing sandpaper wore the mountain down. In time, too, the small ragged-edged, V-shaped ravines became widened, deepened, and extended into enormous U-shaped glaciated gorges.

Glaciers have gouged or scooped many basins in the solid rock. These commonly are made at the bottom of a deep slope where the descending ice bore heavily on the lever or against a reverse incline. The size of the basin thus made is determined by the size, width, and weight of the glacier and by other factors. In the Rocky Mountains these excavations vary in size from a few acres to a few thousand. They became lake-basins on the disappearance of the ice.

More than a thousand lakes of glacial origin dot the upper portions of the Rocky Mountains of Colorado. Most of these are above the altitude of nine thousand feet, and the largest, Grand Lake, is three miles in length. Landslides and silt have filled many of the old glacier lake basins, and these, overgrown with grass and sedge, are called glacier meadows.

Vast was the quantity of material picked up and transported by these glaciers. Mountains were moved piecemeal, and ground to boulders, pebbles, and rock-flour in the moving. In addition to the material which the glacier gathered up and excavated, it also carried the wreckage brought down by landslides and the eroded matter poured upon it by streams from the heights. Most of the material which falls upon the top of the upper end of the glacier ultimately works its way to the bottom, where, with the other gathered material, it is pressed against the bottom and sides and used as a cutting or grinding tool until worn to a powder or pebbles.

Train-loads of débris often accumulate upon the top of the glacier. On the lower course this often is a hundred feet or more above the surface, and as the glacier descends and shrivels, enormous quantities of this rocky débris fall off the sides and, in places, form enormous embankments; these often closely parallel long stretches of the glacier like river levees.

The large remainder of the material is carried to the end of the glacier, where the melting ice unloads and releases it. This accumulation, which corresponds to the delta of a river, is the terminal moraine. For years the bulk of the ice may melt away at about the same place; this accumulates an enormous amount of débris; an advance of the ice may plough through this and repile it, or the retreat of the ice or a changed direction of its flow may pile the débris elsewhere and over wide areas. Many of these terminal moraines are an array of broken embankments, small basin-like holes and smooth, level spaces. The débris of these moraines embraces rock-flour, gravel, pebbles, a few angular rock-masses, and enormous quantities of many-sized boulders,—rocks rounded by the grind of the glacial mill.