During my first boyish exploring trip in the Rocky Mountains I was impressed with the stupendous changes which the upper slope of these mountains had undergone. In places were immense embankments and wild deltas of débris that plainly had come from elsewhere. In other places the rough edges of the ca?ons and ridges had been trimmed and polished; their cliffs and projections were gone and their surfaces had been swept clean of all loose material. Later, I tried vainly to account for some ca?on walls being trimmed and polished at the bottom while their upper parts were jagged. In most ca?ons the height of the polishings above the bottom was equal on both walls, with the upper edge of the polish even or level for the entire length of the ca?on. In one ca?on, in both floor and walls, were deep lateral scratches in the rocks.[Pg 248]
One day I found some polished boulders perched like driftwood on the top of a polished rock dome; they were porphyry, while the dome was flawless granite. They plainly had come from somewhere else. How they managed to be where they were was too much for me. Mountain floods were terrible but not wild enough in their fiercest rushes to do this. Upon a mountainside across a gorge about two miles distant, and a thousand feet above the perched boulders on the dome, I found a porphyry outcrop; but this situation only added to my confusion. I did not then know of the glacial period, or the actions of glaciers. It was a delightful revelation when John Muir told me of these wonders.
Much of the earth\'s surface, together with most mountain-ranges, have gone through a glacial period or periods. There is extensive and varied evidence that the greater portion of the earth has been carved and extensively changed by the Ice King. Substantial works, blurred and broken records, and impressive ruins in wide array over the earth show long and active possession by the Ice King, as eloquently as the[Pg 249] monumental ruins in the Seven Hills tell of their intense association with man.
Both the northern and the southern hemispheres have had their heavy, slow-going floods of ice that appear to have swept from the polar world far toward the equator. During the great glacial period, which may have lasted for ages, a mountainous flood of ice overspread America from the north and extended far down the Mississippi Valley. This ice may have been a mile or more in depth. It utterly changed the topography and made a new earth. Lakes were filled and new ones made. New landscapes were formed: mountains were rubbed down to plains, morainal hills were built upon plains, and streams were moved bodily.
It is probable that during the last ice age the location and course of both the Ohio and the Missouri Rivers were changed. Originally the Missouri flowed east and north, probably emptying into a lake that had possession of the Lake Superior territory. The Ice King deliberately shoved this river hundreds of miles toward the south. The Ohio probably had a sim[Pg 250]ilar experience. These rivers appear to mark the "Farthest South" of the ice; their position probably was determined by the ice. Had a line been traced on the map along the ragged edge and front of the glacier at its maximum extension, this line would almost answer for the present position of the Missouri and Ohio Rivers.
THE HALLETT GLACIER THE HALLETT GLACIER
The most suggestive and revealing words concerning glaciers that I have ever read are these of John Muir in "The Mountains of California": "When we bear in mind that all the Sierra forests are young, growing upon moraine soil recently deposited, and that the flank of the range itself, with all its landscapes, is new-born, recently sculptured, and brought to light of day from beneath the ice mantle of the glacial winter, then a thousand lawless mysteries disappear and broad harmonies take their places."
"A glacier," says Judge Junius Henderson, in the best definition that I have heard, "is a body of ice originating in an area where the annual accumulation of snow exceeds the dissipation, and moving downward and outward to an area where dissipation exceeds accumulation."
[Pg 251]
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,[Pg 252] 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[Pg 253] 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 addi[Pg 254]tion 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[Pg 255] 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 ............