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.