As one climbs out of the main valleys, especially on the western side of the Rockies, when timber-line is passed, snow begins to show itself, and at length there are snow-fields draining into glaciers, which creep thousands of feet down into the valleys. Finally the warmth of the lower elevation balances their slow advance and from an ice cave at the end there flows a milky mountain torrent, loaded with the stones and gravel and rock flour ground from the rock floor of the glacier above. Here there is a splendid chance to study the carving power of ice in its downward motion urged by gravity. Where the mountain torrent cuts sharp-walled canyons or V-shaped gorges, the glacier carves broad U-shaped valleys with smoothly rounded surfaces; and one notices that these broad U valleys often run far below the present glacier and are crossed by crescent-shaped moraines, perhaps now tree-covered, monuments of former ice extension. In general our glaciers seem to be retreating as if the warming up of the climate after the Ice Age were still slowly going on.
In many cases the old ice-carved valleys have been hollowed into rock basins or have their outlet blocked by moraines; and this gives rise to some of our most delightful mountain scenery, where forest slopes and precipices and snow-fields are reflected in lakes of the most marvellous turquoise blue in deeper parts, running into clear green in the shallows. These ravishing colors appear to be due to the last remnants of glacial mud from the ice-fed streams flowing into the lakes, the finest possible particles settling almost infinitely slowly, and reflecting the short blue rays of light, just as infinitessimal particles in the air give the paler blue to the sky. The intense blue or green of these mountain lakes contrasts strongly with the much paler blue of clear lakes like Superior or Ontario,, unfed with glacial mud, and makes it certain that the minute remaining particles are the real cause of the color.
