It is supposed that all these valleys have been formed by glaciers, which during the ice age, thousands of years ago, filled the cañons and swept over the mountains. These masses of ice, moving very slowly, ground and tore up the rocks under and around them till deep gorges and steep, high cliffs were left in their tracks. Most of the glaciers melted long ago, but on Mount Lyell, on Shasta, and a few of the Sierra summits may still be found those ever-living ice-rivers, the one on Mount Lyell being the source of the Tuolumne River. California is rich in lakes, especially in the mountains where the melting snows gather in every hollow and form lakelets in chains or groups, or in one large body of water like Tahoe, Donner, or Tenaya lakes. One of the most beautiful lakes in the world is Lake Tahoe. It is six thousand feet above sea-level, and the mountains around it rise four thousand feet higher. On these peaks snow-drifts lie the year round above the "snow-line," as a height over eight or nine thousand feet is called. Nevada, treeless and barren, is on the eastern side of Lake Tahoe, while the western or California side is green and thickly wooded with beautiful pines. But the first thing one would notice, perhaps, is the wonderful clearness of the lake water. As one stands on the wharf the steamer _Tahoe_ seems to be hanging in the clear green depths with her keel and twin propellers in plain sight. The fish dart under her and all about as in some large aquarium. There a big lake-trout shoots by like a silver streak of light, or here is a school of hundreds of little fingerlings. Every stick or stone shows on the bottom as one starts out on the steamer, and as one sails along where the water is sixty or seventy feet deep. In the middle the lake's depth is fifteen hundred feet and the water is a dark indigo-blue. At the edge and along shallow places the color is bright green, as at Emerald Bay, a beautiful inlet three miles long. Lake Tahoe is twenty miles in length and about five wide, and its icy cold waters are of crystal clearness and very pure. Fallen Leaf Lake is a smaller Tahoe, and Donner Lake, not far from Truckee, and now the camping-place of many a summer visitor, is the place where years ago the Donner overland party spent a terrible winter in the Sierra snows.