Later the Ice Age bore downward from the north, and through immeasurable years carved the fractured granite into shapes of enduring beauty. It rounded the great jutting fronts, it insured them against the tooth of time with the keen icy polish with which they shine still against the morning. It gouged narrow wall-sided cañons, cut the course of rivers, and sinking like a graver's tool into the heart of the range, scooped out deep wells of pleasantness. Afterward, when the ice was old, it must have moved more slowly, for the lines it left, retreating northward, are more flowing, the hill-crowns rounder. And then the mountain was besieged with trees. They stormed it, scaled its free precipices; you can see by the thick mould of the valleys what ranks and ranks of them went down, and along the snow-line how by the persistence of assault they are bent and contorted. This is the whole effect of the sombre swathes of pine that mask the Sierra slopes. They march along the water-courses, they climb up sheer precipices in staggering files, trooping in the passes; across the smooth meadow spaces they lock arms, they await the word of command. By a very little observation they are seen to be ranged in orderly companies. Here a warm current of air travelling steadily from the superheated valleys carries the life zone higher, there a defiant bony ridge drops it a few thousand feet, but the relative arrangement of species does not greatly vary. The broad oaks, like reverend grandsires, from the foothills see the procession go by, they follow as far as the gates of the mountain, crutched and bowed. All the lower cañons are full of a rabble of deciduous trees, chinquapins, scrub oak, madroño, full of gay camp-followers, lilac, dogwood, azaleas, strumpet penstemons, flaunting lupins, monk's-hood, columbine. [Illustration: YOSEMITE FALLS] The grey nut pine, wide-branched, unwarlike but serviceable, opens the ranks of conifers. Then the long-leaved pines begin, _ponderoso_, _Coulteri_, and the slender, arrowy, fire-resisting _attenuata_. On the western slope, increasing as they go northward, the redwood holds all the open country, but it is no climber like _monticola_, the largest of all true pines, the captain of the Sierra forests. The firs usurp the water-borders and the low moraines; clannish, incommunicable, they seem not to find it worth while to grow unless they grow stately. Above all these range the thin-barked pines, the lodgepole, Douglas spruce, librocedrus, and hardy junipers in windy passes. About the meadows and lake-borders the quaking asps push like children between the knees along the line; and highest, most persistent, the creeping-limbed, wind-depressed white-barked pine, under whose matted boughs the wild sheep bed.