Post 24220
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After the forest times and fire times had passed away, and the volcanic furnaces were banked and held in abeyance, another great change occurred. The glacial winter came on. The sky was again darkened, not with dust and ashes, but with snow which fell in glorious abundance, piling deeper, deeper, slipping from the overladen heights in booming avalanches, compacting into glaciers, that flowed over all the landscape, wiping off forests, grinding, sculpturing, fashioning the comparatively featureless lava beds into the beautiful rhythm of hill and dale and ranges of mountains we behold to-day; forming basins for lakes, channels for streams, few soils for forests, gardens, and meadows. While this ice-work was going on, the slumbering volcanic fires were boiling the subterranean waters, and with curious chemistry decomposing the rocks, making beauty in the darkness; these forces, seemingly antagonistic, working harmoniously together. How wild their meetings on the surface were we may imagine. When the glacier period began, geysers and hot springs were playing in grander volume, it may be, than those of to-day. The glaciers flowed over them while they spouted and thundered, carrying away their fine sinter and travertine structures, and shortening their mysterious channels.
The soils made in the down-grinding required to bring the present features of the landscape into relief are possibly no better than were some of the old volcanic soils that were carried away, and which, as we have seen, nourished magnificent forests, but the glacial landscapes are incomparably more beautiful than the old volcanic ones were. The glacial winter has passed away, like the ancient summers and fire periods, though in the chronology of the geologist all these times are recent. Only small residual glaciers on the cool northern slopes of the highest mountains are left of the vast all-embracing ice-mantle, as solfataras and geysers are all that are left of the ancient volcanoes.
Now the post-glacial agents are at work on the grand old palimpsest of the park region, inscribing new characters; but still in its main telling features it remains distinctly glacial. The moraine soils are being leveled, sorted, refined, re-formed, and covered with vegetation; the polished pavements and scoring and other superficial glacial inscriptions on the crumbling lavas are being rapidly obliterated; gorges are being cut in the decomposed rhyolites and loose conglomerates, and turrets and pinnacles seem to be springing up like growing trees; while the geysers are depositing miles of sinter and travertine. Nevertheless, the ice-work is scarce blurred as yet. These later effects are only spots and wrinkles on the grand glacial countenance of the park.