Page:Popular Science Monthly Volume 69.djvu/133

Rh flowed in much their present channels. At times their flood was mighty, comparable only with such streams as Amazon and Nilus. Once its waters reflected the foliage of oak and plane-tree; the fig and the tulip tree flourished on its banks and the heights beyond were dark with forests of Sequoia. Later, its soft alluvial flats trembled under the ponderous tread of the hairy mammoth, while the wild horse grazed upon its verdant savannas. The bison knew its prairieland and the mazama its foothills. With the wane of the Age of Ice the musk-ox sought pasturage upon the Yukon tundra.

Strangely enough, during the height of the great Ice Age when the northeastern part of the continent as well as southern Alaska were buried deep under a continental ice-sheet, the greater part of the Yukon basin remained open to the sun. The traces of the glaciers are plain to see, about its head waters, on the Alaskan mountains to the south and the Yukon mountains to the north, but the terminal moraines are there to show where the deadly creeping of the ice was stayed, far above the present valley. During this time, perhaps, the muddy torrents bore to the river and the sea the alluvium which now composes the vast delta of the Yukon and the submarine flats, covering thousands of square miles, which are the characteristic feature of the eastern half of Bering Sea. With the shrinking of the glacier-sheet vast floods of water were let loose upon the alluvium of the lowlands, gradually shaping the features of the valley, concentrating the metallic contents of the gravels, and hurrying seaward the 'mountain meal' or impalpable white silt of the glacial grist.

From the volcanic craters of the mountain ranges to the south and west fine white ashes on one occasion poured in such volume as to cover the ground with a fleecy blanket, several feet thick, for many hundred square miles. Though covered by later accretions, this continuous layer of white ashes may still be traced for many leagues along the steep bluffs of the right bank of the river where it is under-cut by the current.

As the glaciers receded, the water supply became less profuse, the river settled between its banks, while the flats and prairies were invaded by willow and poplar, birch and spruce. The flora of the north, delicate and abundant, spread over the land, followed by the bee and the butterfly. Singing birds found nesting places, and with them all the small wild things which populate the wilderness, to gather sustenance from seed or berry, or seek refuge from the fox or hawk. And so at last the valley lay complete as first we knew it.

A brave domain, well defended, stretching some two thousand miles. On the north broad tundras hardly divided by low hills from the inviolate Arctic floes where they push upon the low sandy coast. To the northwest a turmoil of mountains, with hardly any game, kept off the explorer; while to the west, before the flatlands of the delta, lay many