Page:Popular Science Monthly Volume 2.djvu/682

662 step-like plains." Roads are carried up the Cordillera on elevated terraces to a height of 9,000 feet.

These formations, so widely distributed and so uniform in their aspects, have an important geological significance. They are evidently among the latest results of the dynamic agents which have modified, and are still modifying, the surface of the globe. Those along the banks of rivers have been formed during the erosion of the valleys. Their history, therefore, begins with the development of the present



river-systems, and comprises what is known in geology as the "Terrace Epoch." They are most abundant and perfect in the drift latitudes—that is, where the continental floors are deeply covered by the waste and débris of the Glacial Period, which closely preceded that of the Terraces. If we examine the valley of a gently-flowing river, we may study all the processes by which it was formed, and step-like terraces distributed along its banks.

There is the channel along which the stream is flowing. By the side of it, at intervals, are verdure-covered meadows and deposits of shingle and sand, overflowed during periods of rain and freshet. These constitute what may be termed the river-flats or flood-plains. Something is added to it during each overflow. Meanwhile, the river-channel is deepening by the wearing action of the current and transportation of the materials of its bed. At length the waters are discharged along the channel, and no longer overflow the flood-plain, which becomes at once a terrace, the last formed and newest of the series, the oldest of which may be more than a thousand feet up the bank. Fig. 2 shows a section of a river-valley with terraces on one side only, a circumstance which frequently arises from sinuosities of the stream.

The newly-made terrace now really forms the bank or banks of the