Page:Popular Science Monthly Volume 2.djvu/683

Rh stream, and is itself slowly worn away and distributed elsewhere by the abrasion of annual freshets. Portions of it may thus disappear, but other portions remain.



It is obvious that terrace-formations occur in greatest perfection where the stream is not very rapid. Where it flows as a torrent, a flood-plain or delta may form only at its mouth. Sometimes, however, a swift stream is checked by the accumulations of débris or by rocky gorges, forming lake-like basins around which terrace-formations occur with great uniformity and beauty. The Connecticut River is 1,589 feet higher at its source than at its mouth; and, according to Prof. Hitchcock's excellent report on the Surface Geology of New England, twenty-two such basins, or levels, occur in its descent.

It is evident, as we have observed, that the highest terrace of a series is the one first formed and the oldest, but, when formed, was equally, with the last one, the flats, or flood-plain, of the river; whence it follows that the river was then much higher as regards the general level of the land than now. Its present deep valley was not excavated, but it by no means follows that the river was any higher as regards the level of the sea. A change of level has, indeed, taken place, but it has been of the land, not of the ocean. No truth in geology is better established than this perpetual oscillation of the crust of the globe, and from the unchanging ocean-level is measured the extent of the movement.

The process by which a river-valley is excavated, and terraces formed upon its banks, is directly connected with this elevation of the land. Indeed, it could occur only during a period of elevation, and may have commenced with the emergence of the land above the waters, for then would begin the flowing of streams and their concentration into larger ones, forming at last our magnificent system of rivers. During a period of subsidence, however, the rivers disappear, as their valleys are filled, and the land is overflowed by the invading ocean. Nor is proof wanting of submergence of a very large portion of this continent, especially that which is north of the fortieth parallel, directly following the Glacial and preceding the Terrace Epoch; and nowhere is that fact more apparent than in New England.