RIVER AND LAKE TERRACES. * 663 



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. 



Fig. 2. 



Tkeeaced Rivee- Valley. 1, 2', 8', 4', and 5, are Terraces, 



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 debris 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 concen- 

 tration 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 j^roof 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. 



