CENOZOIC TIME — QUATERNARY. 



991 



The tributaries of a river, when torrential, often carried into the valley 

 great quantities of gravel and stones, and made river-border deltas on a 

 continuous series of levels, as the deposits rose in height. The materials 

 thus received were stratified in part by the waters of the river, the finer 

 portions being taken up and drifted on to make the finer deposits down 

 stream. The delta-like deposits give local coarseness and irregularity to the 

 beds and somewhat greater firmness, so that, under erosion, they sometimes 

 are made to stand out and look a little kame-like, although strictly of 

 fluvial origin. 



Under the abundant supply of water, the width of the flood grounds or 

 river flats in many large valleys became increased to miles, and in some 

 cases to scores of miles. Over such flood plains, through all the progressing 

 deposition and varying velocities of flow in the river channel, there were, as 

 in modern flood plains, areas of relatively quiet waters, where beds of clay 

 or fine earth were formed, giving the valley-formations a great diversity of 

 constitution. Further: rivers in some places became dammed by floating 

 ice and whatever else the waters transported, as now in modern floods ; and 

 these dams were the cause of quiet deposits in the Avaters above them, — 

 that is, of extensive beds of clay and fine sands or earth. Through the two 

 agencies, subsidence and dams, and perhaps in a few cases elevation of the 

 land toward the mouth of the stream or elsewhere, nearly all the transi- 

 tions in the nature of the fluvial deposits, from clays to the coarsest kinds, 

 have their explanation. The height of the highest flood plain gives approxi- 

 mately the height of the maximum flood. 



The Champlain deposits along valleys or about lakes are usually terraced. 

 A view of the terraces on the Connecticut below Hanover is given on page 195. 



The following figure is a generalized section of a terraced valley. 



1554. 



wMmmmmm. 



Section of a valley, with its terraces completed. 



In this figure, the channel which the river occupies at low water is at R ; 

 a6, a'6', are the flats either side which become flooded in modern high 

 freshets, — in other words, the flood-grounds ; e/, e^f\ are the flood-grounds 

 of the river (or what is left of them) during the great Champlain floods. 

 The intermediate terrace-plains are other levels, formed either during the 

 rise of the flood, the water while on the increase flowing long, it may be, at 

 certain levels ; or during the decline, which also may have taken place by 

 stages, and have been long in progress. Part may be under-water levels ; 

 for great streams and lakes, or lake-borders, often have shoals at two or 



