RESIDUAL, GRAVITY-LAID, WATER-LAID DEPOSITS 39 



represent the former Hood plains of streams which now flow at a 

 lower level; second, first bottom lands, or present flood plains; third, 

 deltas; and fourth, alluvial cones and fans. 



Terraces originate in three ways : ( 1 ) those formed hy depo- 

 sition of material from overloaded streams giving rise to sand, 

 gravel, or silt terraces (Fig. 29). These occur principally along 

 the streams that carried the d-ainage from the melting glaciers. 



V.'iuMi the current decreased, the 

 load was dropped and the valleys 

 were filled to a greater or less 

 height with gravel and sand. In 

 some cases the valleys were filled 

 almost to a level with the upland 

 (Figs. 30 and 31). Farther 

 down the stream the terrace be- 

 came lower and the finer material 

 was deposited. When the glacier 

 retreated, the stream, having no 

 load to carry, would begin to cut 

 down through the gravel and 

 soon this formation would be 

 much above the stream and con- 

 stitute a terrace, second bottom, 

 or bench land. The same action 

 might take place down the stream 

 where the finer material was 

 deposited. (2) Those formed 

 through deration of land and 

 consequent rejuvenation of the 

 stream, thus causing it to cut 



Fl ' 31 ' C ' i< ' W n flmvn lnro "- n nn<1 abandon the 

 r old flood plain and form a new 



1 hose formed by i>o,,<Knri of tributary stream* due to the 

 building up of the Hood plain of fbe main stream more rapidly than 

 that of the tributaries. Tn this wnv the lower part of the tributary 

 valley is formed into a lake which would receive a deposit of (im- 

 material from the tributary but conrse from ihe inrushing waters 

 from the main stream during floods. A reduction of the wafer 

 supply and the amount of sediment carried by the main stream 

 will enable the tributary to cut down into the flood plain, drain the 

 lake, and form a. new valley in the fill. (Jood examples of this are 



