I2 8 PHYSICAL GEOLOGY 



as alluvial terraces. The latter are fragments of sediments which 

 once filled the valleys to their level, and may be accounted for by 

 meandering and swinging streams, slowly degrading valleys which 

 had previously been aggraded ; in other words, by streams slowly 

 eroding their flood plains. Such a change from deposition to erosion 

 may be the result of one or more of several causes, (i) If a region 

 is elevated so as to increase the velocity of the streams, deposition is 

 succeeded by erosion. (2) This is also true if the volume of water 

 in a stream increases without a corresponding increase of sediment. 

 Such a condition may result when a moist climate follows a dry 

 one, or when a stream captures the headwaters of another stream. 

 Alluvial terraces in many dry regions appear to indicate oscillations 

 between dry conditions, when soil and rock waste were washed down 

 from the mountain sides into the valleys, and moist conditions, when 

 the deposits formed in the valley bottoms were dissected because the 

 load of the streams had been diminished. This resulted from the 

 fact that during wet years the soil was held in place by the flourish- 

 ing vegetation ; while during the dry years, although the rainfall was 

 less, the amount of waste removed was great because of the disap- 

 pearance of the vegetation which formerly bound the weathered 

 rock. 



(3) If the quantity of sediment is decreased, as occurs when a 

 stream ceases to erode at its head, deposition gives place to erosion. 



(4) As a valley lengthens, so 

 much of its load may be 

 dropped in the upper and 

 newer portions of its flood 

 plain that it is enabled to 

 degrade its older flood plain. 



Fig. 118. - Rock terraces due to uplift. (5) When a region suffers 



successive uplifts, so that a 

 stream is unable to cut away its former flood plain before its grade 

 is again increased, terraces will be formed which correspond on the 

 two sides of the valley (Fig. 118). (6) If a degrading stream 

 decreases in volume, it. will not be able to occupy the full width of 

 its valley and will cut a narrower valley in the older one. The last- 

 mentioned cause, although perhaps the one which first suggests itself, 

 appears to have been rarely effective in the formation of terraces. 



J he close of glacial times (p. 663) seems to have been especially 

 favorable for valley filling, because of the overloading of the streams 



