188 



GEOLOGY 



cut through. A later stage in the process is shown in Fig. 2, 

 PI. VIII. 



When the stream has cut off a meander, the abandoned part 

 of the channel often remains unfilled with sediment. If it contains 

 standing water, as it often does, it becomes the site of a lake (Fig. 

 156). Such lakes sometimes have the form of an ox-bow, and so 

 are called ox-bow lakes (Pis. XIV and VIII). 



3. At debouchures. Where a swift stream flows into the sea 

 or a lake, its current is promptly checked and soon destroyed alto- 



Fig. 159. The delta of the Nile. 



gether. Its load is accordingly dropped. If not washed away by 

 waves, etc., the deposits of river-borne sediment in such pi; 

 make deltas (Figs. 146 and 157 to 159). 



The delta has some points in common with the alluvial fan. 

 In both cases the principal deposit is concentrated at the point when- 

 the velocity is suddenly checked. In the case of the delta, how- 

 ever, the current is checked more completely, and the debris accu- 

 mulates (at the outset) below the surface of the standing water. 

 In form, the delta differs from the alluvial fan in that its edre lias 

 a steep slope (compare Figs. 160 and 161). 



Once a delta is started below water, deposition takes place upon 

 its surface, which may be built up to, and even above, the water- 



