Mr Hufjh Miller on River- Terr acinrj. 293 



The distinctive characters of fan terraces are obvious — 

 (1.) Their surface-planes slope radially away from the "gate." 

 (2.) They are of e([ual lieight on the two sides of the stream 

 issuing from it. (o.) Their terrace-fronts, as viewed from the 

 main valley, tend to thin down at each end. They are best 

 formed when a tributary enters upon the terraced side of a 

 persistent bend. Wlien the main river no longer encourages 

 fan-formation, and the side-stream is allowed to work its 

 way after its own fashion, it may of course set up its pro- 

 cesses of planation and terracing within the heart of its own 

 fans (see Fig. 16, in which the shaded surface is the plane of 

 an old delta terrace). 



Delta Terroxes. 



The fan terrace just described is the best exemplar of the 

 terraces that may result from the movements of a river near 

 its mouth. The valley which receives the tributary, and the 

 coast-line which terminates the course of the river, are both 

 alike liable to be arranged in stages of descent — the latter by 

 subsidence, the former by erosion — so as to communicate 

 periodical impulses to the stream which enters them. The 

 formation of delta terraces and fan terraces, in fact, can be 

 almost equally w^ell pictured to the mind by means of Fig. 15. 

 The delta in both cases is cut in two when the acceleration 

 comes; the margins of the cut face each other at the same 

 level, and the terrace-flats representing the tw^o halves, or, as 

 Chambers phrased it, the toings of the intersected delta, have 

 the same outward slope and gentle arch.* 



The greater size and gentler slope of delta terraces render 

 them more liable than fan terraces to form " cone within 

 cone." Hence they are more frequently complicated w^itli 

 other varieties of terrace. Upheavals alternated with periods 

 of rest, such as produce old coast-lines, may each obviously 

 result in a kind of ci/cle of river action containing tln-ee 

 stages : 1st. The stage of the delta terrace (answering to that 



* Strictly speaking, of course, the margins of the intersected delta have 

 relation to this arcuated form of its transverse section. If, therefore, one of 

 the wings were removed as far as its tip, while the other remained intact, the 

 marofius would 7iot stand at the same level. 



