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Transactions. 



If now the landward dunes have become 

 fixed by vegetation and somewhat con- 

 solidated, they will in time have a 

 normal drainage system developed on 

 them, and their irregularity of surface 

 will be reduced by the filling of the 

 hollows and the wearing-down of the 

 initial hills, so that their surface will 

 become a peneplain (fig. 2, C). 



If now after a period of slow pro- 

 gradation, perhaps followed by a period 

 of stationary shore-line, retrograda- 

 tion begins again, the seaward-sloping 

 coastal lowland, as its toe is cut back 

 to a line of cliffs of growing height, 

 will be dissected owing to the shorten- 

 ing of its streams. Since the dune 

 sand areas lie between the larger streams 

 from the old land, they will be traversed 

 only by systems of small streams arising 

 on them or heading on the cliffs behind. 

 The energy of such streams is slight, 

 but they work on very weak material 

 and so erode quite rapidly (fig. 2, D). 

 The larger streams from the old land 

 will now trench the fans they formerly 

 built, and they may destroy the fan- 

 surface altogether or reduce it to frag- 

 mentary terraces, developing stream- 

 eroded plains at lower levels, which may 

 in places be widened so as to plane 

 down parts of the dune-sand areas. 



The lowland will now resemble in a 

 general way a dissected coastal plain 

 of subaqueous origin, but there will be 

 differences in detail, for the initial form 

 in this case is a subaerially graded sur- 

 face, and when the streams entrenched 

 below it have again become mature the 

 slopes of their valley-plains will ap- 

 proximate to that of the interfluves, and 

 also such of the interfluvial areas as are 

 underlain by dune sandstone will not be 

 quite flat, but will retain the small relief 

 of a peneplain. 



There may be pauses in the re- 

 trogradation of the coast, and even 

 reversals from time to time, and thus 

 several series of terraces or valley-in- 

 valley forms may be developed (fig. 2, 

 E). 



It may be postulated that after the 

 coastal lowland has been cut back bv 



