P. A. Paly — Problems of the Pacific Islands. Y17 



would cut a wide bench, with an increase in the detrital shelf, 

 as shown in section 18. If the island rocks were weak, the 

 waves of the Glacial period might destroy the remnant of the 

 island, forming the smooth platform of section 19. Or, if the 

 island were large and the rocks stronger, section 18 might be 

 taken to represent the total amount of wave-cutting possible at 

 the end of the Glacial period. Colonization by post-Glacial 

 corals would lead to fringing reefs at Jf, barrier reefs at Y, 

 and atoll reefs at Z. 



The melting of the ice-caps meant a warmer Earth climate 

 and the re-establishment of marine conditions once more favor- 



Figs. 16-19. 





^^^^—^ -^ii| ■'■ 19 



lliiiii^^. 



Figs. 16-19. Sections illustrating the development of barrier reefs and 

 atolls. 



] 6. A normal volcanic island. 



17. The same island largely peneplained, with the necessary formation 

 of an encircling embankment of detritus (stippled). It is here arbitrarily 

 assumed that there has been no marine abrasion. 



1 8. The same island extensively benched by the waves, involving some 

 increase of the embankment. Such benching is expected in very old islands 

 which have been exposed to active abrasion, either because of the Pleistocene 

 chilling of the ocean or because of temporary failure of reef protection in 

 pre-Glacial time. 



19. Complete truncation of the island by continued marine abrasion, 

 with a slight broadening of the embankment. In many instances this was 

 a stage possibly attained during pre-Glacial periods as well as during the 

 Pleistocene. 



In figs. 17, 18, and 19, the size of the embankment, as drawn, corresponds 

 merely to the bulk of purely inorganic detritus. If intermixed reef and 

 other organic material were allowed for, the embankment must be repre- 

 sented as broader. After the abrasion, fringing, barrier, and atoll reefs 

 would be favorably located at X, Y, and Z, respectively. Shifts of sea-level 

 are not shown. 



The sections are drawn to scale and are also intended to show the great 

 areal extent of the weak embankment materials, laid down around old 

 oceanic volcanoes in pre-Glacial time. About one-half of the platform repre- 

 sented in fig. 19 is underlain by these materials, which would offer little 

 resistance to the benching surf of the Pleistocene period. 



