Daly — The Coral-Reef Zone. U5 



the waves expended much of their energy offshore, on the 

 shelving flat, which acted as a somewhat prolonged 

 defence against rapid abrasion of the land. For a con- 

 siderable time both the fringing reef and the underlying 

 bed-rock would be left nearly untouched by the waves 

 when these were beating at lowest sea-level. Farther 

 out to sea wind-driven ocean currents were scouring, 

 attempting to reduce the outer part of the shelf once 

 more to "current base" (usually at about 40 fathoms or 

 75 meters). The resulting steepening of the bottom gra- 

 dient ultimately increased the striking power of the 

 shore breakers, and hence accelerated the benching of the 

 coastal-plain sediments. The bluffs of dead f ringing- 

 reef would be attacked in their turn, and finally the waves 

 would discover the hard, volcanic or other, rocks beneath 

 the reef. The cliffing of these more resistant rocks must 

 have been very slow. It is probable that the retrogres- 

 sion of the cliffs cut in massive lavas by the waves of the 

 first glacial stage was normally no more than a few hun- 

 dred meters. If so, very few of these cliffs could now be 

 seen above sea-level. 



(3) A third result of the lowering of sea-level must 

 have been the deepening of valleys on the land. The 

 bays due to drowning of valleys in pre-Glacial time were 

 more or less filled with inorganic detritus or weak cal- 

 careous deposits. In such materials the streams would 

 speedily bring their channels close to the new base-level, 

 and subsequent widening of the young valleys would be 

 comparatively rapid. Moreover, the fillings of the old 

 drowned-valley bays in open-ocean islands w^ould be 

 attacked by the ocean waves. Hence there would be a 

 tendency toward the removal of the old valley-filling at 

 the pre-Glacial sea-level and for some distance below it. 

 If enough time for the erosion were available, this tend- 

 ency may have gone to the limit in some cases. The 

 return of the sea to its pre-Glacial level would give such 

 a valley an embayed termination very similar to the 

 original bay, so far as the ground-plan is concerned. 



Doubtless much more numerous are the instances 

 where narrow bays were formed by the Recent drowning 

 of valleys which had never been drowned in pre-Glacial 

 time. 



In general, the deductions just mentioned apply in 

 principle to the different glacial stages, but probably new 



Am. Jour. Sci. — Fourth Series, Vol. XLVIII, No. 284. — August, 1919. 

 10 



