﻿GEOLOGY AND PALEONrOI.OGY OF THE CANAL ZONE. 257 



The character of the entrenching within an estabhshed valley after 

 lowering of sea level will depend upon the off shore slope of the sea 

 bottom previous to the lowering of sea level. As any acceleration 

 of headward erosion by a stream depends upon increase in steepness 

 of the longitudmal profile of the stream bed. unless the gradient of 

 the lower course of the stream is considerably increased there will 

 be no visible vaUey-in-valley landscape after submergence following 

 deglaciation. Subsequently I will show that in the West Indies there 

 is abundant evidence of another kind that during Pleistocene time 

 sea level was lowered, and that at the close of Pleistocene time it was 

 raised. Valley-in-valley arangement is a criterion of very doubtful 

 value. 



Professor Davis also insists that if the Glacial-control hypothesis 

 is correct, the spurs of islands within barrier reefs should be cliffed — 

 the cliffs cut during Pleistocene glaciation. As promulgated in print 

 by Professor Davis, I doubt the vaUdity of this criterion. Perhaps 

 the following hypothetical explanation may apply in some instances: 



Around volcanic islands, the centers of which are far enough from 

 the shore for the surface profile of the ejecta to have assumed the 

 theoretic catenary curve, marine planation may proceed without at 

 first cutting pronounced cliffs. If the material on the higher slopes 

 is not greatly consolidated, alluviation and surface creep may deliver 

 detritus more rapidly than the sea can remove it by marginal cutting 

 and b}^ undertow and other transporting agents. The sea may thus 

 be held back from the interiorly situated harder volcanic rocks and 

 the development of well-marked sea cliffs may thereby be prevented 

 while the sea bottom would be aggraded near shore and a submarine 

 flat produced. Should sea level then fall so that the shore line would 

 shift to the outer edge of the previously formed flat, erosion al processes 

 might obliterate the low scarp carved into unconsolidated coUuvial 

 and alluvial material. Under such circumstances, should the sea- 

 bottom gradient be less than that of the stream profiles, the lowering 

 of sea level would not lead to the development of vaUeys-wi thin- 

 valleys, and alluvial plains might be pushed forward beyond the ends 

 of the interstream spurs. Should sea level rise back to its former 

 stand reef corals might establish themselves on the submerged flat 

 at any place where the proper ecologic conditions might be found 

 and develop into a barrier reef, off a land area on which there would 

 be no valle^^-in-valley arrangement of stream courses and along whose 

 shores there would be no cliffed spurs. This is an hypothetical in- 

 stance, but that it is possible is apparently shown by the island of 

 St. Christopher, West Indies, where such an arrangement of central 

 volcanic mountains and relatively flat areas underlain by volcanic 

 ejecta and colluvial and aUuvial material intervene between them 



