20 FERGUSON. 



of storms is it as effective as a degrading stream. Marine erosion 

 may be tremendously effective during the progress of a tropical cyclone. 

 This is shown by the fact that in the vortex of the typhoon the elevation 

 of the water level, due solely to decrease in pressure of the atmosphere, 

 may amount to nearly a meter." A long period without storms has the 

 same effect on coastal erosion as a similar one without floods has upon 

 stream erosion. The waves, having less force, are no longer as effective 

 upon the cliffs and the weaker undertow is compelled to deposit its ma- 

 terial nearer shore, building up an additional bench of loose detritus 

 which is carried away by the next storm. Just as the deposits made by a 

 stream of small transporting power are carried away in times of flood. 

 AVhen elevation of the land occurs, no matter how small, the delicate rela- 

 tion between bench and cliff is destroyed, waves are tripped before they 

 can do effective w^ork upon the cliff and the result is first, a protecting 

 reef which must be planed down before the cliff can again be attacked, 

 and, if elevation continues, a raised bench. If, on the other hand the 

 land is depressed, the waves beat directly upon the cliff. In the deeper 

 water the undertow is less effective as a transporting agent and must 

 deposit the material fed to it by the waves until it has built up its bench 

 to "grade." 



The material deposited by the undertow at the end of the bench 

 conies' into the power of the shore current. The action of this current 

 is extremely variable, dejDending upon the prevalence of onshore winds 

 and the shape of the coast line. Its action is always to simplify the coast 

 line by depositing its load in and across the deeper reentrants. On the 

 coast of Batan this current should be strong during the prevalence of 

 steady monsoon winds, although it is complicated by and subordinate to 

 the tidal currents. Fig. 3 (p. 9) modified from Gilbert,^* illustrates 

 the formation of a sea cliff, on a coast having an original outline of AB. 

 Beginning with the sea cliff (a) in this diagram, we have fiist, an upper 

 bench (&) composed of large blocks broken off from the cliff and of 

 slightly rounded bowlders. This is only reached by the waves of the 

 heaviest storms. Below this bench there is another ledge (c) the mate- 

 rial of which is worked over by waves of ordinary storms and is conse- 

 quently composed of smaller bowlders. Beyond tliis is the beach (d) 

 of material progressively smaller towards the sea. This beach is being 

 gradually built up in ordinaiy weather, but in time of stonn it is moved 

 by the undertow which erodes the ledge (e), to a point where the waves 

 when breaking do their most effective work on the cliff. Farther sea- 

 ward at (/) is the extension of the terrace built by the storm undertow ; 



" Algiie, Rev. J., S. .J. : The Cyclones of the Far East. Weather Bureau, 

 Manila (1904), 173. 

 ^'Loc. cit., 84. 



