Mr Black's Remarks on the Chessil Bank, JVei/movth. 127 



plunging action of the surf breaking on it, and has a steeper 

 slope than the landward face, which is more level in confor- 

 mation, so that the crest of the bank is seen to lie nearer the 

 sea half than the land half. 



14:tJi, This would lead to the inference that the bank might 

 be gradually advancing inwards on the land, and that it 

 formerly existed much more out to sea than it does now, but 

 of this there is no historical evidence yet forthcoming ; but 

 Professor Prestwich's discovery of the existence of raised 

 beaches on Portland and the Devonshire coast gives a geo- 

 logical support to the idea. 



15th, This conjecture may derive some further confirma- 

 tion from the fact of the twenty-fathom line of soundings in 

 Lyme Bay, stretching in an unbroken curve from Portland 

 Ledges to Start Point, indicating that the bay might formerly 

 have been dry land, and had since been scooped out by the 

 denuding action of the sea, and had left these stones and 

 pebbles as the remains and records of its action. 



If a straight line be drawn between Portland Point and 

 Charmouth, representing the chord of the arc of Chessil Bay, 

 the perpendicular raised upon this to the circumference of 

 the arc at the pebble bank may be assumed to be proportional 

 to the effects of the storm waves at each spot. These lines 

 will be seen to be larger in the middle of the bank, and re- 

 present therefore the greater force in driving back the shingle, 

 and shorter at the sides, where the effect of the wind waves 

 would be less. Again, if a tangent line be drawn on the 

 outside of the arc of the bay perpendicular to the middle of 

 the bank, then the perpendiculars erected on this may be 

 taken to represent the landward resistance to the invasion of 

 the sea. This will then be seen to be strongest at the sides, 

 where the bank rests on rocky cliffs, and weaker in the 

 middle, where the cliffs are absent, and the superficial strata 

 and deposits reach the level of the sea. 



