﻿of 
  the 
  Northern 
  Appalachians. 
  355 
  

  

  fore 
  in 
  this 
  case, 
  the 
  quantity 
  of 
  material 
  eroded 
  for 
  equal 
  

   vertical 
  distances 
  is 
  the 
  same. 
  But 
  the 
  erosion 
  surface 
  

   is 
  very 
  much 
  greater 
  for 
  the 
  deeper 
  offshore 
  part 
  of 
  the 
  

   profile 
  (DE) 
  than 
  it 
  is 
  for 
  the 
  shallower 
  nearshore 
  part 
  

   (AB), 
  which 
  corresponds 
  to 
  the 
  very 
  much 
  weaker 
  forces 
  

   of 
  erosion 
  on 
  the 
  deeper 
  parts. 
  The 
  profile 
  of 
  equi- 
  

   librium, 
  then, 
  for 
  stationary 
  conditions 
  of 
  land 
  and 
  sea 
  

   is 
  such 
  that 
  its 
  slope 
  becomes 
  flatter 
  in 
  proportion 
  to 
  the 
  

   progressive 
  weakness 
  of 
  erosion 
  with 
  depth. 
  This 
  is 
  not 
  

   the 
  same 
  as 
  the 
  curve 
  of 
  decreasing 
  amplitude 
  of 
  wave 
  

   motion 
  with 
  depth, 
  for 
  rapidity 
  of 
  erosion 
  is 
  a 
  function 
  of 
  

   several 
  factors 
  of 
  which 
  wave 
  motion, 
  however, 
  is 
  the 
  

   most 
  important. 
  

  

  The 
  depth 
  to 
  which 
  rock 
  platforms 
  can 
  be 
  cut 
  must 
  be 
  

   distinctly 
  less 
  than 
  the 
  depth 
  at 
  which 
  wave 
  motion 
  pre- 
  

   vents 
  the 
  settling 
  of 
  the 
  finest 
  silts. 
  Current 
  action, 
  

   cooperating 
  with 
  waves, 
  is 
  important 
  in 
  moving 
  sands 
  

   over 
  the 
  abrasion 
  platform 
  at 
  depths 
  where 
  wave 
  motion 
  

   alone 
  has 
  but 
  little 
  influence. 
  As 
  to 
  the 
  depths 
  at 
  which 
  

   silt 
  can 
  be 
  moved 
  we 
  may 
  quote 
  from 
  D. 
  W. 
  Johnson's 
  

   recent 
  book 
  : 
  29 
  

  

  "Thomas 
  Stevenson 
  has 
  made 
  a 
  very 
  interesting 
  comparison 
  

   between 
  the 
  depths 
  at 
  which 
  mud 
  reposes 
  on 
  the 
  floor 
  of 
  differ- 
  

   ent 
  parts 
  of 
  the 
  North 
  Sea, 
  and 
  the 
  vigor 
  of 
  wave 
  action 
  in 
  those 
  

   places. 
  He 
  finds 
  that 
  there 
  is 
  a 
  direct 
  relation 
  between 
  these 
  

   two 
  phenomena, 
  the 
  depth 
  of 
  the 
  level 
  at 
  which 
  mnd 
  accumulates 
  

   increasing 
  in 
  much 
  the 
  same 
  proportion 
  as 
  the 
  violence 
  of 
  the 
  

   waves. 
  From 
  this 
  we 
  may 
  infer 
  that 
  the 
  upper 
  limit 
  of 
  mud 
  

   accumulation 
  is 
  a 
  measure 
  of 
  the 
  maximum 
  depth 
  of 
  wave 
  dis- 
  

   turbance 
  in 
  any 
  given 
  locality. 
  Applying 
  this 
  rule 
  to 
  the 
  North 
  

   Sea, 
  we 
  find 
  that 
  in 
  protected 
  areas, 
  as 
  the 
  inner 
  parts 
  of 
  Moray 
  

   Firth 
  and 
  the 
  Firth 
  of 
  Forth, 
  and 
  along 
  the 
  Holland 
  coast 
  in 
  

   the 
  narrow 
  southern 
  part 
  of 
  the 
  sea, 
  wave 
  action 
  reaches 
  to 
  a 
  

   depth 
  of 
  from 
  25, 
  50, 
  or 
  100 
  feet, 
  while 
  in 
  exposed 
  places 
  the 
  

   disturbance 
  is 
  appreciable 
  to 
  a 
  depth 
  of 
  from 
  300 
  to 
  500 
  feet 
  

   or 
  more. 
  According 
  to 
  J. 
  N. 
  Douglas, 
  the 
  fishermen 
  off 
  Land's 
  

   End 
  bring 
  up 
  stones 
  one 
  pound 
  in 
  weight, 
  which 
  have 
  been 
  

   washed 
  into 
  their 
  lobster 
  pots 
  at 
  a 
  depth 
  of 
  180 
  feet 
  by 
  the 
  action 
  

   of 
  the 
  ground-swell, 
  while 
  coarse 
  sand 
  is 
  often 
  washed 
  from 
  a 
  

   depth 
  of 
  150 
  feet 
  by 
  storm 
  waves 
  and 
  hurled 
  to 
  the 
  lantern 
  

   gallery 
  of 
  the 
  Bishop 
  Rock 
  lighthouse, 
  120 
  feet 
  above 
  low- 
  

   water. 
  ' 
  ' 
  

  

  Johnson 
  concludes 
  that 
  there 
  is 
  no 
  theoretical 
  reason 
  

   why 
  appreciable 
  oscillatory 
  wave 
  action 
  should 
  not 
  extend 
  

  

  29 
  Shore 
  Processes 
  and 
  Shoreline 
  Development, 
  p. 
  79. 
  

  

  