﻿352 
  

  

  J. 
  Barrell 
  — 
  The 
  Piedmont 
  Terraces 
  

  

  erosion 
  could 
  be 
  observed 
  at 
  work 
  over 
  the 
  broad 
  surfaces 
  

   of 
  the 
  continents, 
  in 
  marked 
  contrast 
  to 
  the 
  narrow 
  zone 
  

   over 
  which 
  the 
  sea 
  worked, 
  thus 
  making 
  it 
  possible 
  to 
  

   formulate 
  concisely 
  and 
  graphically 
  a 
  normal 
  cycle 
  of 
  

   fluvial 
  denudation. 
  Nevertheless, 
  marine 
  planation 
  has 
  

   been 
  an 
  important 
  process 
  in 
  the 
  past 
  and 
  in 
  favorably 
  

   situated 
  regions 
  is 
  still 
  notably 
  effective 
  in 
  producing 
  

   plane 
  surfaces. 
  Consequently, 
  in 
  order 
  to 
  determine 
  the 
  

   expectability 
  of 
  ancient 
  peneplanes 
  of 
  marine 
  or 
  fluvial 
  

   origin 
  it 
  is 
  necessary 
  to 
  examine 
  the 
  way 
  in 
  which 
  sub- 
  

   marine 
  and 
  subaerial 
  forces 
  carry 
  on 
  their 
  own 
  work 
  and 
  

   also 
  cooperate. 
  

  

  It 
  has 
  been 
  held 
  by 
  some 
  geologists 
  that 
  marine 
  denu- 
  

  

  FiG. 
  14. 
  

  

  Offshore 
  

  

  -*6borefacej«-5bore 
  ^ 
  Coast 
  

  

  

  Fig. 
  14. 
  — 
  The 
  chief 
  divisions 
  of 
  the 
  marine 
  profile. 
  After 
  D. 
  W. 
  John- 
  

   son. 
  

  

  dation 
  is 
  effective 
  only 
  at 
  the 
  shore. 
  The 
  shore, 
  however, 
  

   is 
  simply 
  the 
  zone 
  of 
  visible 
  and 
  impressive 
  action; 
  

   erosion 
  is 
  there 
  dominant 
  only 
  on 
  a 
  young 
  and 
  bold 
  coast- 
  

   line. 
  Marine 
  erosion 
  passes 
  quickly 
  to 
  a 
  stage 
  of 
  matur- 
  

   ity 
  when 
  bottom 
  erosion, 
  to 
  a 
  depth 
  approaching 
  wave 
  

   base, 
  has 
  to 
  remove 
  as 
  much 
  or 
  more 
  material 
  as 
  does 
  the 
  

   breaking 
  surf. 
  

  

  Marine 
  erosion, 
  in 
  fact, 
  works 
  to 
  a 
  graded 
  profile 
  com- 
  

   parable 
  to 
  that 
  of 
  a 
  stream. 
  This 
  is 
  evident 
  both 
  from 
  

   observation 
  and 
  theory. 
  The 
  profile, 
  as 
  shown 
  in 
  fig. 
  14, 
  

   becomes 
  horizontal 
  at 
  the 
  depth 
  of 
  wave 
  base, 
  namely, 
  

   the 
  depth 
  at 
  which 
  the 
  wave 
  action 
  of 
  the 
  heavier 
  storms 
  

   just 
  ceases 
  to 
  stir 
  the 
  bottom 
  material. 
  Moving 
  toward 
  

   the 
  shore 
  the 
  waves 
  lose 
  some 
  of 
  their 
  power 
  through 
  

  

  