﻿of 
  the 
  Northern 
  Appalachians. 
  411 
  

  

  grade 
  and 
  that 
  where 
  they 
  have 
  been 
  diverted 
  the 
  portion 
  

   of 
  their 
  channels 
  (wind 
  gaps) 
  across 
  the 
  outstanding 
  

   ridges 
  will 
  endure 
  for 
  a 
  long 
  period 
  because 
  the 
  agent 
  

   which 
  formed 
  it 
  was 
  more 
  competent 
  than 
  any 
  other 
  to 
  

   act 
  on 
  resistant 
  rocks. 
  

  

  Likewise, 
  waves 
  are 
  an 
  effective 
  erosive 
  agent 
  espe- 
  

   cially 
  for 
  cutting 
  headlands 
  and 
  when 
  working 
  at 
  shal- 
  

   low 
  depths 
  of 
  water. 
  The 
  headlands 
  tend 
  to 
  be 
  cut 
  on 
  

   belts 
  of 
  resistant 
  rock 
  determined 
  for 
  that 
  reason 
  as 
  

   interfluvial 
  ridges 
  during 
  a 
  previous 
  cycle 
  of 
  fluvial 
  de- 
  

   nudation. 
  The 
  action 
  of 
  the 
  sea 
  is 
  thus 
  concentrated 
  

   upon 
  the 
  hard 
  rocks 
  spared 
  by 
  the 
  rivers. 
  The 
  power 
  of 
  

   the 
  sea, 
  however, 
  ceases 
  to 
  act 
  at 
  a 
  given 
  locality 
  after 
  

   an 
  emergence 
  of 
  the 
  land 
  and 
  consequently 
  the 
  benches 
  

   and 
  cliffs 
  which 
  have 
  been 
  cut 
  will 
  endure 
  for 
  a 
  long 
  

   period, 
  though 
  in 
  modified 
  form. 
  

  

  A 
  cliff 
  is 
  especially 
  enduring 
  when 
  cut 
  in 
  very 
  resist- 
  

   ant 
  rock. 
  Streams 
  may 
  trench 
  it. 
  All 
  the 
  land 
  slopes 
  

   may 
  become, 
  in 
  fact, 
  the 
  result 
  of 
  later 
  fluvial 
  action 
  and 
  

   yet 
  the 
  old 
  coast-line 
  back 
  of 
  the 
  younger 
  shore 
  may 
  

   remain 
  as 
  a 
  definite 
  break 
  in 
  the 
  continuity 
  of 
  the 
  upland, 
  

   better 
  visible 
  from 
  a 
  distance 
  than 
  when 
  viewed 
  from 
  

   among 
  its 
  remnants. 
  Looked 
  at 
  across 
  the 
  direction 
  of 
  

   the 
  drainage 
  the 
  old 
  interfluvial 
  ridge 
  will 
  be 
  seen 
  slop- 
  

   ing 
  down 
  at 
  the 
  place 
  of 
  the 
  old 
  sea 
  cliff 
  from 
  a 
  higher 
  

   and 
  more 
  rugged 
  country 
  to 
  one 
  of 
  a 
  different 
  character. 
  

  

  The 
  effect 
  of 
  marine 
  planation, 
  as 
  expressed 
  in 
  cliff 
  

   cutting, 
  will 
  be 
  at 
  a 
  minimum 
  when 
  a 
  fluvially 
  base- 
  

   leveled 
  land 
  is 
  but 
  slightly 
  submerged. 
  It 
  will 
  be 
  at 
  a 
  

   maximum 
  when 
  submergence 
  to 
  as 
  much 
  as 
  200 
  feet 
  

   affects 
  a 
  land 
  whose 
  surface 
  had 
  been 
  previously 
  re- 
  

   duced 
  to 
  a 
  maturity 
  characterized 
  by 
  rather 
  strong 
  relief. 
  

   In 
  a 
  region 
  of 
  complex 
  structure, 
  such 
  as 
  southern 
  New 
  

   England, 
  this 
  topographic 
  stage 
  would 
  mean 
  that 
  many 
  

   of 
  the 
  interfluvial 
  ridges 
  were 
  prominent 
  because 
  of 
  

   greater 
  resistance. 
  The 
  previous 
  fluvial 
  erosion, 
  by 
  

   widening 
  the 
  valleys, 
  gives 
  the 
  waves 
  room 
  for 
  attack. 
  

   They 
  may 
  cut 
  platforms 
  miles 
  in 
  width 
  in 
  front 
  of 
  resid- 
  

   uals 
  which 
  may 
  rise 
  as 
  dissected 
  plateaus 
  several 
  hun- 
  

   dred 
  feet 
  in 
  height. 
  The 
  effect 
  of 
  this 
  contrast 
  in 
  relief 
  

   will 
  remain 
  even 
  after 
  subsequent 
  uplift 
  and 
  erosion 
  of 
  

   the 
  combined 
  features 
  of 
  fluvial 
  and 
  marine 
  denudation 
  

   to 
  advanced 
  maturity. 
  

  

  To 
  sum 
  up, 
  the 
  most 
  favorable 
  conditions 
  for 
  pro- 
  

  

  