﻿of 
  the 
  Northern 
  Appalachians. 
  351 
  

  

  undercutting 
  much 
  more 
  rapidly 
  than 
  the 
  abrasion 
  of 
  the 
  

   offshore 
  bottom. 
  

  

  The 
  effective 
  deterrents 
  to 
  marine 
  planation 
  are 
  an 
  

   abundance 
  of 
  land 
  waste, 
  especially 
  coarse 
  waste, 
  deliv- 
  

   ered 
  by 
  rivers. 
  Even 
  if 
  the 
  waves 
  are 
  powerful 
  enough 
  

   to 
  prevent 
  the 
  extensive 
  growth 
  of 
  deltas, 
  their 
  energy 
  

   may 
  be 
  almost 
  entirely 
  absorbed 
  in 
  grinding 
  up 
  and 
  

   removing 
  the 
  waste. 
  Also, 
  a 
  decreased 
  depth 
  of 
  water 
  

   due 
  to 
  emergence 
  of 
  the 
  land 
  has 
  a 
  deterrent 
  effect 
  

   because 
  the 
  energy 
  of 
  the 
  waves 
  is 
  then 
  absorbed 
  offshore. 
  

   This 
  condition, 
  however, 
  is 
  but 
  temporary 
  and 
  the 
  supply 
  

   of 
  coarse 
  river 
  waste 
  must 
  also 
  come 
  to 
  an 
  end. 
  

  

  As 
  will 
  be 
  shown, 
  even 
  a 
  small 
  rise 
  of 
  sea-level 
  is 
  effec- 
  

   tive 
  in 
  promoting 
  marine 
  denudation. 
  It 
  seems 
  clear, 
  

   therefore, 
  that 
  marine 
  denudation 
  becomes 
  relatively 
  

   more 
  effective 
  when 
  the 
  cycle 
  of 
  fluvial 
  denudation 
  has 
  

   passed 
  its 
  maturity. 
  During 
  the 
  late 
  stages 
  of 
  the 
  

   erosion 
  cycle 
  low 
  sea 
  cliffs 
  instead 
  of 
  high 
  ones 
  would 
  be 
  

   characteristic. 
  The 
  development 
  of 
  the 
  marine 
  pene- 
  

   plane 
  would 
  be 
  chiefly 
  by 
  subaqueous 
  erosion 
  and 
  to 
  

   transform 
  the 
  broad 
  outer 
  margin 
  of 
  a 
  fluvial 
  peneplane 
  

   into 
  a 
  marine 
  peneplane 
  would 
  not 
  require 
  the 
  removal, 
  

   on 
  the 
  average, 
  of 
  over 
  100 
  to 
  200 
  feet 
  of 
  rock. 
  

  

  There 
  would 
  seem 
  to 
  be 
  no 
  room 
  to 
  doubt 
  the 
  prob- 
  

   ability 
  of 
  the 
  existence 
  of 
  marine 
  peneplane 
  s 
  many 
  tens 
  

   of 
  miles 
  or 
  more 
  in 
  breadth. 
  In 
  studying 
  a 
  seaward- 
  

   facing 
  peneplane, 
  therefore, 
  the 
  possibility 
  of 
  its 
  origin, 
  

   either 
  in 
  part 
  or 
  in 
  whole, 
  through 
  marine 
  agencies 
  

   should 
  be 
  thoroughly 
  considered. 
  

  

  The 
  Process 
  of 
  Marine 
  Denudation. 
  

  

  Ancient 
  peneplanes 
  become 
  either 
  erosion 
  surfaces 
  

   covered 
  by 
  later 
  deposits 
  and 
  constituting 
  unconform- 
  

   ities 
  or 
  erosion 
  surfaces 
  without 
  protecting 
  covers 
  now 
  

   uplifted 
  and 
  undergoing 
  destruction. 
  And 
  whereas 
  these 
  

   surfaces 
  were 
  formerly 
  considered 
  as 
  made 
  only 
  by 
  

   marine 
  action, 
  in 
  recent 
  years 
  they 
  have 
  been 
  conceived 
  

   of 
  almost 
  exclusively 
  as 
  having 
  been 
  formed 
  by 
  fluvial 
  

   denudation. 
  The 
  truth 
  is 
  that 
  they 
  may 
  be 
  formed 
  not 
  

   only 
  by 
  marine 
  and 
  by 
  fluvial 
  denudation 
  but 
  also, 
  even 
  if 
  

   less 
  commonly, 
  by 
  eolian 
  erosion. 
  

  

  It 
  was 
  natural 
  that 
  fluvial 
  denudation 
  should 
  supplant 
  

   marine 
  denudation 
  as 
  the 
  explanation 
  of 
  the 
  origin 
  of 
  

   extensive 
  erosion 
  planes, 
  for 
  the 
  forces 
  of 
  subaerial 
  

  

  