﻿232 
  J. 
  Barrell 
  — 
  The 
  Piedmont 
  Terraces 
  

  

  stage 
  where 
  lie 
  had 
  organized 
  or 
  perhaps 
  could 
  organize 
  

   the 
  work 
  as 
  a 
  whole. 
  Further, 
  it 
  'will 
  assist 
  those 
  who 
  

   are 
  approaching 
  the 
  general 
  problem 
  for 
  the 
  first 
  time 
  

   if 
  there 
  is 
  also 
  presented 
  a 
  summary 
  outline 
  of 
  the 
  older 
  

   interpretation 
  up 
  to 
  1911, 
  when 
  Professor 
  Barrell 
  began 
  

   his 
  work. 
  For 
  a 
  comprehensive 
  summary 
  of 
  Appala- 
  

   chian 
  research 
  up 
  to 
  the 
  same 
  year 
  the 
  reader 
  may 
  refer 
  

   to 
  Dr. 
  Bowman's 
  Forest 
  Physiography, 
  Part 
  II, 
  Physi- 
  

   ography 
  of 
  the 
  United 
  States, 
  Chapters 
  XXV 
  and 
  

   XXVIII-XXXII. 
  

  

  The 
  scientific 
  interpretation 
  of 
  the 
  later 
  physiographic 
  

   history 
  of 
  the 
  Appalachian 
  Province, 
  based 
  on 
  the 
  con- 
  

   ception 
  of 
  a 
  normal, 
  or 
  pluvial, 
  erosion 
  cycle, 
  began 
  with 
  

   the 
  classic 
  work 
  of 
  W. 
  M. 
  Davis 
  presented 
  in 
  three 
  

   papers, 
  the 
  first 
  of 
  which 
  appeared 
  in 
  1889. 
  3 
  And 
  it 
  is 
  

   a 
  tribute 
  to 
  Prof. 
  Davis' 
  keenness 
  of 
  thought 
  and 
  ability 
  

   in 
  exposition 
  that 
  his 
  underlying 
  conception 
  of 
  the 
  prob- 
  

   lem 
  quickly 
  gained 
  acceptance 
  and 
  has 
  been 
  the 
  founda- 
  

   tion, 
  it 
  may 
  be 
  said 
  almost 
  without 
  qualification, 
  of 
  all 
  

   later 
  research. 
  

  

  In 
  general, 
  then, 
  the 
  interpretation 
  of 
  the 
  physio- 
  

   graphic 
  history 
  of 
  the 
  Appalachian 
  region 
  has 
  rested 
  on 
  

   the 
  recognition 
  of 
  a 
  topographic 
  plane 
  of 
  reference, 
  com- 
  

   monly 
  known 
  as 
  the 
  Cretaceous 
  peneplain, 
  above 
  which 
  

   rose 
  older 
  residual 
  masses, 
  below 
  which 
  were 
  locally 
  cut 
  

   secondary 
  topographic 
  levels 
  of 
  Tertiary 
  age 
  represent- 
  

   ing 
  temporary 
  stillstands 
  of 
  the 
  land 
  following 
  periods 
  

   of 
  uplift, 
  and 
  on 
  the 
  outer 
  margin 
  of 
  which 
  rested 
  the 
  

   great 
  unlithified 
  series 
  of 
  Mesozoic 
  and 
  Neozoic 
  sedi- 
  

   ments. 
  The 
  underlying 
  assumption 
  was 
  that 
  all 
  pene- 
  

   planed 
  surfaces 
  were 
  the 
  result 
  of 
  fluvial 
  denudation 
  and 
  

   that 
  remnants 
  of 
  all 
  remain 
  so 
  little 
  reduced 
  as 
  to 
  permit 
  

   the 
  former 
  surfaces 
  to 
  be 
  restored 
  with 
  assurance. 
  4 
  

  

  In 
  most 
  parts 
  of 
  the 
  province 
  three 
  cycles 
  of 
  pene- 
  

   planation 
  were 
  recognized 
  and 
  correlated 
  in 
  a 
  general 
  

   way, 
  as 
  the 
  Cretaceous, 
  the 
  early 
  Tertiary, 
  and 
  the 
  late 
  

   Tertiary. 
  In 
  some 
  localities 
  an 
  older 
  erosion 
  surface, 
  

   the 
  Jurassic 
  peneplain, 
  was 
  recognized. 
  Keith 
  went 
  

  

  3 
  The 
  Rivers 
  and 
  Valleys 
  of 
  Pennsylvania, 
  Nat. 
  Geogr. 
  Mag., 
  vol. 
  1, 
  

   183-253, 
  1889. 
  

  

  The 
  Rivers 
  of 
  Northern 
  New 
  Jersey 
  with 
  Notes 
  on 
  the 
  Classification 
  

   of 
  Rivers 
  in 
  General, 
  ibid., 
  vol. 
  2, 
  81-110, 
  1890. 
  

  

  The 
  Geologic 
  Dates 
  of 
  Origin 
  of 
  Certain 
  Topographic 
  Forms 
  on 
  the 
  

   Atlantic 
  Slope 
  of 
  the 
  United 
  States, 
  Bull. 
  Geol. 
  Soe. 
  Am., 
  vol. 
  2, 
  545-586,. 
  

   1891. 
  

  

  4 
  See 
  sections 
  on 
  back 
  of 
  Delaware 
  Watergap 
  topographic 
  map. 
  

  

  