﻿420 
  J. 
  Barrell 
  — 
  The 
  Piedmont 
  Terraces 
  

  

  the 
  determination 
  of 
  dates 
  on 
  the 
  basis 
  of 
  physiographic 
  

   evidence 
  alone 
  is 
  more 
  or 
  less 
  unsatisfactory, 
  depending 
  

   upon 
  the 
  extent 
  to 
  which 
  unknown 
  factors 
  of 
  importance 
  

   outnumber 
  the 
  known. 
  The 
  present 
  confusion 
  as 
  to 
  the 
  

   dating 
  of 
  Appalachian 
  erosion 
  surfaces 
  is 
  traceable 
  to 
  

   this 
  situation 
  and 
  it 
  should 
  be 
  noted 
  that 
  the 
  tendency 
  to 
  

   consider 
  these 
  surfaces 
  younger 
  than 
  they 
  were 
  thought 
  

   to 
  be, 
  or 
  presumably 
  are 
  still 
  thought 
  to 
  be 
  by 
  most 
  geol- 
  

   ogists, 
  is 
  due 
  not 
  so 
  much 
  to 
  additional 
  evidence 
  bearing 
  

   on 
  the 
  problem 
  as 
  to 
  a 
  reinterpretation 
  of 
  existing 
  evi- 
  

   dence 
  on 
  the 
  basis 
  of 
  new 
  conceptions 
  of 
  a 
  general 
  nature, 
  

   such 
  as 
  an 
  increased 
  length 
  of 
  geologic 
  time 
  and 
  a 
  greater 
  

   effectiveness 
  of 
  erosion 
  processes. 
  

  

  Editorial 
  note 
  on 
  the 
  physiographic 
  history 
  of 
  the 
  region. 
  

  

  Professor 
  Barrell 
  states 
  in 
  the 
  abstract 
  of 
  his 
  paper 
  on 
  

   '" 
  Post- 
  Jurassic 
  History 
  of 
  the 
  Northern 
  Appalachians" 
  

   that 
  the 
  sequence 
  of 
  events 
  therein 
  outlined 
  "rests 
  

   largely 
  on 
  an 
  analysis 
  of 
  the 
  profiles." 
  The 
  main 
  pro- 
  

   files 
  referred 
  to 
  are 
  (1) 
  the 
  general 
  profile 
  of 
  the 
  terraces 
  

   of 
  western 
  Masschusetts 
  and 
  Connecticut 
  (PL 
  V), 
  (2) 
  

   the 
  profile 
  showing 
  the 
  relation 
  of 
  the 
  unconformities 
  

   between 
  the 
  formations 
  of 
  the 
  Costal 
  Plain 
  to 
  the 
  ero- 
  

   sion 
  surfaces 
  of 
  the 
  Piedmont 
  and 
  South 
  Mountains 
  (PL 
  

   VI), 
  and 
  (3) 
  presumably 
  the 
  profile 
  of 
  the 
  Piedmont 
  

   terraces 
  of 
  Maryland 
  (PL 
  VI), 
  although 
  in 
  part 
  this 
  is 
  

   practically 
  a 
  duplicate 
  of 
  the 
  section 
  shown 
  by 
  the 
  pre- 
  

   ceding 
  profile. 
  No 
  doubt 
  the 
  sequence 
  of 
  events 
  outlined 
  

   was 
  intended 
  to 
  hold 
  for 
  the 
  entire 
  region 
  from 
  New 
  Eng- 
  

   land 
  to 
  Maryland 
  ; 
  on 
  the 
  other 
  hand, 
  certain 
  unfinished 
  

   figures 
  and 
  notes 
  indicate 
  that 
  it 
  applies 
  most 
  definitely 
  

   to 
  the 
  former 
  region. 
  

  

  The 
  Jurassic 
  and 
  Cretaceous 
  erosion 
  cycles 
  were 
  

   sharply 
  differentiated 
  by 
  Professor 
  Barrell 
  for 
  reasons 
  

   which 
  are 
  stated 
  in 
  the 
  sections 
  "The 
  Jurassic 
  Erosion 
  

   Cycle" 
  and 
  "The 
  Post- 
  Jurassic 
  Erosion 
  Cycles" 
  of 
  his 
  

   paper 
  on 
  the 
  "Upper 
  Devonian 
  Delta 
  of 
  the 
  Appa- 
  

   lachian 
  Geosyncline. 
  ' 
  ' 
  35 
  

  

  On 
  the 
  basis 
  of 
  physiographic 
  evidence 
  he 
  considered 
  

   that 
  the 
  Cretaceous 
  sea 
  covered 
  Massachusetts 
  and 
  Con- 
  

   necticut 
  to 
  the 
  base 
  of 
  the 
  Green 
  Mountains, 
  whereas 
  in 
  

   Maryland 
  and 
  Pennsylvania 
  it 
  extended 
  over 
  both 
  the 
  

  

  25 
  This 
  Journal, 
  37, 
  102-105, 
  1914. 
  

  

  