﻿238 
  J. 
  Barrel! 
  — 
  The 
  Piedmont 
  Terraces 
  

  

  superposition 
  come 
  to 
  exist 
  as 
  is 
  shown 
  by 
  the 
  presence 
  

   of 
  many 
  wind 
  gaps 
  in 
  Pennsylvania? 
  

  

  It 
  is 
  not 
  conceivable 
  that 
  tilting 
  of 
  the 
  peneplain 
  could 
  

   have 
  brought 
  about 
  general 
  superposition 
  of 
  the 
  drain- 
  

   age. 
  The 
  angle 
  of 
  tilt 
  necessary 
  for 
  this 
  purpose 
  would 
  

   have 
  had 
  to 
  be 
  steeper 
  than 
  the 
  slopes 
  of 
  the 
  ridges. 
  

   Such 
  a 
  tilt, 
  before 
  it 
  reached 
  the 
  elevation 
  implied 
  on 
  the 
  

   landward 
  side, 
  of 
  the 
  axis, 
  would 
  have 
  caused 
  entrench- 
  

   ment 
  of 
  the 
  drainage 
  in 
  soft 
  formations 
  rather 
  than 
  de- 
  

   flection 
  across 
  the 
  most 
  resistant 
  ones. 
  

  

  Davis 
  has 
  suggested 
  that 
  the 
  discordance 
  to 
  structure 
  

   shown 
  by 
  the 
  tributaries 
  of 
  the 
  Susquehanna 
  north 
  of 
  

   Harrisburg 
  is 
  due 
  to 
  alluvial 
  upbuilding 
  and 
  superposi- 
  

   tion 
  during 
  the 
  latest 
  stages 
  of 
  development 
  of 
  the 
  pene- 
  

   plane. 
  This 
  supposition, 
  however, 
  can 
  not 
  account 
  for 
  

   the 
  numerous 
  wind 
  gaps 
  unless 
  the 
  whole 
  region 
  were 
  

   mantled 
  with 
  sediments. 
  Such 
  a 
  mantling, 
  indeed, 
  is 
  

   equivalent 
  to 
  a 
  landward 
  extension 
  of 
  the 
  Coastal 
  Plain 
  

   deposits 
  across 
  the 
  Kittatinny 
  ridge, 
  an 
  extension 
  of 
  

   from 
  60 
  to 
  70 
  miles 
  and 
  more 
  beyond 
  the 
  present 
  limits 
  

   of 
  the 
  Coastal 
  Plain. 
  The 
  conclusion 
  was 
  reached 
  by 
  the 
  

   writer 
  in 
  1896 
  that 
  after 
  the 
  completion 
  of 
  the 
  Kittatinny 
  

   peneplain, 
  a 
  movement 
  of 
  submergence 
  took 
  place 
  which 
  

   caused 
  the 
  sea 
  to 
  advance 
  far 
  inland 
  beyond 
  the 
  present 
  

   position 
  of 
  the 
  Coastal 
  Plain, 
  cutting 
  into 
  and 
  burying 
  

   the 
  low 
  Kittatinny 
  ridge 
  under 
  sediments. 
  This 
  conclu- 
  

   sion, 
  it 
  should 
  be 
  said, 
  was 
  not 
  published, 
  but 
  served 
  with 
  

   later 
  observations 
  to 
  raise 
  questions 
  in 
  regard 
  to 
  the 
  

   validity 
  of 
  current 
  interpretations. 
  6 
  

  

  Geological 
  work 
  in 
  western 
  Massachusetts 
  in 
  1911 
  

   showed 
  remnants 
  of 
  four 
  baseleveled 
  surfaces 
  at 
  eleva- 
  

   tions 
  of 
  2,250, 
  1,840, 
  1,400, 
  and 
  1,100 
  feet. 
  In 
  Vermont 
  

   traces 
  of 
  one 
  or 
  more 
  still 
  higher 
  levels 
  were 
  observed. 
  

   Which 
  of 
  these 
  should 
  be 
  regarded 
  as 
  the 
  southern 
  New 
  

   England 
  peneplain, 
  supposed 
  to 
  be 
  of 
  Cretaceous 
  age, 
  

   and 
  which 
  the 
  Harrisburg 
  peneplain 
  of 
  early 
  Tertiary 
  

   age! 
  If 
  two 
  of 
  these 
  represented 
  Cretaceous 
  and 
  early 
  

   Tertiary 
  levels, 
  what 
  were 
  the 
  other 
  two 
  levels 
  ? 
  

  

  In 
  a 
  preliminary 
  attempt 
  to 
  answer 
  these 
  questions 
  the 
  

   various 
  erosion 
  surfaces 
  were 
  traced 
  toward 
  the 
  sea 
  and 
  

   along 
  the 
  Appalachians 
  to 
  the 
  southwest 
  in 
  order 
  to 
  

   obtain 
  a 
  broader 
  correlation. 
  But 
  instead 
  of 
  simplify- 
  

  

  6 
  [It 
  was 
  not 
  printed. 
  It 
  is 
  stated 
  in 
  the 
  Master 
  of 
  Science 
  thesis 
  re- 
  

   ferred 
  to 
  in 
  an 
  earlier 
  paragraph 
  and 
  is 
  here 
  given 
  in 
  the 
  following 
  sec- 
  

   tion. 
  — 
  Editor.] 
  

  

  