﻿33S 
  J. 
  Barrel! 
  — 
  The 
  Piedmont 
  Terraces 
  

  

  one-half 
  mile 
  from 
  the 
  river 
  and 
  600 
  feet 
  above 
  it. 
  

   These 
  differences 
  may 
  be 
  partly 
  accounted 
  for 
  by 
  assum- 
  

   ing 
  that 
  uplift 
  has 
  been 
  greater 
  in 
  the 
  vicinity 
  of 
  the 
  

   Delaware 
  than 
  near 
  the 
  Susquehanna 
  thus 
  permitting 
  

   later 
  erosion 
  to 
  more 
  completely 
  and 
  deeply 
  dissect 
  the 
  

   peneplain 
  at 
  the 
  former 
  locality. 
  On 
  the 
  other 
  hand, 
  it 
  

   is 
  probable 
  that 
  the 
  Harrisburg 
  peneplain 
  was 
  never 
  as 
  

   well 
  developed 
  on 
  the 
  slate 
  in 
  the 
  region 
  between 
  the 
  

   Schuylkill 
  and 
  Delaware 
  as 
  it 
  was 
  in 
  the 
  vicinity 
  of 
  the 
  

   Susquehanna. 
  

  

  Campbell 
  ascribed 
  large 
  importance 
  to 
  the 
  Harrisburg 
  

   peneplain 
  as 
  indicating 
  a 
  stillstand 
  of 
  the 
  land 
  through 
  

   the 
  early 
  part 
  of 
  Tertiary 
  time. 
  The 
  difficulty 
  in 
  clearly 
  

   identifying 
  it 
  in 
  the 
  region 
  from 
  the 
  Schuylkill 
  to 
  the 
  

   Delaware 
  suggests 
  that 
  its 
  importance 
  has 
  been 
  exagger- 
  

   ated. 
  Several 
  equally 
  important 
  cycles 
  of 
  erosion 
  may 
  

   have 
  elapsed 
  between 
  the 
  level 
  marked 
  by 
  the 
  Kittatinny 
  

   ridge 
  and 
  that 
  of 
  the 
  Harrisburg 
  peneplain 
  as 
  developed 
  

   on 
  the 
  Ordovician 
  slates 
  near 
  the 
  Susquehanna. 
  

  

  Evidence 
  of 
  Baselevels 
  given 
  by 
  Wind 
  Gaps. 
  

  

  Kittatinny 
  Ridge, 
  or 
  North 
  Mountain, 
  the 
  outcrop 
  of 
  

   resistant 
  Silurian 
  quartzite, 
  forms 
  a 
  prominent 
  and 
  per- 
  

   sistent 
  ridge 
  extending 
  from 
  southern 
  New 
  York 
  to 
  Ala- 
  

   bama, 
  as 
  already 
  described. 
  It 
  is 
  cut 
  at 
  intervals 
  by 
  

   what 
  are 
  obviously 
  old 
  stream 
  valleys, 
  many 
  of 
  which 
  

   have 
  long 
  been 
  recognized 
  as 
  such, 
  whose 
  floors 
  are 
  now 
  

   high 
  above 
  the 
  adjacent 
  country. 
  The 
  significance 
  of 
  

   these 
  as 
  indicative 
  of 
  an 
  originally 
  superposed 
  drainage 
  

   across 
  the 
  ridge 
  has 
  been 
  discussed 
  in 
  a 
  preceding 
  sec- 
  

   tion. 
  Similar 
  gaps 
  cross 
  the 
  Blue 
  Ridge 
  in 
  Virginia 
  and 
  

   have 
  been 
  long 
  regarded 
  as 
  indicators 
  of 
  river 
  piracy. 
  

   It 
  seems 
  that 
  they 
  may 
  be 
  used 
  also 
  as 
  records 
  of 
  base- 
  

   levels 
  of 
  which 
  no 
  other 
  clear 
  evidence 
  remains. 
  As 
  

   wind 
  gaps 
  have 
  not 
  been 
  used 
  before 
  in 
  a 
  comprehensive 
  

   manner 
  for 
  this 
  purpose, 
  the 
  argument 
  should 
  be 
  ampli- 
  

   fied. 
  It 
  should 
  be 
  said 
  that 
  the 
  term 
  as 
  here 
  used 
  is 
  

   restricted 
  to 
  what 
  are 
  clearly 
  ancient 
  river 
  valleys 
  cross- 
  

   ing 
  a 
  resistant 
  formation 
  through 
  which 
  no 
  river 
  now 
  

   flows 
  because 
  of 
  capture 
  of 
  the 
  headwaters 
  by 
  a 
  stream 
  

   more 
  favorably 
  situated. 
  

  

  The 
  most 
  obvious 
  thing 
  about 
  a 
  wind 
  gap 
  is 
  that 
  it 
  

   definitely 
  fixes 
  the 
  position 
  of 
  a 
  point 
  on 
  an 
  ancient 
  

   stream 
  course. 
  But 
  in 
  relation 
  to 
  baseleveling 
  it 
  is 
  more 
  

  

  