﻿340 
  

  

  J. 
  Barrell 
  — 
  The 
  Piedmont 
  Terraces 
  

  

  advanced 
  maturity 
  or 
  old 
  age 
  of 
  the 
  streams 
  which 
  once 
  

   flowed 
  through 
  the 
  gaps. 
  The 
  cliffs 
  of 
  the 
  Delaware 
  

   Water 
  Gap 
  are 
  well 
  known. 
  The 
  Lehigh 
  Gap 
  below 
  the 
  

   level 
  of 
  the 
  wind 
  gaps 
  shows 
  two 
  facets, 
  an 
  upper 
  at 
  

   about 
  the 
  angle 
  of 
  repose, 
  a 
  lower 
  marked 
  by 
  a 
  rock 
  cliff. 
  

   The 
  Pen 
  Argyl 
  wind 
  gap 
  shows 
  two 
  facets, 
  as 
  illustrated 
  

   with 
  exaggerated 
  vertical 
  scale 
  in 
  fig. 
  9. 
  The 
  slope 
  of 
  

   the 
  lower 
  two-thirds 
  of 
  the 
  gap 
  is 
  about 
  18 
  degrees, 
  

   whereas 
  for 
  the 
  upper 
  one-third 
  it 
  is 
  about 
  8 
  degrees. 
  

   The 
  upper 
  and 
  flatter 
  slope 
  does 
  not 
  appear 
  to 
  be 
  a 
  mere 
  

   rounding 
  off 
  of 
  the 
  lower 
  one, 
  instead 
  it 
  belongs 
  to 
  the 
  

   older 
  and 
  higher 
  series 
  of 
  gaps 
  and 
  denotes 
  a 
  baselevel 
  

   in 
  an 
  erosion 
  cycle 
  which 
  reached 
  a 
  far 
  more 
  advanced 
  

   stage 
  than 
  did 
  the 
  cycle 
  marked 
  by 
  the 
  lower 
  slopes 
  of 
  

  

  Fig. 
  9. 
  

  

  Fig. 
  9. 
  — 
  Pen 
  Argyl 
  Wind 
  Gap 
  showing 
  composite 
  slopes. 
  

  

  the 
  gap. 
  The 
  restored 
  elevation 
  of 
  this 
  old 
  baselevel, 
  

   indicated 
  by 
  the 
  dotted 
  line 
  in 
  fig. 
  9, 
  about 
  1,350 
  feet, 
  

   which 
  agrees 
  with 
  the 
  upper 
  set 
  of 
  wind 
  gaps 
  on 
  the 
  

   Kittatinny 
  ridge 
  and 
  also 
  with 
  the 
  baselevel 
  of 
  about 
  

   1,400 
  found 
  farther 
  north. 
  

  

  It 
  is 
  clear 
  on 
  comparing 
  both 
  the 
  upper 
  and 
  lower 
  

   slopes 
  of 
  the 
  Pen 
  Argyl 
  gap 
  with 
  those 
  of 
  the 
  gorges 
  cut 
  

   by 
  the 
  present 
  through-flowing 
  rivers 
  that 
  all 
  moderately 
  

   resistant 
  formations 
  must 
  have 
  been 
  very 
  thoroughly 
  

   peneplaned 
  during 
  the 
  cycles 
  represented 
  by 
  those 
  

   slopes. 
  Especially 
  in 
  the 
  older 
  cycle 
  even 
  the 
  massif 
  of 
  

   the 
  Kittatinny 
  ridge 
  must 
  have 
  been 
  reduced 
  to 
  a 
  chain 
  

   of 
  hills 
  whose 
  slopes 
  were 
  not 
  over 
  ten 
  degrees. 
  All- 
  

   subsequent 
  time 
  has 
  been 
  so 
  brief 
  in 
  comparison 
  that 
  the 
  

  

  