﻿of 
  the 
  Northern 
  Appalachians. 
  341 
  

  

  upper 
  slopes 
  belonging 
  to 
  this 
  older 
  cycle 
  have 
  been 
  

   flattened 
  but 
  little 
  further 
  through 
  weathering 
  and 
  creep. 
  

  

  It 
  may 
  be 
  questioned 
  why 
  any 
  of 
  the 
  stream 
  adjust- 
  

   ment 
  was 
  left 
  to 
  later 
  cycles 
  when 
  the 
  first 
  cycle 
  was 
  so 
  

   prolonged. 
  The 
  answer 
  would 
  appear 
  to 
  turn 
  on 
  the 
  

   amount 
  of 
  uplift. 
  The 
  uplift 
  which 
  inaugurated 
  the 
  

   second 
  cycle 
  of 
  erosion 
  was 
  greater 
  than 
  that 
  of 
  the 
  first 
  

   cycle 
  and 
  consequently 
  a 
  tributary 
  did 
  not 
  have 
  to 
  de- 
  

   velop 
  so 
  flat 
  a 
  grade 
  before 
  it 
  could 
  appropriate, 
  at 
  a 
  

   given 
  distance, 
  the 
  headwaters 
  of 
  a 
  neighboring 
  stream. 
  

   The 
  drainage 
  became 
  adjusted 
  to 
  structure 
  early 
  in 
  the 
  

   cycle, 
  whereas 
  in 
  the 
  first 
  cycle, 
  because 
  of 
  the 
  smaller 
  

   uplift, 
  it 
  most 
  probably 
  became 
  adjusted 
  late 
  in 
  the 
  cycle. 
  

   The 
  wind 
  gaps 
  of 
  the 
  second 
  cycle, 
  as 
  represented 
  by 
  the 
  

   lower 
  slopes 
  of 
  the 
  Pen 
  Argyl 
  gap, 
  are 
  much 
  less 
  numer- 
  

   ous 
  than 
  are 
  those 
  of 
  the 
  first 
  cycle, 
  although 
  they 
  are 
  

   much 
  clearer 
  in 
  origin. 
  

  

  There 
  is 
  seen 
  to 
  be 
  evidence, 
  therefore, 
  of 
  fluvial 
  base- 
  

   levels 
  at 
  about 
  400 
  feet 
  (Somerville), 
  520 
  (Harrisburg), 
  

   750 
  (Sunbury), 
  940 
  (?), 
  21 
  1,350 
  (Schooley?), 
  and 
  1,600 
  

   feet 
  (Kittatinny), 
  the 
  higher 
  levels 
  being 
  only 
  approxi- 
  

   mate. 
  And 
  most 
  of 
  these 
  are 
  present 
  within 
  a 
  rather 
  

   limited 
  area 
  in 
  the 
  vicinity 
  of 
  the 
  Susquehanna 
  water 
  

   gap. 
  The 
  750 
  (Sunbury) 
  level 
  is 
  made 
  more 
  definite 
  by 
  

   the 
  evidence 
  from 
  wind 
  gaps. 
  The 
  940 
  level 
  is 
  disclosed 
  

   in 
  the 
  foregoing 
  locality 
  only 
  by 
  them 
  and 
  the 
  1,350 
  level 
  

   is 
  less 
  definitely 
  registered. 
  

  

  It 
  will 
  be 
  shown 
  later 
  that 
  the 
  granite-gneisses 
  of 
  

   northern 
  New 
  Jersey 
  and 
  southern 
  New 
  York 
  are 
  of 
  a 
  

   breadth 
  and' 
  resistance 
  which 
  permitted 
  the 
  1,350 
  level 
  

   to 
  be 
  developed 
  upon 
  them 
  and 
  also 
  to 
  be 
  preserved, 
  

   forming, 
  in 
  part, 
  the 
  Schooley 
  peneplain 
  determined 
  by 
  

   Davis. 
  Remnants 
  of 
  the 
  940 
  level 
  may 
  be 
  identified 
  in 
  

   the 
  same 
  region 
  but 
  they 
  are 
  so 
  fragmentary 
  that 
  they 
  

   could 
  hardly 
  be 
  used 
  by 
  themselves 
  as 
  evidence 
  of 
  a 
  

   former 
  erosion 
  surface, 
  for 
  they 
  might 
  also 
  be 
  explained 
  

   as 
  residuals 
  from 
  an 
  older 
  cycle 
  partly 
  reduced 
  toward 
  

   one 
  of 
  the 
  lower 
  baselevels. 
  

  

  It 
  is 
  evident 
  that 
  a 
  fluvial 
  peneplane 
  might 
  be 
  so 
  com- 
  

   pletely 
  destroyed 
  by 
  the 
  formation 
  of 
  a 
  younger 
  and 
  

   lower 
  one 
  that 
  the 
  only 
  evidence 
  of 
  its 
  existence 
  would 
  be 
  

   preserved 
  in 
  wind 
  gaps. 
  The 
  reason 
  is 
  that 
  with 
  the 
  

   completion 
  (the 
  old 
  age) 
  of 
  a 
  cycle, 
  stream 
  channeling 
  

  

  21 
  See 
  following 
  section. 
  

  

  