﻿of 
  the 
  Northern 
  Appalachians. 
  335 
  

  

  feet. 
  The 
  result 
  of 
  these 
  inequalities 
  of 
  resistance 
  

   shown 
  by 
  the 
  Newark 
  formations 
  is 
  that 
  the 
  Harrishurg 
  

   peneplain, 
  although 
  rather 
  broadly 
  developed 
  over 
  the 
  

   Triassic 
  on 
  the 
  Susquehanna, 
  is 
  not 
  so 
  clearly 
  evident 
  as 
  

   it 
  is 
  on 
  the 
  more 
  uniformly 
  resistant 
  Ordovician 
  slate. 
  

   If 
  rock 
  formations 
  were 
  all 
  equally 
  resistant 
  the 
  problem 
  

   of 
  tracing 
  peneplains, 
  which 
  are 
  now 
  in 
  an 
  eroded 
  condi- 
  

   tion, 
  would 
  be 
  greatly 
  simplified. 
  In 
  nature, 
  however, 
  

   rock 
  formations 
  differ 
  much 
  in 
  their 
  resistance 
  to 
  ero- 
  

   sion 
  and 
  generally 
  the 
  same 
  formation 
  will 
  differ 
  from 
  

   place 
  to 
  place, 
  consequently 
  it 
  is 
  of 
  first 
  importance 
  to 
  

   evaluate 
  these 
  differences 
  and 
  allow 
  for 
  them 
  in 
  tracing 
  

   a 
  peneplain, 
  or 
  series 
  of 
  peneplains, 
  from 
  one 
  region 
  to 
  

   another. 
  

  

  With 
  this 
  idea 
  in 
  mind 
  the 
  Harrisburg 
  peneplain 
  may 
  

   be 
  examined 
  more 
  carefully. 
  The 
  Mauch 
  Chunk 
  shale 
  

   outcrops 
  north 
  of 
  Cove 
  Mountain 
  (Harrisburg 
  and 
  New 
  

   Bloomfield 
  quadrangles) 
  and 
  shows 
  the 
  Harrisburg 
  sur- 
  

   face 
  at 
  about 
  520 
  feet 
  as 
  well 
  as 
  does 
  the 
  Martinsburg 
  

   shale 
  south 
  of 
  Blue 
  Mountain. 
  The 
  same 
  formation 
  out- 
  

   crops 
  farther 
  up 
  the 
  river 
  in 
  the 
  vicinity 
  of 
  Millersburg 
  

   where 
  it 
  is 
  seen 
  to 
  have 
  been 
  smoothly 
  peneplaned 
  with 
  

   respect 
  to 
  a 
  baselevel 
  at 
  the 
  Susquehanna 
  of 
  about 
  540 
  

   feet. 
  Between 
  these 
  two 
  localities 
  Devonian 
  shales 
  and 
  

   sandstones 
  outcrop. 
  They 
  are 
  more 
  resistant 
  than 
  the 
  

   Mauch 
  Chunk 
  and 
  evidently 
  were 
  not 
  baseleveled 
  in 
  the 
  

   Harrisburg 
  cycle, 
  as 
  lines 
  of 
  hills 
  rise 
  from 
  650 
  to 
  730 
  

   feet 
  even 
  near 
  the 
  river. 
  The 
  surface 
  at 
  this 
  locality 
  was 
  

   reduced, 
  perhaps, 
  to 
  a 
  mature 
  form 
  during 
  that 
  cycle 
  and 
  

   the 
  valleys 
  since 
  cut 
  in 
  the 
  Devonian 
  formations 
  are 
  still 
  

   in 
  a 
  state 
  of 
  youth 
  even 
  near 
  such 
  a 
  large 
  river 
  as 
  the 
  

   Susquehanna. 
  The 
  Devonian 
  strata 
  outcrop 
  again 
  far- 
  

   ther 
  north 
  (northwestern 
  part 
  of 
  Millersburg 
  and 
  cen- 
  

   tral 
  parts 
  of 
  the 
  Millerstown 
  and 
  Sunbury 
  quadrangles) 
  

   where 
  the 
  country 
  is 
  hilly 
  and 
  shows 
  no 
  trace 
  of 
  the 
  val- 
  

   ley 
  flats 
  belonging 
  to 
  the 
  Harrisburg 
  baselevel. 
  The 
  

   accordance 
  of 
  flat 
  hilltops 
  at 
  a 
  level 
  of 
  800 
  to 
  860 
  feet 
  

   suggests 
  that 
  this 
  region 
  of 
  Devonian 
  formations 
  was 
  

   peneplaned 
  in 
  an 
  earlier 
  cycle 
  than 
  the 
  Harrisburg 
  with 
  

   respect 
  to 
  a 
  river 
  level 
  of 
  about 
  750 
  feet. 
  This 
  peneplain 
  

   was 
  then 
  dissected 
  during 
  the 
  Harrisburg 
  cycle 
  but 
  not 
  

   so 
  extensively 
  as 
  to 
  prevent 
  its 
  recognition 
  at 
  the 
  present 
  

   day. 
  

  

  In 
  the 
  southern 
  part 
  of 
  the 
  Sunbury 
  quadrangle 
  the 
  

  

  