﻿200 
  F. 
  Leverett 
  — 
  Pleistocene 
  Fluvial 
  Planes 
  of 
  

  

  Art. 
  XVIII. 
  — 
  Pleistocene 
  Fluvial 
  Planes 
  of 
  Western 
  Penn- 
  

   sylvania 
  ; 
  by 
  Frank 
  Leverett. 
  

  

  In 
  the 
  November 
  number 
  of 
  this 
  Journal 
  there 
  appeared 
  a 
  

   paper 
  by 
  Mr. 
  P. 
  Max 
  Foshay, 
  entitled 
  " 
  Preglacial 
  Drainage 
  

   and 
  Recent 
  Geological 
  History 
  of 
  Western 
  Pennsylvania," 
  in 
  

   which 
  certain 
  views 
  are 
  expressed 
  which 
  do 
  not 
  seem 
  consistent 
  

   with 
  facts 
  in 
  my 
  possession 
  ; 
  and 
  in 
  which, 
  although 
  valuable 
  

   suggestions 
  are 
  made 
  regarding 
  the 
  possibilities 
  of 
  changes 
  in 
  

   drainage 
  which 
  the 
  region 
  may 
  have 
  undergone 
  since 
  the 
  be- 
  

   ginning 
  of 
  the 
  glacial 
  period, 
  adequate 
  data 
  are 
  not 
  presented 
  

   to 
  sustain 
  the 
  broad 
  and 
  positive 
  conclusions 
  drawn. 
  It 
  there- 
  

   fore 
  seems 
  important 
  that 
  the 
  subject 
  be 
  further 
  considered 
  

   and 
  that 
  attention 
  be 
  called 
  to 
  facts 
  which 
  render 
  the 
  problem 
  

   more 
  complex 
  and 
  its 
  solution 
  less 
  certain 
  than 
  the 
  paper 
  

   leaves 
  the 
  readers 
  to 
  suppose. 
  

  

  A 
  few 
  general 
  statements 
  respecting 
  the 
  fluvial 
  planes 
  of 
  the 
  

   region 
  (following 
  essentially 
  President 
  Chamberlin*) 
  will 
  aid 
  

   in 
  showing 
  the 
  bearing 
  of 
  the 
  facts 
  I 
  wish 
  to 
  present. 
  In 
  the 
  

   district 
  immediately 
  southeast 
  of 
  the 
  drift-covered 
  portion 
  of 
  

   western 
  Pennsylvania 
  there 
  are 
  three 
  well-developed 
  fluvial 
  

   planes 
  distinct 
  from 
  the 
  present 
  flood-planes 
  of 
  the 
  streams, 
  

   representing 
  as 
  many 
  distinct 
  episodes 
  in 
  recent 
  geological 
  

   history. 
  The 
  lowest 
  fluvial 
  plane 
  is 
  the 
  rock 
  floor 
  of 
  the 
  val- 
  

   leys, 
  which 
  in 
  much 
  of 
  the 
  region 
  is 
  at 
  a 
  lower 
  level 
  than 
  the 
  

   beds 
  of 
  the 
  present 
  streams. 
  The 
  middle 
  fluvial 
  plane 
  is 
  rep- 
  

   resented 
  by 
  the 
  moraine-headed 
  terraces 
  which 
  take 
  their 
  rise 
  

   in 
  the 
  bulky 
  outer 
  moraine 
  of 
  the 
  " 
  Grand 
  River 
  lobe." 
  These 
  

   are 
  at 
  a 
  somewhat 
  higher 
  level 
  than 
  the 
  present 
  flood-planes, 
  

   for 
  the 
  altitude 
  of 
  the 
  region 
  now 
  is 
  such 
  that 
  the 
  streams 
  are 
  

   deepening 
  their 
  channels. 
  The 
  highest 
  fluvial 
  plane 
  is 
  repre- 
  

   sented 
  by 
  elevated 
  terraces 
  250 
  feetdb 
  above 
  the 
  present 
  streams. 
  

   This 
  is 
  much 
  broader 
  than 
  the 
  middle 
  and 
  lowest 
  planes. 
  Its 
  

   remnant 
  is 
  a 
  rocky 
  shelf 
  capped 
  with 
  distinctly 
  fluvial 
  material 
  

   varying 
  in 
  depth 
  up 
  to 
  40 
  feet 
  or 
  more. 
  Abandoned 
  loops 
  or 
  

   "oxbows" 
  occasionally 
  occur, 
  giving 
  a 
  complete 
  cross 
  profile. 
  

  

  The 
  time 
  sequence 
  of 
  these 
  planes 
  is 
  as 
  follows 
  : 
  the 
  high 
  

   terraces 
  are 
  the 
  oldest, 
  the 
  moraine-headed 
  terraces 
  are 
  the 
  

   youngest, 
  while 
  the 
  rock 
  floors 
  of 
  the 
  buried 
  channels 
  are 
  of 
  

   intermediate 
  age, 
  for 
  they 
  represent 
  the 
  limit 
  of 
  an 
  erosion 
  

   and 
  deepening 
  that 
  took 
  place 
  after 
  the 
  high 
  terraces 
  were 
  

   formed 
  and 
  before 
  the 
  moraine-headed. 
  terraces 
  were 
  built 
  up. 
  

  

  High-level 
  terraces. 
  — 
  President 
  Chamberlin 
  has 
  set 
  forth 
  in 
  

   the 
  bulletin 
  referred 
  to 
  the 
  fact 
  that 
  the 
  high 
  terraces 
  were 
  

  

  * 
  Bulletin 
  No. 
  58, 
  U. 
  S. 
  Geological 
  Survey, 
  pp. 
  24-37. 
  

  

  