﻿202 
  F 
  Leverett 
  — 
  Pleistocene 
  Fluvial 
  Planes 
  of 
  

  

  the 
  erosion 
  of 
  the 
  lower 
  300 
  feet 
  of 
  the 
  Allegheny 
  valley 
  

   within 
  the 
  earlier 
  glacial 
  and 
  the 
  interglacial 
  epoch, 
  and 
  

   throws 
  doubt 
  upon 
  the 
  preglacial 
  age 
  of 
  the 
  buried 
  channels, 
  

   although 
  they 
  are 
  plainly 
  older 
  than 
  the 
  moraine-headed 
  ter- 
  

   races 
  of 
  this 
  region. 
  

  

  The 
  fact 
  that 
  the 
  Allegheny 
  and 
  Monongahela 
  river-beds 
  

   had 
  become 
  coated 
  to 
  considerable 
  depth 
  with 
  fluvial 
  debris 
  

   (40 
  ft.±) 
  by 
  the 
  close 
  of 
  the 
  earlier 
  glacial 
  period 
  is 
  evidence 
  

   that 
  the 
  streams 
  had 
  reached 
  a 
  base 
  level 
  at 
  a 
  still 
  earlier 
  date, 
  

   and 
  makes 
  it 
  probable, 
  though 
  not 
  demonstrative, 
  that 
  the 
  ex- 
  

   cavation 
  to 
  the 
  level 
  of 
  the 
  upper 
  rock 
  plane 
  of 
  the 
  several 
  

   lines 
  of 
  drainage 
  which 
  subsequently 
  united 
  to 
  form 
  the 
  

   Allegheny 
  was 
  preglacial. 
  If 
  not 
  preglacial, 
  it 
  must 
  have 
  

   been 
  accomplished 
  during 
  the 
  earlier 
  part 
  of 
  the 
  first 
  glacial 
  

   epoch. 
  

  

  In 
  Mr. 
  Foshay's 
  paper 
  the 
  high 
  level 
  terrace 
  ' 
  along 
  the 
  Bea- 
  

   ver 
  river 
  is 
  discussed 
  as 
  " 
  an 
  old 
  base-level 
  plane," 
  and 
  Pro- 
  

   fessor 
  I. 
  C. 
  White, 
  in 
  his 
  report 
  on 
  Beaver 
  County, 
  Pennsyl- 
  

   vania, 
  calls 
  it 
  the 
  " 
  fourth 
  terrace."* 
  

  

  It 
  has 
  been 
  identified 
  by 
  these 
  writers 
  as 
  far 
  north 
  as 
  the 
  

   mouth 
  of 
  the 
  Connoquenessing 
  where 
  the 
  terminal 
  moraine 
  of 
  

   the 
  later 
  drift 
  lies 
  upon 
  it. 
  Mr. 
  Foshay 
  calls 
  attention 
  to 
  the 
  

   important 
  fact 
  that 
  this 
  terrace 
  has 
  a 
  northward 
  descent 
  from 
  

   the 
  mouth 
  of 
  Beaver 
  river 
  to 
  the 
  terminal 
  moraine, 
  and 
  

   President 
  Chamber! 
  in 
  has 
  shown 
  that 
  the 
  high 
  terrace 
  of 
  the 
  

   lower 
  Allegheny 
  and 
  the 
  Monongahela 
  descends 
  with 
  the 
  

   present 
  streams 
  to 
  their 
  junction 
  not 
  far 
  above 
  the 
  mouth 
  of 
  

   the 
  Beaver 
  river 
  ; 
  all 
  of 
  which 
  evidence 
  favors 
  the 
  hypothesis 
  

   that 
  the 
  lower 
  factors 
  of 
  the 
  Allegheny 
  river 
  and 
  the 
  Monon- 
  

   gahela 
  discharged 
  toward 
  the 
  Lake 
  Erie 
  basin 
  along 
  the 
  course 
  

   of 
  the 
  Beaver 
  river 
  before 
  the 
  first 
  glacial 
  epoch. 
  This 
  de- 
  

   cline 
  of 
  the 
  high 
  level 
  terrace 
  from 
  the 
  mouth 
  of 
  the 
  Beaver 
  

   north 
  to 
  the 
  point 
  where 
  it 
  is 
  lost 
  under 
  the 
  moraine 
  seems 
  in 
  

   itself 
  to 
  be 
  good 
  evidence 
  that 
  the 
  old 
  river 
  took 
  this 
  north- 
  

   ward 
  course 
  instead 
  of 
  that 
  now 
  followed 
  by 
  the 
  Ohio, 
  and 
  this 
  

   determination 
  by 
  Mr. 
  Foshay 
  is 
  a 
  valuable 
  addition 
  to 
  our 
  

   knowledge. 
  It 
  is 
  somewhat 
  short 
  of 
  a 
  conclusive 
  demonstra- 
  

   tion 
  of 
  the 
  northward 
  course 
  of 
  the 
  stream 
  in 
  the 
  fact 
  that 
  the 
  

   decline 
  is 
  only 
  25 
  feet, 
  that 
  the 
  distance 
  is 
  only 
  ten 
  miles, 
  that 
  

   the 
  observations 
  are 
  few, 
  (apparently 
  only 
  two), 
  that 
  the 
  two 
  

   remnants 
  may 
  not 
  belong 
  to 
  strictly 
  identical 
  planes, 
  that 
  the 
  

   decline 
  is 
  not 
  greater 
  than 
  the 
  possible 
  differential 
  northward 
  

   depression 
  of 
  the 
  region, 
  and 
  that 
  the 
  non-continuance 
  of 
  the 
  

   high-level 
  plane 
  down 
  the 
  present 
  course 
  of 
  the 
  Ohio 
  has 
  not 
  

   been 
  demonstrated. 
  If 
  it 
  shall 
  be 
  shown 
  that 
  no 
  such 
  high 
  ter- 
  

   races 
  follow 
  down 
  the 
  Ohio, 
  the 
  presumption 
  in 
  favor 
  of 
  the 
  

  

  * 
  2nd 
  Geol. 
  Survey 
  of 
  Penn. 
  Q, 
  pp. 
  11, 
  12. 
  

  

  