﻿212 
  F. 
  Lever 
  ett 
  — 
  Pleistocene 
  Fluvial 
  Planes, 
  etc. 
  

  

  The 
  excavation 
  that 
  intervened 
  between 
  the 
  two 
  epochs 
  in 
  

   other 
  portions 
  of 
  the 
  Allegheny, 
  Monongahela, 
  and 
  Upper 
  

   Ohio 
  valleys 
  is 
  closely 
  comparable 
  with 
  this. 
  

  

  In 
  view 
  of 
  these 
  facts 
  it 
  seems 
  scarcely 
  less 
  than 
  proven 
  that 
  

   it 
  was 
  the 
  earlier 
  invasion 
  of 
  ice 
  that 
  reversed 
  the 
  drainage 
  

   and 
  partially 
  tilled 
  the 
  valleys 
  with 
  debris, 
  forming 
  the 
  capping 
  

   of 
  glacial 
  gravel 
  that 
  rests 
  upon 
  the 
  upper 
  terrace." 
  

  

  " 
  From 
  the 
  fact 
  that 
  the 
  fluvial 
  material 
  in 
  these 
  abandoned 
  

   channels 
  and 
  on 
  the 
  corresponding 
  terraces 
  in 
  the 
  Monongahela 
  

   valley 
  is 
  wholly 
  local, 
  or 
  southern, 
  while 
  among 
  the 
  analogous 
  

   material 
  of 
  the 
  Allegheny 
  there 
  mingle 
  crystalline 
  erratics 
  of 
  

   Canadian 
  derivation, 
  and 
  from 
  the 
  evidence 
  given 
  above, 
  we 
  

   draw 
  the 
  inference 
  that 
  the 
  partial 
  filling 
  was 
  coincident 
  with 
  

   some 
  stage 
  of 
  the 
  earlier 
  glaciation, 
  presumably 
  a 
  late 
  stage. 
  

   This 
  view 
  gathers 
  some 
  support 
  from 
  the 
  now 
  well 
  sustained 
  

   belief 
  that 
  a 
  general 
  depression 
  and 
  slackening 
  of 
  drainage 
  

   accompanied 
  the 
  earlier 
  glaciation. 
  

  

  Following 
  this 
  episode 
  of 
  valley-filling 
  and 
  earlier 
  glacia- 
  

   tion 
  there 
  was 
  a 
  prolonged 
  epoch 
  of 
  rapid 
  erosion 
  of 
  the 
  valley 
  

   bottom, 
  which 
  was 
  apparently 
  coincident 
  with 
  an 
  interglacial 
  

   epoch, 
  and 
  was, 
  perhaps 
  the 
  result 
  of 
  the 
  resilience 
  of 
  the 
  

   land 
  after 
  the 
  glacial 
  depression. 
  During 
  this 
  epoch 
  the 
  rock 
  

   gorges 
  were 
  cut 
  down 
  to 
  the 
  rock 
  bottoms 
  that 
  now 
  lie 
  forty 
  

   feet 
  or 
  more 
  below 
  the 
  present 
  river 
  bottoms. 
  Then 
  came 
  the 
  

   later 
  invasion 
  that 
  halted 
  at 
  the 
  outer 
  terminal 
  moraine, 
  whose 
  

   overloaded 
  floods, 
  like 
  those 
  of 
  the 
  preceding 
  glacial 
  incursion, 
  

   filled 
  the 
  valley 
  bottoms 
  with 
  glacial 
  alluvium 
  ; 
  only, 
  in 
  this 
  

   instance, 
  in 
  harmony 
  with 
  the 
  more 
  vigorous 
  character 
  of 
  the 
  

   later 
  glaciation, 
  the 
  filling 
  reached, 
  at 
  some 
  points, 
  300 
  feet. 
  

   Since 
  that 
  time 
  there 
  has 
  been 
  another 
  stage 
  of 
  reexcavation, 
  

   giving 
  origin 
  to 
  the 
  lower 
  gravel 
  terraces. 
  

  

  This 
  is 
  doubtless 
  far 
  from 
  being 
  the 
  whole 
  history 
  of 
  events, 
  

   and 
  may 
  be 
  divergent 
  from 
  the 
  truth 
  in 
  minor 
  phases, 
  but 
  I 
  

   believe 
  with 
  some 
  confidence 
  that 
  it 
  represents 
  the 
  general 
  

   truth 
  respecting 
  the 
  history 
  of 
  the 
  abandoned 
  channels 
  and 
  

   chief 
  terrace 
  deposits 
  of 
  the 
  system 
  of 
  benches 
  under 
  consid- 
  

   eration." 
  

  

  Madison, 
  Wis., 
  March 
  14, 
  1891. 
  

  

  