﻿208 
  F. 
  Leverett 
  — 
  Pleistocene 
  Fluvial 
  Planes 
  of 
  

  

  valley 
  floor 
  has 
  its 
  highest 
  point 
  near 
  Titusville, 
  Penn. 
  (No. 
  2), 
  

   and 
  there 
  is 
  a 
  descent 
  of 
  Q6 
  feet 
  in 
  eight 
  miles 
  upstream. 
  The 
  

   valley 
  floor 
  of 
  French 
  creek 
  rises 
  for 
  Ave 
  or 
  six 
  miles 
  upstream, 
  

   but 
  near 
  Meadville 
  (No. 
  3), 
  25 
  miles 
  above 
  its 
  mouth, 
  the 
  

   rock 
  floor 
  is 
  about 
  150 
  feet 
  lower 
  than 
  at 
  the 
  mouth 
  of 
  the 
  

   creek. 
  The 
  floor 
  of 
  Big 
  Sandy 
  creek 
  rises 
  from 
  its 
  mouth 
  to 
  

   the 
  vicinity 
  of 
  Raymilton 
  (No. 
  17), 
  but 
  descends 
  above 
  that 
  

   village, 
  being 
  fully 
  40 
  feet 
  lower 
  at 
  Sandy 
  Lake 
  than 
  at 
  Ray- 
  

   milton. 
  Borings 
  are 
  sufficiently 
  numerous 
  to 
  show 
  a 
  strong 
  

   probability 
  that 
  these 
  valley 
  floors 
  have 
  no 
  channels 
  deep 
  

   enough 
  to 
  drain 
  them 
  southward, 
  but 
  unfortunately 
  they 
  are 
  

   not 
  sufficient 
  to 
  demonstrate 
  whether 
  or 
  not 
  there 
  is 
  a 
  continu- 
  

   ous 
  descent 
  to 
  the 
  Lake 
  Erie 
  basin 
  from 
  any 
  of 
  the 
  points 
  

   noted. 
  Mr. 
  Carll 
  has 
  shown 
  that 
  there 
  appears 
  to 
  be 
  no 
  

   obstacle 
  to 
  the 
  northward 
  continuation 
  of 
  the 
  Conewango 
  val- 
  

   ley 
  past 
  Cassadaga 
  lake 
  into 
  Lake 
  Erie, 
  though 
  it 
  is 
  necessary 
  

   to 
  assume 
  about 
  500 
  feet 
  of 
  drift 
  filling 
  at 
  the 
  watershed. 
  

   Similarly, 
  to 
  give 
  French 
  creek 
  a 
  northern 
  outlet 
  by 
  way 
  of 
  

   Conneaut 
  lake 
  and 
  Conneaut 
  creek 
  a 
  drift 
  filling 
  of 
  over 
  300 
  

   feet 
  at 
  the 
  watershed 
  north 
  of 
  Conneaut 
  lake 
  must 
  be 
  assumed. 
  

  

  Inasmuch 
  as 
  the 
  northward 
  drainage 
  of 
  these 
  buried 
  chan- 
  

   nels 
  remains 
  an 
  open 
  question, 
  a 
  brief 
  consideration 
  of 
  other 
  

   hypotheses 
  to 
  account 
  for 
  the 
  phenomena 
  seems 
  called 
  for. 
  A 
  

   certain 
  amount 
  of 
  northward 
  descent 
  may 
  prove 
  to 
  be 
  due 
  

   to 
  crust-deformation. 
  The 
  beaches 
  about 
  Lakes 
  Erie 
  and 
  

   Ontario, 
  and 
  those 
  of 
  the 
  G-lacial 
  Lake 
  Agassiz, 
  as 
  is 
  well 
  

   known, 
  indicate 
  clearly 
  a 
  northward 
  differential 
  uplift 
  accom- 
  

   panying 
  the 
  retreat 
  of 
  the 
  ice, 
  but 
  they 
  indicate 
  nothing 
  as 
  to 
  

   the 
  depression 
  that 
  preceded 
  this 
  uplift. 
  In 
  the 
  opinion 
  of 
  

   those 
  who 
  have 
  given 
  most 
  attention 
  to 
  these 
  beaches, 
  the 
  

   uplift 
  was 
  due 
  mainly 
  to 
  the 
  withdrawal 
  of 
  the 
  load 
  of 
  ice. 
  

   This 
  hypothesis 
  involves 
  a 
  previous 
  depression 
  occasioned 
  by 
  

   its 
  accumulation 
  and 
  an 
  imperfect 
  restoration, 
  owing 
  to 
  the 
  

   removal 
  of 
  material 
  from 
  portions 
  of 
  the 
  glaciated 
  district 
  and 
  

   the 
  presence 
  of 
  a 
  load 
  of 
  drift 
  and 
  large 
  bodies 
  of 
  water 
  after 
  the 
  

   ice 
  withdrew 
  in 
  parts 
  of 
  the 
  glaciated 
  district 
  not 
  thus 
  encum- 
  

   bered 
  in 
  preglacial 
  times. 
  So 
  far 
  as 
  we 
  may 
  reason 
  from 
  theo- 
  

   retical 
  grounds, 
  there 
  should 
  be 
  expected 
  a 
  residuum 
  of 
  north- 
  

   ward 
  depression 
  in 
  the 
  region 
  under 
  discussion, 
  since 
  a 
  large 
  

   amount 
  of 
  drift 
  was 
  deposited 
  here. 
  And 
  this 
  may 
  prove 
  to 
  

   have 
  been 
  an 
  important 
  factor 
  in 
  giving 
  these 
  valley 
  floors 
  a 
  

   northward 
  slope, 
  though 
  it 
  is 
  hardly 
  probable 
  that 
  it 
  was 
  the 
  

   chief 
  one. 
  

  

  A 
  more 
  important 
  factor 
  in 
  the 
  production 
  of 
  the 
  peculiar 
  

   valley 
  phenomena 
  of 
  this 
  region 
  may 
  prove 
  to 
  have 
  been 
  

   erosion 
  effected 
  beneath 
  the 
  ice 
  either 
  by 
  the 
  ice 
  itself 
  or 
  by 
  

   subglacial 
  waters. 
  Whether 
  the. 
  ice 
  greatly 
  deepened 
  valleys 
  

  

  