﻿50 
  NEW 
  YORK 
  STATE 
  MUSEUM 
  

  

  velopment 
  of 
  the 
  Mississippi 
  drainage 
  system, 
  considers 
  that 
  the 
  

   flow 
  of 
  the 
  Laurentian 
  drainage 
  system 
  was 
  southward 
  into 
  the 
  pre- 
  

   decessor 
  of 
  the 
  Ohio 
  river. 
  As 
  has 
  already 
  been 
  shown, 
  Spencer's 
  

   eastward 
  flowing 
  river 
  system 
  can 
  be 
  originated 
  only 
  by 
  a 
  com- 
  

   plete 
  readjustment 
  of 
  the 
  drainage, 
  resulting 
  from 
  a 
  great 
  relative 
  

   depression 
  of 
  the 
  eastern 
  uplands. 
  Such 
  a 
  system 
  could 
  only 
  come 
  

   into 
  existence 
  after 
  the 
  valleys 
  had 
  been 
  formed 
  for 
  it, 
  and 
  hence, 
  

   as 
  far 
  as 
  the 
  history 
  of 
  the 
  lake 
  basins 
  is 
  considered, 
  no 
  such 
  river 
  

   system 
  is 
  required, 
  and, 
  unless 
  positive 
  proof 
  of 
  its 
  former 
  existence 
  

   Is 
  forthcoming, 
  it 
  may 
  be 
  dismissed 
  as 
  hypothetic. 
  One 
  of 
  the 
  most 
  

   important 
  theories 
  of 
  the 
  origin 
  of 
  the 
  Ontario 
  and 
  other 
  lake 
  basins, 
  

   and 
  one 
  which 
  has 
  had, 
  and 
  still 
  has 
  many 
  prominent 
  advocates, 
  is 
  

   that 
  of 
  glacial 
  erosion, 
  either 
  entire 
  or 
  preceded 
  by 
  river 
  erosion. 
  

   This 
  explanation 
  was 
  first 
  most 
  strongly 
  urged 
  by 
  Prof. 
  Newberry, 
  

   and 
  it 
  has 
  found 
  its 
  most 
  recent 
  able 
  supporter 
  in 
  Prof. 
  Tarr. 
  It 
  is 
  

   impossible 
  to 
  do 
  full 
  justice 
  to 
  this 
  view 
  in 
  the 
  present 
  limited 
  space. 
  

   Ice 
  erosion 
  is 
  a 
  factor 
  the 
  potency 
  of 
  which 
  has 
  often 
  been 
  over- 
  

   looked, 
  but 
  of 
  the 
  importance 
  of 
  which 
  there 
  can 
  be 
  no 
  question. 
  

   We 
  may 
  however 
  question 
  whether 
  a 
  valley 
  which, 
  like 
  that 
  of 
  On- 
  

   tario, 
  lies 
  transverse 
  to 
  the 
  general 
  direction 
  of 
  ice 
  movement 
  in 
  

   this 
  region, 
  can 
  owe 
  much 
  of 
  its 
  depth 
  to 
  this 
  agent. 
  The 
  following 
  

   considerations 
  will 
  be 
  helpful 
  in 
  understanding 
  the 
  influence 
  of 
  

   glacial 
  erosion 
  on 
  preexisting 
  topography. 
  If 
  a 
  valley 
  like 
  that 
  of 
  

   Lake 
  Ontario 
  is 
  occupied 
  by 
  a 
  glacier 
  the 
  motion 
  of 
  which 
  is 
  parallel 
  

   to 
  the 
  trend 
  of 
  the 
  valley, 
  the 
  topographic 
  relief 
  is 
  likely 
  to 
  be 
  

   accentuated 
  by 
  ice 
  erosion. 
  If 
  the 
  motion 
  of 
  the 
  ice 
  is 
  transverse 
  

   to 
  the 
  direction 
  of 
  the 
  valley, 
  the 
  erosion 
  tends 
  to 
  obliterate 
  or 
  at 
  

   least 
  reduce 
  the 
  reHef 
  features. 
  If,- 
  however, 
  a 
  mass 
  of 
  ice 
  remains 
  

   stagnant 
  in 
  the 
  valley, 
  the 
  upper 
  strata 
  of 
  ice 
  may 
  override 
  it, 
  and 
  

   the 
  amount 
  of 
  glacial 
  erosion 
  is 
  reduced 
  to 
  a 
  minimum. 
  The 
  striae 
  

   in 
  this 
  region, 
  together 
  with 
  the 
  direction 
  of 
  slopes 
  from 
  the 
  old- 
  

   land, 
  point 
  to 
  a 
  southward 
  movement 
  of 
  the 
  ice, 
  and 
  Gilbert 
  has 
  

   shown 
  that 
  the 
  amount 
  of 
  erosion 
  on 
  the 
  edge 
  of 
  the 
  escarpment 
  in 
  

   western 
  New 
  York 
  is 
  comparatively 
  slight.^ 
  Hence 
  we 
  may 
  assume 
  

   that 
  the 
  basin 
  of 
  Ontario 
  was 
  mainly 
  occupied 
  by 
  ice 
  during 
  the 
  

  

  ^Bul. 
  geol. 
  soc. 
  Am. 
  11:121. 
  

  

  