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  NEW 
  YORK 
  STATE 
  MUSEUM 
  

  

  Great 
  Lakes 
  history. 
  The 
  Great 
  Lakes 
  certainly 
  did 
  not 
  exist 
  

   before 
  the 
  Ice 
  age, 
  but 
  instead 
  the 
  depressions 
  in 
  that 
  region 
  were 
  

   occupied 
  by 
  stream 
  channels. 
  During 
  the 
  very 
  long 
  erosion 
  period 
  

   (already 
  discussed) 
  from 
  the 
  Paleozoic 
  to 
  the 
  Cenozoic, 
  no 
  lakes, 
  

   except 
  possibly 
  a 
  few 
  very 
  small 
  ones 
  due 
  to 
  landslides, 
  beaver 
  

   dams, 
  etc., 
  could 
  have 
  existed. 
  Compared 
  with 
  such 
  an 
  immense 
  

   length 
  of 
  time 
  lakes 
  are, 
  at 
  most, 
  only 
  ephemeral 
  features 
  of 
  the 
  

   earth's 
  surface 
  because 
  they 
  are 
  soon 
  destroyed 
  either 
  by 
  being 
  

   filled 
  with 
  sediments, 
  or 
  by 
  having 
  their 
  outlets 
  cut 
  down, 
  or 
  both. 
  

   Since 
  the 
  Great 
  Lakes 
  are 
  of 
  postglacial 
  origin 
  it 
  is, 
  then, 
  proper 
  to 
  

   ask 
  how 
  they 
  came 
  into 
  existence. 
  During 
  preglacial 
  time, 
  as 
  we 
  

   have 
  learned, 
  broad 
  valleys 
  were 
  cut 
  out 
  along 
  belts 
  of 
  weak 
  rock 
  

   in 
  the 
  Great 
  Lakes 
  region, 
  and 
  these 
  old 
  valleys, 
  to 
  a 
  considerable 
  

   extent 
  at 
  least, 
  account 
  for 
  the 
  present 
  depressions, 
  but 
  not 
  for 
  

   the 
  closed 
  lake 
  basins. 
  This 
  idea 
  of 
  preglacial 
  stream 
  valleys 
  is 
  

   not 
  at 
  all 
  opposed 
  by 
  the 
  fact 
  that 
  some 
  of 
  the 
  lake 
  bottoms 
  are 
  

   now 
  well 
  below 
  sea 
  level 
  because 
  there 
  has 
  been 
  a 
  notable 
  sub- 
  

   sidence 
  of 
  the 
  region 
  since 
  preglacial 
  time. 
  The 
  surface 
  of 
  Lake 
  

   Erie 
  is 
  573 
  feet 
  and 
  its 
  deepest 
  point 
  369 
  feet 
  above 
  sea 
  

   level, 
  while 
  the 
  surface 
  of 
  Lake 
  Ontario 
  is 
  247 
  feet 
  above 
  

   and 
  its 
  deepest 
  point 
  is 
  491 
  feet 
  below 
  sea 
  level. 
  The 
  

   greatest 
  depth 
  (738 
  feet) 
  of 
  Lake 
  Ontario 
  is 
  well 
  toward 
  the 
  

   east 
  end 
  and 
  not 
  far 
  from 
  the 
  south 
  shore, 
  and 
  if 
  we 
  consider 
  this 
  

   deep 
  place 
  as 
  due 
  to 
  preglacial 
  erosion, 
  we 
  ought 
  to 
  find 
  an 
  outlet 
  

   channel. 
  But 
  no 
  such 
  outlet 
  channel 
  exists 
  because 
  the 
  whole 
  

   eastern 
  end, 
  at 
  least, 
  of 
  the 
  lake 
  is 
  certainly 
  rock-rimmed. 
  As 
  

   Tarr 
  has 
  said: 
  "There 
  could 
  hardly 
  be 
  a 
  valley 
  over 
  700 
  feet 
  

   deep 
  and 
  broad 
  enough 
  to 
  form 
  the 
  continuation 
  of 
  the 
  preglacial 
  

   Ontario 
  valley, 
  which 
  is 
  so 
  completely 
  obscured 
  by 
  drift 
  that 
  not 
  

   the 
  least 
  trace 
  of 
  it 
  has 
  been 
  found 
  on 
  the 
  surface." 
  1 
  To 
  assume 
  

   that 
  this 
  deep 
  part 
  of 
  the 
  basin 
  was 
  produced 
  by 
  warping 
  of 
  the 
  

   land 
  is 
  not 
  borne 
  out 
  by 
  examining 
  the 
  exposed 
  strata 
  on 
  all 
  sides. 
  

   It 
  therefore 
  seems 
  quite 
  certain 
  that 
  the 
  preglacial 
  Ontario 
  depres- 
  

   sion 
  was 
  here 
  considerably 
  deepened 
  by 
  ice 
  erosion. 
  The 
  conditions 
  

   were 
  very 
  favorable 
  for 
  such 
  erosion 
  because 
  the 
  rocks 
  were 
  chiefly 
  

   soft 
  Ordovicic 
  shales 
  ; 
  because 
  the 
  ice 
  flowed 
  through 
  a 
  deep 
  pre- 
  

   glacial 
  valley; 
  and 
  because 
  there 
  was. 
  unusual 
  crowding 
  of 
  ice 
  into 
  

   this 
  valley 
  due 
  to 
  the 
  pronounced 
  deflection 
  of 
  a 
  great 
  ice 
  current 
  

   around 
  the 
  Adtfrondacks 
  on 
  the 
  west 
  side. 
  Strong 
  arguments 
  might 
  

   be 
  adduced 
  to 
  show 
  that 
  by 
  ice 
  erosion, 
  portions, 
  at 
  least, 
  of 
  all 
  the 
  

  

  1 
  Tarr's 
  Physical 
  Geography 
  of 
  New 
  York 
  State, 
  p. 
  235. 
  

  

  