﻿THE 
  GEOLOGICAL 
  HISTORY 
  OF 
  NEW 
  YORK 
  STATE 
  

  

  113 
  

  

  shale, 
  and 
  Medina 
  shale 
  and 
  sandstone. 
  These 
  formations 
  show 
  

   only 
  a 
  slight 
  southward 
  dip 
  or 
  tilt. 
  

  

  The 
  Genesee 
  river 
  from 
  its 
  source 
  to 
  Portageville, 
  Wyoming 
  

   county, 
  appears 
  to 
  be 
  in 
  a 
  mature 
  preglacial 
  valley. 
  Near 
  Portage- 
  

   ville, 
  however, 
  the 
  river 
  plunges 
  into 
  a 
  deep, 
  narrow, 
  rock 
  gorge 
  

   of 
  postglacial 
  origin, 
  which 
  continues 
  for 
  25 
  miles 
  to 
  Mount 
  Morris. 
  

   This 
  gorge 
  has 
  been 
  cut 
  through 
  soft 
  Devonic 
  shales 
  and 
  shaly 
  

   sandstones, 
  and 
  its 
  walls 
  are 
  mostly 
  nearly 
  vertical, 
  often 
  rising 
  to 
  

   heights 
  of 
  several 
  hundred 
  feet. 
  The 
  three 
  noted 
  Portage 
  falls 
  

   (see 
  plate 
  51) 
  are 
  situated 
  just 
  below 
  Portageville, 
  the 
  upper 
  falls 
  

  

  

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  ™3 
  i 
  r 
  

  

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  K 
  ?! 
  

  

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  ■■' 
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  1 
  hi- 
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  1, 
  f 
  Adv. 
  

  

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  Fig. 
  38 
  Section 
  at 
  Niagara 
  Falls, 
  showing 
  

   the 
  character 
  and 
  position 
  of 
  the 
  rock 
  forma- 
  

   tions 
  and 
  the 
  depth 
  of 
  water 
  below 
  the 
  falls. 
  

  

  After 
  Gilbert 
  

  

  plunging 
  66 
  feet, 
  the 
  middle 
  falls 
  no 
  feet, 
  and 
  the 
  lower 
  falls 
  96 
  

   feet. 
  According 
  to 
  Grabau, 
  the 
  preglacial 
  course 
  between 
  Portage- 
  

   ville 
  and 
  Mount 
  Morris 
  was 
  farther 
  westward 
  along 
  the 
  present 
  

   Oatka 
  creek. 
  A 
  second 
  postglacial 
  gorge 
  is 
  entered 
  at 
  Rochester, 
  

   and 
  this 
  continues 
  for 
  7 
  miles 
  to 
  the 
  mouth 
  of 
  the 
  river. 
  Here, 
  

   also, 
  are 
  three 
  falls, 
  the 
  first 
  over 
  Niagara 
  limestone 
  being 
  98 
  feet, 
  

   the 
  second 
  over 
  Clinton 
  shale 
  and 
  limestone 
  being 
  20 
  feet, 
  and 
  the 
  

   third 
  over 
  Medina 
  sandstone 
  being 
  105 
  feet. 
  The 
  preglacial 
  chan- 
  

   nel 
  here 
  was 
  probably 
  a 
  little 
  to 
  the 
  east 
  and 
  through 
  Irondequoit 
  

   bay. 
  

  

  