﻿THE 
  GEOLOGICAL 
  HISTORY 
  OF 
  NEW 
  YORK 
  STATE 
  

  

  115 
  

  

  advances 
  and 
  retreats 
  of 
  the 
  ice 
  sheets 
  etc. 
  Although 
  a 
  closer 
  

   calculation 
  is 
  well 
  nigh 
  impossible 
  because 
  of 
  the 
  variability 
  of 
  the 
  

   factors 
  involved 
  from 
  time 
  to 
  time, 
  it 
  is 
  nevertheless 
  certain 
  that, 
  

   from 
  the 
  geological 
  standpoint, 
  the 
  Ice 
  age 
  was 
  of 
  short 
  duration, 
  

   while, 
  from 
  the 
  standpoint 
  of 
  known 
  human 
  history, 
  it 
  was 
  

   immensely 
  long. 
  

  

  Estimates 
  of 
  the 
  length 
  of 
  time 
  since 
  the 
  close 
  of 
  the 
  Ice 
  age 
  

   are 
  perhaps 
  more 
  satisfactory, 
  though 
  it 
  must 
  be 
  remembered 
  that 
  

   the 
  close 
  of 
  the 
  Ice 
  age 
  was 
  not 
  at 
  the 
  same 
  time 
  for 
  all 
  places. 
  

   The 
  ice 
  retreated 
  northward 
  very 
  slowly 
  and 
  when, 
  for 
  example, 
  

   southern 
  New 
  York 
  was 
  free 
  from 
  the 
  ice, 
  northern 
  New 
  York 
  was 
  

   still 
  glaciated. 
  The 
  best 
  estimates 
  for 
  the 
  length 
  of 
  time 
  since 
  the 
  

  

  Fig. 
  39 
  Sketch 
  map 
  showing 
  the 
  relation 
  of 
  the 
  crest 
  of 
  

   Niagara 
  Falls 
  in 
  1842 
  to 
  that 
  of 
  1905. 
  Based 
  upon 
  actual 
  

   surveys. 
  The 
  retreat 
  of 
  the 
  inner 
  portion 
  of 
  the 
  Horseshoe 
  

   Fall 
  was 
  more 
  than 
  300 
  feet. 
  

  

  Modified 
  after 
  Gilbert, 
  U. 
  S. 
  G. 
  S. 
  Bui. 
  306, 
  p. 
  20 
  

  

  close 
  of 
  the 
  Ice 
  age 
  in 
  New 
  York 
  State 
  are 
  based 
  upon 
  the 
  rate 
  

   of 
  recession 
  of 
  Niagara 
  falls. 
  We 
  have 
  learned 
  that 
  the 
  Niagara 
  

   river 
  began 
  its 
  work 
  about 
  the 
  time 
  the 
  glacial 
  waters 
  in 
  the 
  Erie- 
  

   Ontario 
  regions 
  had 
  dropped 
  to 
  the 
  Iroquois 
  level, 
  and 
  that 
  the 
  

   falls 
  were 
  first 
  formed 
  by 
  the 
  plunging 
  of 
  the 
  river 
  over 
  the 
  Niagara 
  

   limestone 
  escarpment 
  at 
  Queenston 
  and 
  Lewiston. 
  Studies 
  based 
  

   upon 
  actual 
  surveys, 
  drawings, 
  daguerreotypes, 
  photographs 
  etc. 
  

   made 
  between 
  the 
  years 
  1842 
  and 
  1905, 
  have 
  shown 
  that 
  the 
  Horse- 
  

   shoe 
  fall 
  had 
  receded 
  about 
  5 
  feet 
  a 
  year, 
  while 
  the 
  American 
  fall 
  

   between 
  1827 
  and 
  1905, 
  had 
  receded 
  about 
  3 
  inches 
  a 
  year. 
  Thus 
  

   the 
  gorge 
  cutting 
  is 
  clearly 
  taking 
  place 
  on 
  the 
  Canadian 
  side. 
  The 
  

  

  