﻿90 
  NEW 
  YORK 
  STATE 
  MUSEUM 
  

  

  New 
  York 
  is 
  literally 
  strewn 
  with 
  thousands 
  of 
  glacial 
  boulders 
  or 
  

   erratics 
  which 
  Were 
  transported 
  from 
  the 
  Adirondacks 
  by 
  the 
  ice, 
  

   and 
  similar 
  boulders 
  are 
  occasionally 
  found 
  as 
  far 
  south 
  as 
  Bing- 
  

   hamton 
  or 
  well 
  down 
  the 
  Hudson 
  valley. 
  Evidences 
  of 
  glaciation 
  

   also 
  occur 
  high 
  up 
  in 
  the 
  Adirondacks 
  and 
  the 
  Catskills, 
  so 
  that 
  the 
  

   greatest 
  depth 
  of 
  ice 
  over 
  New 
  York 
  State 
  could 
  not 
  have 
  been 
  

   less 
  than 
  several 
  thousand 
  feet. 
  In 
  fact, 
  we 
  have 
  every 
  reason 
  to 
  

   believe 
  that 
  the 
  Adirondacks, 
  if 
  not 
  the 
  Catskills, 
  were 
  completely 
  

   buried. 
  The 
  reader 
  may 
  wonder 
  how 
  an 
  ice 
  sheet 
  a 
  mile 
  thick 
  in 
  

   northern 
  New 
  York 
  could 
  have 
  thinned 
  out 
  to 
  disappearance 
  at 
  or 
  

   near 
  the 
  southern 
  border 
  of 
  the 
  State, 
  but 
  observations 
  on 
  existing 
  

   glaciers 
  show 
  that 
  it 
  is 
  quite 
  the 
  habit 
  of 
  extensive 
  ice 
  bodies 
  to 
  

   thin 
  out 
  very 
  rapidly 
  near 
  the 
  margins, 
  thus 
  producing 
  steep 
  slopes 
  

   along 
  the 
  ice 
  fronts. 
  

  

  Successive 
  ice 
  invasions. 
  The 
  front 
  of 
  the 
  great 
  ice 
  sheet, 
  like 
  

   that 
  of 
  ordinary 
  valley 
  glaciers, 
  must 
  have 
  shown 
  many 
  advances 
  

   and 
  retreats. 
  In 
  the 
  northern 
  Mississippi 
  valley, 
  however, 
  we 
  have 
  

   positive 
  proof 
  for 
  several 
  (perhaps 
  five 
  or 
  six) 
  important 
  advances 
  

   and 
  retreats 
  of 
  the 
  ice 
  which 
  gave 
  rise 
  to 
  true 
  interglacial 
  stages. 
  

   The 
  strongest 
  evidence 
  is 
  the 
  presence 
  of 
  successive 
  layers 
  of 
  glacial 
  

   debris, 
  a 
  given 
  layer 
  often 
  having 
  been 
  oxidized, 
  eroded, 
  and 
  cov- 
  

   ered 
  with 
  vegetation 
  before 
  the 
  next 
  (overlying) 
  layer 
  was 
  

   deposited. 
  In 
  drilling 
  wells 
  through 
  the 
  glacial 
  deposits 
  of 
  Iowa, 
  

   for 
  example, 
  two 
  distinct 
  deposits 
  or 
  layers 
  of 
  vegetation 
  are 
  often 
  

   encountered 
  at 
  depths 
  of 
  from 
  ioo 
  to 
  200 
  feet. 
  Near 
  Toronto, 
  

   Canada, 
  plants 
  which 
  actually 
  belong 
  much 
  farther 
  south 
  in 
  a 
  

   warmer 
  climate 
  have 
  been 
  found 
  between 
  two 
  layers 
  of 
  glacial 
  

   debris. 
  Thus 
  we 
  know 
  that 
  some, 
  at 
  least, 
  of 
  the 
  ice 
  retreats 
  pro- 
  

   duced 
  interglacial 
  stages 
  with 
  warmer 
  climate 
  and 
  were 
  sufficient 
  

   greatly 
  to 
  reduce 
  the 
  size 
  of 
  the 
  continental 
  ice 
  sheet 
  or 
  possibly 
  

   to 
  cause 
  its 
  entire 
  disappearance. 
  

  

  In 
  New 
  York 
  State 
  no 
  very 
  positive 
  evidence 
  has 
  as 
  yet 
  been 
  

   found 
  to 
  prove 
  truly 
  multiple 
  glaciation, 
  though 
  some 
  phenomena 
  

   as, 
  for 
  example, 
  certain 
  buried 
  gorges, 
  are 
  very 
  difficult 
  to 
  account 
  

   for 
  except 
  on 
  the 
  basis 
  of 
  more 
  than 
  one 
  advance 
  and 
  retreat 
  of 
  

   the 
  ice. 
  At 
  any 
  rate, 
  there 
  appears 
  to 
  be 
  no 
  good 
  reason 
  whatever 
  

   to 
  believe 
  that 
  there 
  were 
  more 
  than 
  two 
  advances 
  and 
  retreats 
  of 
  

   the 
  ice 
  over 
  the 
  State, 
  and 
  for 
  our 
  purpose 
  in 
  considering 
  only 
  the 
  

   general 
  effects 
  of 
  glaciation 
  we 
  may 
  practically 
  disregard 
  the 
  prob- 
  

   lem 
  of 
  multiple 
  glaciation 
  because 
  the 
  final 
  effects 
  would 
  have 
  been 
  

   essentially 
  the 
  same 
  as 
  a 
  result 
  of 
  a 
  single 
  great 
  glacial 
  advance 
  and 
  

   retreat. 
  

  

  