﻿GEOLOGY 
  OF 
  THOUSAND 
  ISLANDS 
  REGION 
  1 
  59 
  

  

  The 
  pitted 
  clays 
  are 
  a 
  link 
  between 
  the 
  ice 
  marginal 
  deposits 
  and 
  

   the 
  open 
  lake 
  deposits. 
  They 
  might 
  be 
  classed 
  with 
  the 
  morainal 
  

   or 
  peripheral 
  drift, 
  since 
  they 
  were 
  associated 
  with 
  remnants 
  of 
  the 
  

   ice 
  front, 
  but 
  the 
  aqueous 
  origin 
  is 
  here 
  regarded 
  as 
  the 
  more 
  im- 
  

   portant 
  element. 
  

  

  Glacial 
  erosion 
  

  

  General 
  character. 
  The 
  abrasional 
  work 
  of 
  the 
  glacier 
  in 
  this 
  

   area 
  is 
  more 
  conspicuous 
  in 
  the 
  northern 
  district 
  where 
  the 
  hard 
  

   Precambric 
  and 
  Potsdam 
  rocks 
  are 
  in 
  high 
  relief 
  and 
  the 
  drift 
  is 
  

   mostly 
  in 
  the 
  hollows. 
  Over 
  the 
  southern 
  district 
  where 
  horizontal 
  

   limestones 
  form 
  the 
  floor 
  the 
  ice 
  erosion 
  was 
  probably 
  greater 
  than 
  

   farther 
  north, 
  but 
  the 
  evidences 
  are 
  Inore 
  concealed. 
  The 
  origin 
  

   of 
  the 
  plains, 
  plateaus 
  and 
  mesas, 
  by 
  preglacial 
  weathering, 
  glacial 
  

   planing 
  and 
  stream 
  trenching, 
  has 
  been 
  discussed 
  in 
  a 
  former 
  chapter 
  

   [p. 
  146]. 
  

  

  The 
  more 
  vigorous 
  erosion 
  on 
  the 
  limestones 
  is 
  shown 
  by 
  flutings 
  

   or 
  ribbing, 
  the 
  lighter 
  and 
  later, 
  by 
  striation 
  and 
  polish. 
  The 
  Pots- 
  

   dam 
  and 
  crystalline 
  knobs 
  seem 
  to 
  have 
  been 
  little 
  more 
  than 
  '' 
  sand- 
  

   papered 
  " 
  by 
  the 
  latest 
  glaciation. 
  The 
  broader 
  surfaces 
  of 
  the 
  

   more 
  horizontal 
  Potsdam 
  shows 
  effective 
  abrasion 
  in 
  spots 
  only. 
  

   The 
  impression 
  made 
  on 
  the 
  observer 
  is 
  that 
  glaciation 
  of 
  an 
  earlier 
  

   ice 
  invasion 
  was 
  vigorous 
  but 
  that 
  the 
  latest 
  ice 
  sheet 
  was 
  compara- 
  

   tively 
  ineffective.^ 
  

  

  Striations. 
  Occurrence. 
  The 
  limestones 
  exhibit 
  few 
  striae, 
  

   as 
  will 
  be 
  inferred 
  from 
  the 
  lack 
  of 
  arrows 
  on 
  the 
  maps 
  of 
  the 
  

   limestone 
  districts 
  [pi. 
  44-47]. 
  It 
  is 
  uncertain 
  whether 
  this 
  should 
  

   be 
  chiefly 
  attributed 
  to 
  the 
  failure 
  of 
  the 
  latest 
  ice 
  to 
  generally 
  abrade 
  

   the 
  rock 
  surface, 
  due 
  possibly 
  to 
  clayey 
  character 
  of 
  the 
  subglacial 
  

   drift 
  in 
  this 
  district, 
  or 
  to 
  the 
  obliterative 
  effect 
  of 
  solution 
  and 
  

   weathering. 
  The 
  limestones 
  are 
  readily 
  attacked 
  by 
  atmospheric 
  

   waters, 
  as 
  proven 
  by 
  the 
  very 
  numerous 
  areas 
  of 
  solution 
  structures 
  

   and 
  open 
  joints 
  [p. 
  133, 
  pi. 
  26-27, 
  35]. 
  But 
  in 
  many 
  places 
  the 
  

   fresh 
  removal 
  of 
  clay 
  or 
  clayey 
  till 
  that 
  would 
  seem 
  to 
  be 
  sufficient 
  

   protection 
  to 
  the 
  rock 
  reveals 
  unglaciated 
  surface, 
  though 
  usually 
  

   firm 
  and 
  even, 
  as 
  if 
  a 
  glaciated 
  surface 
  had 
  lost 
  its 
  smoothness. 
  This 
  

   feature 
  is 
  emphasized 
  by 
  the 
  finding 
  in 
  the 
  same 
  locality 
  surfaces 
  

  

  1 
  Unf 
  ortunatel}^ 
  we 
  have 
  no 
  standard 
  or 
  measure 
  of 
  the 
  intensity 
  of 
  ice 
  

   abrasion 
  or 
  erosion, 
  or 
  glaciation 
  in 
  general. 
  When 
  a 
  writer 
  says 
  that 
  the 
  

   drift 
  is 
  scanty 
  or 
  abundant, 
  that 
  erosion 
  has 
  been 
  great 
  or 
  small, 
  he 
  ex- 
  

   presses 
  merely 
  his 
  own 
  conception 
  of 
  relative 
  intensity, 
  based 
  on 
  his 
  obser- 
  

   vational 
  experience. 
  It 
  is 
  apparent 
  that 
  different 
  observers 
  might 
  have 
  

   different 
  opinions, 
  according 
  to 
  the 
  range 
  of 
  their 
  work 
  and 
  their 
  mental 
  

   attitude. 
  Moreover, 
  the 
  view 
  of 
  the 
  same 
  student 
  might 
  vary 
  with 
  increas- 
  

   ing 
  experience 
  and 
  changing 
  emphasis 
  on 
  the 
  various 
  elements 
  or 
  factors. 
  

  

  