﻿GEOLOGY 
  OF 
  THOUSAND 
  ISLANDS 
  REGION 
  I47 
  

  

  that 
  the 
  knobby 
  surface 
  of 
  the 
  crystallines 
  is 
  the 
  immensely 
  ancient 
  

   erosion 
  surface 
  of 
  the 
  Precambric 
  land 
  area, 
  which 
  had 
  been 
  buried 
  

   under 
  Potsdam 
  sediments 
  and 
  only 
  recently 
  uncovered. 
  Ice 
  erosion 
  

   seems 
  to 
  have 
  had 
  very 
  little 
  influence 
  in 
  shaping 
  the 
  surface, 
  merely 
  

   rounding 
  and 
  smoothing 
  the 
  knobs. 
  

  

  The 
  major 
  axes 
  of 
  the 
  knobs 
  are 
  roughly 
  parallel 
  with 
  the 
  valley 
  

   and 
  the 
  ice 
  movement, 
  but 
  the 
  relation 
  to 
  the 
  latter 
  is 
  mostly 
  casual 
  

   and 
  not 
  genetic. 
  The 
  struck 
  or 
  northwest 
  side 
  commonly 
  shows 
  

   more 
  erosion, 
  but 
  frequently 
  the 
  difference 
  is 
  not. 
  evident. 
  As 
  a 
  

   rule 
  the 
  crystallines 
  have 
  not 
  retained 
  their 
  striae 
  and 
  polish 
  as 
  well 
  

   as 
  the 
  Potsdam 
  sandstones. 
  

  

  Plains 
  of 
  erosion. 
  The 
  broad 
  plains, 
  either 
  rock 
  or 
  rock 
  floored, 
  

   are 
  regarded 
  as 
  tlie 
  product 
  of 
  long 
  eras 
  of 
  atmospheric 
  erosion 
  

   with 
  later 
  glacial 
  planing 
  and 
  a 
  finishing 
  touch 
  of 
  wave 
  smoothing. 
  

   They 
  are 
  found 
  in 
  districts 
  where 
  the 
  sedimentary 
  rocks 
  are 
  persist- 
  

   ent 
  in 
  considerable 
  thickness 
  so 
  as 
  to 
  cover 
  the 
  Precambric 
  and 
  the 
  

   lower 
  and 
  irregular 
  Potsdam. 
  Broad 
  tracts 
  of 
  this 
  class 
  consisting 
  

   of 
  upper 
  Potsdam 
  occur 
  south 
  of 
  Chippewa 
  Bay 
  and 
  toward 
  Alex- 
  

   andria 
  Bay. 
  Theresa 
  dolomite 
  forms 
  the 
  plain 
  north 
  of 
  Chippewa 
  

   Bay 
  and 
  covers 
  large 
  areas 
  on 
  the 
  parallel 
  of 
  Plessis 
  and 
  Clayton. 
  

   South 
  of 
  the 
  parallel 
  of 
  Lafargeville 
  the 
  plains 
  and 
  plateaus 
  are 
  

   limestones. 
  

  

  The 
  earlier 
  ice 
  sheets 
  seem 
  to 
  have 
  lifted 
  or 
  plucked 
  away 
  the 
  

   weathered 
  and 
  weak 
  superficial 
  layers 
  of 
  these 
  stratified 
  rocks 
  down 
  

   to 
  some 
  firm, 
  less 
  jointed 
  and 
  more 
  resistant 
  bed; 
  but 
  the 
  flatness 
  

   and 
  smoothness 
  of 
  these 
  level 
  stretches 
  is 
  partly 
  due 
  to 
  the 
  latest 
  

   action, 
  the 
  leveling 
  action 
  of 
  the 
  shallowing 
  waters. 
  The 
  glacial 
  

   drift 
  is 
  commonly 
  thin 
  on 
  these 
  plains 
  and 
  patches 
  of 
  bare 
  rock 
  are 
  

   very 
  frequent, 
  sometimes 
  acres 
  in 
  extent, 
  specially 
  on 
  the 
  Potsdam. 
  

   A 
  good 
  example 
  is 
  seen 
  at 
  Plessis, 
  which 
  village 
  was 
  formerly 
  called 
  

   '' 
  Flat 
  Rock." 
  On 
  the 
  highways 
  rock 
  frequently 
  occurs 
  in 
  unex- 
  

   pected 
  manner 
  and 
  often 
  forms 
  the 
  wagon 
  track 
  for 
  considerable 
  

   distance. 
  Although 
  glacial 
  polish 
  and 
  striae 
  occur 
  frequently 
  on 
  the 
  

   Potsdam 
  the 
  majority 
  of 
  exposures 
  have 
  either 
  lost 
  their 
  smooth- 
  

   ness 
  or 
  were 
  never 
  severely 
  rubbed. 
  On 
  the 
  other 
  strata 
  glaciated 
  

   surfaces 
  are 
  not 
  common. 
  

  

  These 
  plains 
  have 
  been 
  trenched 
  by 
  stream 
  erosion 
  and 
  many 
  of 
  

   the 
  valley 
  walls 
  are 
  yet 
  steep, 
  those 
  of 
  Chaumont 
  river 
  for 
  example. 
  

   The 
  dift'erential 
  erosion 
  of 
  the 
  several 
  strata 
  has 
  produced 
  scarps 
  

   or 
  benches 
  about 
  the 
  margins 
  of 
  the 
  higher 
  plains 
  which 
  are 
  fre- 
  

   quently 
  striking 
  features 
  of 
  the 
  landscapes 
  and 
  sometimes 
  are 
  per- 
  

   sistent 
  for 
  long 
  distances. 
  These 
  have 
  been 
  described 
  in 
  a 
  former 
  

  

  