﻿GEOLOGY 
  OF 
  THOUSAND 
  ISLANDS 
  REGION 
  1 
  29 
  

  

  northward, 
  we 
  are 
  in 
  doubt 
  as 
  to 
  the 
  correctness 
  of 
  this 
  view. 
  

   Certain 
  it 
  is, 
  however, 
  that 
  the 
  present 
  course 
  of 
  Indian 
  river 
  is 
  

   a 
  patchwork 
  of 
  various 
  preglacial 
  valleys, 
  the 
  modern 
  character 
  

   of 
  the 
  course 
  being 
  most 
  excellently 
  shown 
  at 
  Theresa 
  where 
  

   the 
  river 
  drops 
  80 
  feet, 
  from 
  a 
  shallow 
  valley 
  into 
  a 
  much 
  deeper 
  

   one, 
  entering 
  this 
  on 
  its 
  east 
  side 
  3 
  miles 
  below 
  its 
  valley 
  head, 
  

   with 
  cutting 
  of 
  a 
  short, 
  postglacial 
  gorge 
  in 
  the 
  old 
  vaUey 
  

   side. 
  

  

  Plateaus, 
  terraces, 
  scarps 
  

  

  With 
  the 
  streams 
  cutting 
  down 
  valleys 
  and 
  exposing 
  rock 
  

   formations 
  of 
  varying 
  age 
  and 
  resistance 
  in 
  their 
  valley 
  walls, 
  

   and 
  with 
  the 
  slow 
  widening 
  of 
  the 
  valleys, 
  the 
  stronger 
  rock 
  

   beds 
  of 
  the 
  region 
  tend 
  to 
  outcrop 
  in 
  cliff 
  form, 
  the 
  scarps 
  run- 
  

   ning 
  across 
  country 
  in 
  the 
  direction 
  of 
  strike, 
  and 
  curving 
  up 
  

   the 
  consequent 
  valleys 
  in 
  the 
  direction 
  of 
  dip. 
  The 
  stronger 
  

   cliffs 
  result 
  where 
  a 
  more 
  resistant 
  rock 
  overlies 
  a 
  considerably 
  

   less 
  resistant 
  one, 
  the 
  more 
  rapid 
  wear 
  of 
  the 
  underlying 
  rock 
  

   tending 
  to 
  keep 
  a 
  tolerably 
  steep 
  and 
  precipitous 
  cliff 
  front. 
  

   Where 
  the 
  differences 
  in 
  resistance 
  are 
  less, 
  or 
  where 
  rapid 
  

   changes 
  in 
  resistance 
  occur, 
  involving 
  no 
  great 
  thickness 
  of 
  

   rock, 
  low, 
  subdued 
  scarps 
  are 
  produced. 
  

  

  Furthermore, 
  where 
  an 
  overlying 
  formation 
  is 
  weaker 
  than 
  that 
  

   beneath, 
  rapid 
  wear 
  is 
  checked 
  at 
  the 
  upper 
  surface 
  of 
  the 
  lower 
  

   rock, 
  the 
  upper 
  rock 
  is 
  stripped 
  away 
  from 
  it 
  and 
  a 
  flat 
  bench 
  of 
  

   varying 
  breadth 
  is 
  produced, 
  separating 
  the 
  cliff 
  fronts 
  of 
  the 
  upper 
  

   and 
  lower 
  formations. 
  In 
  the 
  large 
  way, 
  ignoring 
  minor 
  complica- 
  

   ting 
  factors, 
  the 
  general 
  topography 
  of 
  our 
  district 
  is 
  of 
  this 
  type 
  : 
  

   flat 
  platforms 
  developed 
  on 
  the 
  surfaces 
  of 
  the 
  hard 
  layers, 
  and 
  

   cliff 
  fronts 
  which 
  mark 
  the 
  descent 
  from 
  one 
  rock 
  platform 
  to 
  the 
  

   next, 
  the 
  cliff 
  fronts 
  facing 
  toward 
  the 
  old 
  land 
  area, 
  in 
  this 
  case 
  to 
  

   the 
  north, 
  hence 
  often 
  called 
  infaces. 
  

  

  The 
  most 
  prominent 
  cliffs, 
  and 
  the 
  broadest 
  platforms 
  of 
  the 
  dis- 
  

   trict 
  are 
  those 
  of 
  the 
  Potsdam 
  sandstone, 
  as 
  it 
  usually 
  has 
  consider- 
  

   able 
  thickness, 
  is 
  the 
  strongest 
  or 
  most 
  resistant 
  of 
  the 
  Paleozoic 
  

   rocks, 
  and 
  more 
  enduring 
  than 
  much 
  of 
  the 
  Precambric, 
  on 
  which 
  it 
  

   rests. 
  The 
  Precambric 
  topography 
  has 
  already 
  been 
  described, 
  and 
  

   this 
  does 
  not 
  need 
  repetition. 
  The 
  Potsdam 
  is 
  thickest 
  where 
  the 
  

   underlying 
  Precambric 
  is 
  weakest, 
  the 
  bulk 
  of 
  the 
  remaining 
  Pots- 
  

   dam 
  rests 
  on 
  these 
  weaker 
  rocks, 
  this 
  being 
  notably 
  true 
  in 
  the 
  case 
  

   of 
  the 
  outliers. 
  Potsdam 
  cliffs 
  from 
  20 
  to 
  60 
  feet 
  high 
  are 
  abund- 
  

   ant 
  throughout 
  the 
  district, 
  and 
  are 
  absent 
  only 
  where 
  the 
  under- 
  

   lying 
  rock 
  is 
  granite 
  and 
  the 
  Potsdam 
  very 
  thin. 
  Broad 
  Potsdam 
  

  

  