﻿14 
  NEW 
  YORK 
  STATE 
  MUSEUM 
  

  

  Close 
  of 
  the 
  long 
  period 
  of 
  erosion 
  

  

  Eventually 
  this 
  long" 
  period 
  of 
  surface 
  wear 
  on 
  a 
  land 
  area 
  drew 
  

   to 
  a 
  close, 
  and 
  for 
  a 
  time 
  the 
  history 
  of 
  the 
  region 
  became 
  of 
  very 
  

   different 
  nature, 
  irt 
  other 
  words 
  instead 
  of 
  loss 
  of 
  surface 
  material 
  

   it 
  began 
  to 
  g-ain 
  it 
  in 
  the 
  shape 
  of 
  deposits 
  on 
  the 
  old, 
  worn 
  land 
  

   surface. 
  These 
  deposits 
  blanketed 
  and 
  preserved 
  the 
  old 
  erosion 
  

   surface, 
  and 
  since 
  the 
  wear 
  of 
  today 
  has 
  come 
  down 
  to 
  that 
  precise 
  

   horizon 
  over 
  parts 
  of 
  the 
  district, 
  and 
  the 
  overlying 
  deposits 
  are 
  

   being 
  peeled 
  away 
  from 
  it, 
  it 
  is 
  returning 
  to 
  daylight 
  with 
  precisely 
  

   the 
  characters 
  it 
  possessed 
  when 
  it 
  was 
  buried 
  and 
  preserved 
  ages 
  

   ago. 
  Seldom 
  does 
  a 
  district 
  reveal 
  so 
  abundant 
  and 
  clear 
  evidence 
  

   of 
  the 
  nature 
  of 
  an 
  old 
  fossil 
  land 
  surface. 
  It 
  is 
  clear 
  from 
  its 
  

   study 
  that 
  long 
  wear 
  had 
  reduced 
  it 
  to 
  a 
  surface 
  of 
  comparatively 
  

   slight 
  relief, 
  showing 
  that 
  no 
  considerable 
  elevation 
  of 
  the 
  region 
  

   occurred 
  during 
  the 
  latter 
  portion 
  of 
  the 
  long 
  erosion 
  interval. 
  

   Nevertheless 
  it 
  is 
  very 
  far 
  from 
  being 
  a 
  plane 
  surface, 
  but 
  is 
  of 
  

   considerable 
  minor 
  relief, 
  of 
  low 
  ridges 
  and 
  shallow 
  valleys, 
  or 
  of 
  

   low 
  knobs 
  and 
  basins, 
  the 
  depressions 
  eaten 
  out 
  on 
  the 
  weaker 
  rocks, 
  

   chiefly 
  the 
  Grenville 
  limestones 
  and 
  some 
  of 
  the 
  schists, 
  while 
  the 
  

   more 
  elevated 
  ridges 
  and 
  knobs 
  are 
  due 
  to 
  the 
  resisting 
  qualities 
  of 
  

   the 
  Grenville 
  quartzites 
  and 
  of 
  many 
  of 
  the 
  igneous 
  rocks. 
  The 
  

   knob 
  structure 
  is 
  practically 
  confined 
  to 
  the 
  igneous 
  rock 
  areas, 
  

   chiefly 
  in 
  the 
  Laurentian 
  gneiss. 
  

  

  While 
  the 
  region 
  therefore 
  is 
  quite 
  rugged 
  in 
  a 
  mild 
  fashion, 
  the 
  

   extreme 
  differences 
  in 
  altitude 
  are 
  but 
  slight. 
  One 
  hundred 
  feet 
  is 
  

   about 
  the 
  measure 
  of 
  difference. 
  Seldom 
  does 
  the 
  difference 
  in 
  level 
  

   between 
  valley 
  bottom 
  and 
  ridge 
  crest 
  reach 
  that 
  figure, 
  and 
  rarely 
  

   does 
  it 
  exceed 
  it. 
  This 
  is 
  a 
  small 
  difference, 
  considering 
  the 
  wide 
  

   variation 
  in 
  resisting 
  power 
  to 
  wear 
  which 
  the 
  various 
  rocks 
  present 
  

   and 
  is 
  indicative 
  of 
  a 
  long 
  period 
  of 
  wear 
  under 
  comparatively 
  stable 
  

   conditions 
  of 
  level. 
  

  

  Paleozoic 
  sediments 
  

   Potsdam 
  sandstone. 
  A 
  change 
  in 
  conditions 
  followed 
  and 
  de- 
  

   position 
  of 
  sand 
  commenced 
  upon 
  this 
  old 
  land. 
  surface. 
  It 
  natur- 
  

   ally 
  began 
  on 
  the 
  valley 
  bottoms 
  and 
  encroached 
  on 
  the 
  ridges 
  only 
  

   as 
  the 
  valleys 
  filled. 
  The 
  old 
  limestone 
  surfaces 
  were 
  pitted 
  by 
  

   small 
  depressions, 
  and 
  were 
  somewhat 
  intersected 
  with 
  widened 
  

   joint 
  cracks 
  also, 
  and 
  in 
  these 
  the 
  first 
  materials 
  collected, 
  some- 
  

   times 
  full 
  of 
  coarse 
  fragments 
  of 
  resistant 
  thin 
  quartzite 
  bands 
  or 
  

  

  