﻿46 
  NEW 
  YORK 
  STATE 
  MUSEUM 
  

  

  region 
  southwest 
  of 
  the 
  Adirondacks. 
  Most 
  of 
  the 
  common 
  Adiron- 
  

   dack 
  rock 
  types 
  are 
  strewn 
  over 
  the 
  region 
  and 
  they 
  gradually 
  

   diminish 
  in 
  number 
  as 
  the 
  distance 
  from 
  the 
  mountains 
  becomes 
  

   greater. 
  This 
  subject 
  has 
  been 
  discussed 
  in 
  a 
  paper 
  by 
  Brigham.^ 
  

  

  Thus, 
  bearing 
  in 
  mind 
  all 
  the 
  facts, 
  the 
  writer 
  is 
  strongly 
  of 
  the 
  

   opinion 
  that 
  when 
  the 
  ice 
  in 
  its 
  southward 
  movement 
  struck 
  the 
  

   Adirondacks, 
  it 
  was 
  divided 
  into 
  two 
  currents 
  flowing 
  around 
  the 
  

   mountains 
  and 
  meeting 
  in 
  the 
  Mohawk 
  valley 
  ; 
  that 
  during 
  the 
  time 
  

   of 
  maximum 
  glaciation 
  there 
  was 
  a 
  strong 
  general 
  southwesterlv 
  

   current, 
  but 
  that 
  the 
  border 
  currents 
  continued 
  as 
  under 
  currents 
  

   (more 
  or 
  less 
  checked 
  in 
  velocity) 
  ; 
  and 
  that 
  after 
  the 
  disappearance 
  

   of 
  the 
  ice 
  sheet 
  from 
  the 
  central 
  Adirondacks, 
  border 
  currents 
  were 
  

   maintained. 
  According 
  to 
  this 
  the 
  Port 
  Leyden 
  quadrangle 
  was 
  

   first 
  invaded 
  by 
  a 
  tongue 
  of 
  ice 
  which 
  flowed 
  southeastward 
  up 
  the 
  

   Black 
  river 
  valley. 
  When 
  the 
  general 
  ice 
  sheet 
  had 
  here 
  reached 
  

   a 
  thickness 
  of 
  several 
  thousand 
  feet 
  the 
  main 
  current 
  was 
  south- 
  

   westerly, 
  but 
  with 
  a 
  southeasterly 
  under 
  current 
  in 
  the 
  valley 
  bot- 
  

   tom. 
  The 
  ice 
  first 
  melted 
  from 
  the 
  highlands 
  and 
  left 
  a 
  tongue 
  of 
  

   ice 
  in 
  the 
  valley 
  which 
  gradually 
  melted 
  and 
  retreated 
  northward. 
  

  

  ICE 
  EROSION 
  

  

  Erosion 
  of 
  the 
  Precambric 
  rocks 
  

  

  As 
  the 
  ice 
  moved 
  across 
  the 
  quadrangle, 
  the 
  preglacial 
  rock 
  sur- 
  

   face 
  was 
  more 
  or 
  less 
  scratched, 
  polished 
  and 
  eroded. 
  In 
  the 
  case 
  

   of 
  the 
  Precambric 
  rocks 
  it 
  is 
  doubtful 
  if 
  the 
  ice 
  did 
  any 
  very 
  deej) 
  

   cutting. 
  Its 
  work 
  of 
  erosion 
  involved 
  mostly 
  the 
  removal 
  of 
  masses 
  

   of 
  decayed 
  and 
  weathered 
  rock 
  material 
  near 
  the 
  surface. 
  The 
  evi- 
  

   dence 
  is 
  conclusive 
  that 
  the 
  weathered 
  materials 
  were 
  rathor 
  

   thoroughly 
  scraped 
  off 
  the 
  Precambrics 
  as 
  shown 
  by 
  the 
  remarkable 
  

   freshness 
  of 
  the 
  rocks 
  wherever 
  exposed 
  and 
  by 
  the 
  smoothed 
  and 
  

   rounded 
  character 
  of 
  the 
  outcrops 
  [see 
  pi. 
  3]. 
  The 
  highly 
  jointed 
  

   character 
  of 
  these 
  rocks 
  no 
  doubt 
  greatly 
  aided 
  the 
  ice 
  in 
  its 
  work 
  

   of 
  erosion. 
  Mention 
  should 
  be 
  made 
  of 
  the 
  great 
  number 
  of 
  erratics 
  

   of 
  Precambric 
  rock 
  material 
  strewn 
  over 
  much 
  of 
  the 
  region, 
  

   especially 
  toward 
  the 
  south. 
  One 
  of 
  these 
  erratics 
  measures 
  about 
  

   17 
  feet 
  high 
  and 
  27 
  feet 
  across 
  [see 
  pi. 
  11]. 
  The 
  larger 
  ones 
  are 
  

   mostly 
  of 
  the 
  hard, 
  homogeneous 
  syenite 
  or 
  granite. 
  Probably 
  

   the 
  greatest 
  amount 
  of 
  erosion 
  of 
  the 
  Precambrics 
  occurred 
  along 
  

   Black 
  river 
  between 
  Lyons 
  Falls 
  and 
  Lowville, 
  but 
  this 
  matter 
  will 
  

   be 
  referred 
  to 
  below. 
  

  

  ^ 
  loc. 
  cit. 
  p. 
  213-28. 
  

  

  