﻿56 
  NEW 
  YORK 
  STATE 
  MUSEUM 
  

  

  of 
  glaciers, 
  as 
  is 
  well 
  known, 
  has 
  a 
  certain 
  amount 
  of 
  plasticity 
  and 
  

   will 
  flow 
  under 
  the 
  pressure 
  of 
  its 
  own 
  weight, 
  somewhat 
  after 
  the 
  

   manner 
  of 
  a 
  mass 
  of 
  pitch. 
  The 
  flow 
  of 
  the 
  great 
  Laurentian 
  glacier 
  

   was 
  outward 
  in 
  all 
  directions 
  from 
  the 
  center 
  of 
  accumulation, 
  local 
  

   topographic 
  features 
  exerting 
  a 
  deflecting 
  influence 
  only 
  in 
  the 
  more 
  

   attenuated 
  marginal 
  portions. 
  In 
  its 
  basal 
  portions, 
  the 
  ice 
  was 
  well 
  

   supplied 
  with 
  rock 
  debris, 
  from 
  the 
  finest 
  rock 
  flour 
  and 
  clay 
  to 
  

   boulders 
  often 
  of 
  very 
  great 
  size. 
  This 
  material 
  was 
  derived 
  from 
  

   the 
  surface 
  over 
  which 
  the 
  ice 
  flowed, 
  and 
  it 
  measured 
  in 
  part 
  the 
  

   amount 
  of 
  erosive 
  work 
  which 
  the 
  ice 
  had 
  accomplished. 
  The 
  rock 
  

   fragments 
  frozen 
  into 
  the 
  bottom 
  of 
  the 
  moving 
  ice 
  mass, 
  served 
  as 
  

   efficient 
  tools 
  for 
  grooving 
  and 
  scratching 
  the 
  bedrock 
  over 
  which 
  

   the 
  ice 
  flowed, 
  while 
  at 
  the 
  same 
  time 
  the 
  finer 
  material 
  smoothed 
  

   and 
  polished 
  the 
  rock 
  surfaces. 
  The 
  direction 
  of 
  the 
  grooves 
  and 
  

   striae 
  on 
  the 
  rock 
  surfaces 
  in 
  general 
  indicate 
  the 
  direction 
  of 
  the 
  

   movement 
  of 
  the 
  ice 
  which 
  produced 
  them, 
  but 
  this 
  may 
  not 
  always 
  

   represent 
  the 
  direction 
  of 
  general 
  ice 
  movement 
  for 
  the 
  region, 
  

   since, 
  at 
  the 
  time 
  of 
  making 
  the 
  striae, 
  the 
  ice 
  may 
  have 
  been 
  thin 
  

   enough 
  to 
  be 
  influenced 
  by 
  the 
  local 
  topographic 
  features 
  of 
  the 
  

   region. 
  In 
  the 
  Niagara 
  district 
  the 
  striae 
  have 
  a 
  direction 
  extending 
  

   about 
  30° 
  west 
  of 
  south 
  (Gilbert) 
  which 
  direction, 
  being 
  inhar- 
  

   monious 
  with 
  the 
  trend 
  of 
  the 
  lowlands, 
  indicates 
  that 
  these 
  striae 
  

   were 
  formed 
  by 
  the 
  general 
  movement 
  of 
  the 
  ice, 
  rather 
  than 
  by 
  

   local 
  movements, 
  controlled 
  by 
  topography.^ 
  

  

  While 
  the 
  surface 
  rocks 
  of 
  this 
  region 
  were 
  everywhere 
  scratched 
  

   and 
  polished 
  by 
  the 
  ice, 
  these 
  markings 
  are 
  only 
  exhibited 
  where 
  

   the 
  protecting 
  mantle 
  of 
  loose 
  surface 
  material 
  or 
  drift 
  has 
  been 
  

   recently 
  removed. 
  For 
  where 
  the 
  polished 
  rock 
  surfaces 
  are 
  ex- 
  

   posed 
  for 
  any 
  considerable 
  period 
  of 
  time, 
  weathering 
  usually 
  

   obliterates 
  these 
  superficial 
  markings. 
  The 
  best 
  place 
  in 
  which 
  the 
  

   striae 
  of 
  the 
  region 
  about 
  Niagara 
  river 
  may 
  be 
  studied 
  is 
  near 
  the 
  

   quarries 
  on 
  the 
  edge 
  of 
  the 
  escarpment, 
  a 
  mile 
  or 
  more 
  west 
  of 
  

   Brock's 
  monument, 
  where 
  the 
  ledges 
  are 
  progressively 
  uncovered 
  

   previous 
  to 
  quarrying. 
  

  

  ^For 
  an 
  account 
  of 
  the 
  glacial 
  sculpture 
  in 
  this 
  region, 
  see 
  Gilbert. 
  BuL 
  

   geol. 
  soc. 
  Am. 
  1899. 
  10:121. 
  

  

  