﻿94 
  NEW 
  YORK 
  STATE 
  MUSEUM 
  

  

  When 
  the 
  ice 
  front 
  paused 
  for 
  a 
  considerable 
  time 
  upon 
  a 
  rather 
  

   flat 
  surface, 
  the 
  debris-laden 
  streams 
  emerging 
  from 
  the 
  ice 
  formed 
  

   what 
  is 
  called 
  an 
  overwash 
  plain 
  by 
  depositing 
  layers 
  of 
  sediment 
  

   over 
  the 
  flat 
  surface. 
  The 
  finest 
  illustration 
  of 
  such 
  an 
  overwash 
  

   plain 
  in 
  the 
  State 
  is 
  all 
  of 
  that 
  part 
  of 
  Long 
  island 
  lying 
  just 
  south 
  

   of 
  the 
  great 
  terminal 
  moraine, 
  and 
  known 
  as 
  the 
  Jamaica 
  plain 
  

   toward 
  the 
  east 
  (see 
  plate 
  12). 
  

  

  When 
  the 
  ice 
  front 
  extended 
  across 
  a 
  more 
  rugged 
  country, 
  

   with 
  valleys 
  sloping 
  away 
  from 
  the 
  ice, 
  the 
  large 
  glacial 
  streams, 
  

   heavy 
  laden 
  with 
  debris, 
  caused 
  more 
  or 
  less 
  deposition 
  of 
  materials 
  

   on 
  the 
  valley 
  bottoms 
  often 
  for 
  many 
  miles 
  beyond 
  the 
  ice 
  front. 
  

   Such 
  deposits, 
  known 
  as 
  valley 
  trains, 
  are 
  especially 
  well 
  developed 
  

   along 
  most 
  of 
  the 
  large 
  south-flowing 
  tributaries 
  of 
  the 
  upper 
  

   Susquehanna 
  river 
  in 
  the 
  Southwestern 
  plateau 
  province. 
  

  

  Glacial 
  boulders, 
  or 
  erratics, 
  have 
  already 
  been 
  referred 
  to 
  ; 
  they 
  

   are 
  simply 
  blocks 
  of 
  rock 
  or 
  boulders 
  from 
  the 
  top 
  of 
  the 
  ice 
  or 
  

   within 
  it 
  which 
  have 
  been 
  left 
  strewn 
  over 
  the 
  country 
  as 
  a 
  result 
  

   of 
  the 
  melting 
  of 
  the 
  ice. 
  They 
  vary 
  in 
  size 
  from 
  small 
  pebbles 
  

   to 
  those 
  of 
  many 
  tons 
  weight 
  (see 
  plate 
  40), 
  and 
  are 
  naturally 
  most 
  

   commonly 
  derived 
  from 
  the 
  harder 
  and 
  more 
  resistant 
  rock 
  forma- 
  

   tions. 
  Thus 
  erratics 
  from 
  the 
  Adirondacks 
  are 
  very 
  numerous 
  in 
  

   east-central 
  New 
  York, 
  some 
  having 
  even 
  been 
  transported 
  to 
  the 
  

   southern 
  border 
  of 
  the 
  State. 
  Erratics 
  are 
  often 
  found 
  high 
  up 
  

   on 
  the 
  mountains, 
  and 
  sometimes 
  they 
  have 
  been 
  left 
  stranded 
  in 
  

   remarkably 
  balanced 
  positions. 
  

  

  A 
  very 
  extensive 
  glacial 
  deposit, 
  called 
  the 
  ground 
  moraine, 
  is 
  

   simply 
  the 
  heterogeneous, 
  typically 
  unstratified, 
  debris 
  from 
  the 
  

   bottom 
  oi 
  the 
  ice 
  which 
  was 
  deposited, 
  sometimes 
  during 
  the 
  ice 
  

   advance, 
  but 
  most 
  often 
  during 
  its 
  melting 
  and 
  retreat. 
  When 
  it 
  is 
  

   mostly 
  very 
  fine 
  material 
  with 
  pebbles 
  or 
  boulders 
  scattered 
  through 
  

   its 
  mass, 
  it 
  is 
  known 
  as 
  till 
  or 
  boulder 
  clay. 
  The 
  pebbles 
  or 
  boulders 
  

   of 
  the 
  till 
  are 
  commonly 
  faceted 
  and 
  striated 
  as 
  a 
  result 
  of 
  having 
  

   been 
  rubbed 
  against 
  underlying 
  rock 
  formations. 
  

  

  Another 
  type 
  of 
  glacial 
  deposit 
  of 
  unusual 
  interest 
  is 
  the 
  drumlin 
  

   which 
  is, 
  in 
  reality, 
  only 
  a 
  special 
  form 
  of 
  ground 
  moraine 
  material 
  

   or 
  till. 
  The 
  typical 
  drumlins 
  of 
  New 
  York 
  State 
  are 
  low, 
  rounded 
  

   mounds 
  of 
  till 
  with 
  elliptical 
  bases 
  and 
  steeper 
  slopes 
  on 
  the 
  north 
  

   sides 
  and 
  with 
  long 
  axes 
  parallel 
  to 
  the 
  direction 
  of 
  ice 
  movement 
  

   (see 
  plate 
  42). 
  In 
  height 
  they 
  rarely 
  exceed 
  200 
  feet, 
  being 
  most 
  

   often 
  less 
  than 
  100 
  feet. 
  The 
  origin 
  of 
  the 
  drumlins 
  has 
  not 
  yet 
  

   been 
  satisfactorily 
  determined, 
  though 
  it 
  is 
  known 
  that 
  they 
  formed 
  

   near 
  the 
  margin 
  of 
  the 
  ice 
  either 
  by 
  the 
  erosion 
  of 
  an 
  earlier 
  drift 
  

  

  