﻿PLEISTOCENE 
  GEOLOGY 
  OF 
  NASSAU 
  CO. 
  AND 
  BOBOUGH 
  OF 
  QUEENS 
  633 
  

  

  ing 
  them. 
  If 
  then, 
  we 
  reconstruct 
  the 
  cross-section 
  of 
  the 
  forma- 
  

   tion, 
  it 
  would 
  appear 
  as 
  a 
  wide 
  belt 
  of 
  gravels 
  and 
  sands 
  declining 
  

   soutliward 
  to 
  the 
  sea 
  and 
  rising 
  with 
  a 
  cuestalike 
  bluff 
  from 
  the 
  

   sound 
  on 
  the 
  north. 
  

  

  In 
  ordinary 
  nonglacial, 
  coastwise 
  beds, 
  such 
  a 
  bluff 
  would 
  indicate 
  

   the 
  retreat 
  of 
  the 
  beds, 
  extending 
  originally 
  north 
  to 
  an 
  overlap 
  on 
  

   the 
  mainland, 
  to 
  their 
  present 
  northern 
  limit. 
  But 
  in 
  the 
  case 
  of 
  

   glacial 
  deposits 
  laid 
  down 
  about 
  the 
  border 
  of 
  an 
  ice 
  sheet, 
  it 
  is 
  highly 
  

   probable 
  that 
  the 
  beds 
  never 
  thinned 
  out 
  landward 
  to 
  an 
  overlapping 
  

   series 
  on 
  the 
  ancient 
  gneiss 
  of 
  the 
  region 
  beyond 
  the 
  sound. 
  Some- 
  

   what 
  similar 
  glacial 
  gravels 
  and 
  sands 
  of 
  the 
  last 
  ice 
  epoch, 
  on 
  JSTan- 
  

   tucket, 
  end 
  abruptly 
  on 
  their 
  northern 
  or 
  iceward 
  margins 
  in 
  bluffs 
  

   overlooking 
  lower 
  ground 
  once 
  occupied 
  by 
  the 
  basal 
  portion 
  of 
  tlie 
  

   ice 
  sheet 
  against 
  whose 
  mural 
  front 
  they 
  were 
  laid 
  down.^ 
  Upham 
  ^ 
  

   has 
  expressed 
  his 
  belief 
  in 
  the 
  origin 
  of 
  these 
  gravels 
  and 
  sands 
  in 
  this 
  

   manner, 
  differing 
  only 
  from 
  the 
  view 
  here 
  set 
  forth 
  in 
  that 
  he 
  sup- 
  

   poses 
  the 
  beds 
  to 
  be 
  essentially 
  contemporaneous 
  with 
  the 
  moraines 
  

   which 
  rise 
  above 
  their 
  level. 
  

  

  It 
  is 
  evident 
  that 
  these 
  Columbia 
  beds, 
  exposed 
  in 
  the 
  bluffs 
  and 
  

   rude 
  terraces 
  along 
  the 
  north 
  coast 
  of 
  the 
  island, 
  may 
  once 
  have 
  

   extended 
  much 
  farther 
  to 
  the 
  northward, 
  but 
  how 
  much 
  farther 
  

   into 
  the 
  area 
  of 
  the 
  sound 
  is 
  not 
  now 
  definitely 
  determinable. 
  

   Their 
  occurrence 
  on 
  the 
  Connecticut 
  mainland 
  has 
  not 
  as 
  yet 
  been 
  

   reported, 
  and 
  till 
  that 
  area 
  is 
  carefully 
  studied 
  with 
  this 
  problem 
  in 
  

   mind, 
  it 
  can 
  hardly 
  be 
  satisfactorily 
  settled. 
  The 
  same 
  indefinite 
  

   answer 
  is 
  elicited 
  from 
  a 
  study 
  of 
  the 
  equivalent 
  beds 
  on 
  Block 
  

   island 
  and 
  Marthas 
  Yineyard. 
  In 
  other 
  words, 
  the 
  precise 
  position 
  

   of 
  the 
  ice 
  front 
  and 
  terminal 
  moraine 
  of 
  this 
  earlier 
  ice 
  advance 
  is 
  

   unknown, 
  though 
  it 
  could 
  not 
  have 
  been 
  many 
  miles 
  north 
  of 
  the 
  

   inner 
  limit 
  of 
  these 
  gravels 
  and 
  sands 
  with 
  their 
  intercalated 
  bed 
  of 
  

   true 
  till. 
  

  

  Aside 
  from 
  the 
  disturbances 
  above 
  noted, 
  two 
  classes 
  of 
  changes 
  

   have 
  affected 
  these 
  beds 
  since 
  their 
  deposition 
  : 
  1) 
  the 
  discoloration 
  

   of 
  the 
  beds 
  by 
  local 
  and 
  secular 
  chemical 
  changes 
  in 
  the 
  iron-bear- 
  

  

  1 
  Curtis, 
  G. 
  C. 
  & 
  Woodworth, 
  J. 
  B. 
  Jour. 
  geol. 
  Chicago, 
  1899. 
  7 
  : 
  226-36. 
  

   ^ 
  Upham, 
  Warren. 
  Glacial 
  history 
  of 
  the 
  New 
  England 
  islands. 
  Am, 
  geol. 
  

   1899. 
  24 
  : 
  79-89, 
  with 
  bibliography, 
  p. 
  89-92. 
  

  

  