﻿176 
  E\ 
  H. 
  Williams 
  — 
  Southeim 
  Ice 
  Limit 
  in 
  Pennsylvcmia. 
  

  

  the 
  advancing 
  ice. 
  The 
  glacial 
  discharges 
  and 
  final 
  ablation 
  

   were 
  made 
  under 
  all 
  conditions 
  of 
  freedom 
  or 
  damming, 
  as 
  the 
  

   ice-front 
  faced 
  an 
  ascending 
  or 
  descending 
  valley, 
  or 
  rested 
  

   against 
  a 
  ridge 
  which 
  it 
  could 
  not 
  surmount. 
  

  

  In 
  view 
  of 
  the 
  fact 
  that 
  many 
  writers 
  seem 
  to 
  have 
  forgot- 
  

   ten 
  how 
  ice 
  acts 
  in 
  its 
  first 
  advance 
  over 
  a 
  previously 
  unglaci- 
  

   ated 
  region, 
  it 
  seems 
  necessary 
  to 
  discuss 
  the 
  subject. 
  Ice 
  

   erodes 
  and 
  accumulates 
  and, 
  as 
  the 
  country 
  was 
  generally 
  

   covered, 
  the 
  erosion 
  was 
  of 
  the 
  surface 
  covered, 
  and 
  the 
  accu- 
  

   mulations 
  were 
  also 
  from 
  that 
  surface, 
  which, 
  before 
  the 
  

   advent 
  of 
  the 
  ice, 
  was 
  more 
  or 
  less 
  deeply 
  covered 
  with 
  soil 
  of 
  

   decomposition. 
  What 
  became 
  of 
  this 
  old 
  soil 
  and 
  its 
  more 
  or 
  

   less 
  decomposed 
  fragments? 
  In 
  the 
  west, 
  where 
  drainage 
  

   favored, 
  they 
  were 
  washed 
  away 
  by 
  the 
  glacial 
  floods 
  in 
  the 
  

   shape 
  of 
  mud 
  and 
  sand, 
  and 
  so 
  distributed 
  that 
  their 
  identity 
  

   is 
  lost. 
  It 
  is 
  only 
  where 
  drainage 
  has 
  been 
  opposed 
  by 
  the 
  

   ridges 
  and 
  upward 
  sloping 
  valleys 
  of 
  the 
  Appalachian 
  system, 
  

   and 
  all 
  waters 
  forced 
  to 
  escape 
  sub-glacially, 
  that 
  we 
  can 
  fully 
  

   study 
  the 
  deposits 
  of 
  the 
  first 
  glaciation. 
  

  

  The 
  first 
  advance 
  of 
  the 
  ice 
  carried 
  a 
  burden 
  of 
  rotten 
  mate- 
  

   rial. 
  The 
  soft 
  underlying 
  parts 
  were 
  immediately 
  powdered, 
  

   and 
  whatever 
  resisted 
  immediate 
  destruction 
  was 
  more 
  or 
  less 
  

   rotted 
  and 
  generally 
  oxidized 
  ; 
  but, 
  as 
  it 
  was 
  opposed 
  to 
  a 
  soft 
  

   surface, 
  its 
  fragments 
  were 
  only 
  rounded 
  into 
  a 
  highly 
  oxidized 
  

   gravel 
  that 
  was 
  mixed 
  with 
  the 
  clays 
  and 
  sands 
  of 
  the 
  old 
  soil. 
  

   It 
  was 
  only 
  after 
  a 
  continuance 
  of 
  erosion 
  that 
  the 
  solid 
  rocks 
  

   were 
  reached 
  and 
  attached 
  to 
  the 
  base 
  of 
  the 
  ice. 
  As 
  this 
  took 
  

   place 
  at 
  a 
  distance 
  from 
  the 
  ice 
  front, 
  the 
  fresher 
  fragments 
  

   were 
  carried 
  against 
  the 
  softer 
  surfaces 
  where 
  the 
  ice 
  had 
  as 
  

   much 
  of 
  a 
  cutting 
  effect 
  as 
  its 
  burden, 
  and 
  no 
  scoring 
  or 
  groov- 
  

   ing 
  occurred 
  — 
  only 
  a 
  planing 
  of 
  the 
  surface, 
  as 
  is 
  seen 
  at 
  

   Rauch's 
  gravel 
  pit 
  at 
  Bethlehem. 
  The 
  fresh 
  fragments 
  fir 
  ally 
  

   reached 
  the 
  terminal 
  deposits, 
  and 
  were 
  mixed 
  with 
  the 
  older 
  

   weathered 
  stuff 
  ; 
  but 
  were 
  stirred 
  up 
  with 
  it 
  and 
  are 
  found 
  at 
  

   all 
  depths, 
  as 
  on 
  the 
  crest 
  of 
  the 
  ridge 
  behind 
  South 
  Bethle- 
  

   hem, 
  where 
  in 
  a 
  rotten, 
  unstratified 
  deposit 
  of 
  great 
  thickness, 
  

   a 
  fresh 
  Calciferous 
  bowlder 
  lies 
  near 
  an 
  equally 
  fresh 
  Lower 
  

   Subcarboniferous 
  cobble, 
  on 
  rotten 
  gneiss, 
  and 
  under 
  rotten 
  

   and 
  angular 
  fragments 
  of 
  Potsdam. 
  When 
  a 
  retreat 
  of 
  the 
  

   ice 
  took 
  place, 
  and 
  a 
  halt 
  was 
  made 
  at 
  a 
  distance 
  in 
  the 
  rear, 
  

   the 
  accumulations 
  are 
  generally 
  of 
  fresh 
  fragments, 
  and 
  the 
  

   thoroughly 
  eroded 
  surfaces 
  show 
  an 
  abundance 
  of 
  groovings 
  

   and 
  striations. 
  Those 
  who 
  see 
  in 
  this 
  last 
  all 
  the 
  work 
  of 
  

   glaciation, 
  are 
  unfortunate, 
  in 
  that 
  they 
  accept 
  an 
  incomplete 
  

   sub-stage 
  of 
  the 
  work 
  for 
  its 
  entirety. 
  Secondary 
  advances 
  to 
  

   the 
  former 
  limits 
  of 
  glaciation 
  frequently 
  fail 
  to 
  remove 
  por- 
  

   tions 
  of 
  the 
  former 
  deposits, 
  and 
  cap 
  them 
  with 
  a 
  mass 
  of 
  fresh 
  

  

  