﻿E. 
  H. 
  Williams 
  — 
  Southern 
  Ice 
  Limit 
  in 
  Pennsylvania. 
  177 
  

  

  material 
  gathered 
  from 
  the 
  rear. 
  Although 
  there 
  may 
  be 
  great 
  

   differences 
  in 
  appearance, 
  there 
  may 
  be 
  little 
  variation 
  in 
  age, 
  

   and 
  as 
  it 
  is 
  generally 
  accepted 
  that 
  the 
  erosive 
  power 
  of 
  the 
  

   edge 
  is 
  slight, 
  we 
  may 
  have 
  a 
  succession 
  of 
  deposits 
  varying 
  

   from 
  a 
  rotten 
  gravel 
  at 
  the 
  bottom 
  to 
  a 
  fresh 
  one 
  at 
  the 
  sur- 
  

   face, 
  and 
  all 
  may 
  have 
  been 
  formed 
  within 
  a 
  short 
  period, 
  

   dustiness 
  of 
  gravel, 
  therefore, 
  is 
  no 
  criterion 
  of 
  age, 
  and 
  all 
  

   arguments 
  based 
  upon 
  it 
  must 
  fall. 
  

  

  The 
  bulk 
  of 
  the 
  deposits 
  is 
  also 
  used 
  by 
  some 
  as 
  an 
  evidence 
  

   of 
  the 
  duration 
  of 
  glaciation. 
  This 
  is 
  faulty, 
  as 
  regards 
  the 
  

   first 
  advance, 
  as 
  the 
  first 
  advance 
  evidently 
  removed 
  the 
  rotten 
  

   surface 
  vastly 
  faster 
  than 
  it 
  did 
  the 
  more 
  resisting 
  rocks 
  at 
  

   depth.; 
  so 
  that 
  comparative 
  estimates 
  based 
  on 
  thicknesses 
  of 
  

   fresh 
  material 
  on 
  the 
  one 
  hand, 
  and 
  rotten 
  or 
  rusty 
  stuff 
  on 
  the 
  

   other, 
  are 
  valueless, 
  unless 
  it 
  can 
  be 
  conclusively 
  shown 
  that 
  

   the 
  rotten 
  and 
  rusty 
  deposit 
  was 
  fresh 
  when 
  formed. 
  

  

  Character 
  of 
  the 
  Deposits. 
  

  

  South 
  of 
  Lewis 
  and 
  Wright's 
  moraine 
  the 
  deposits 
  are 
  gener- 
  

   ally 
  angular, 
  or 
  sub 
  angular, 
  and 
  rusty 
  accumulations 
  from 
  the 
  

   old 
  surface. 
  The 
  formations, 
  where 
  exposed, 
  are 
  generally 
  

   unstriated, 
  and 
  the 
  thickness 
  of 
  till 
  dependent 
  on 
  the 
  contour. 
  

   In 
  general 
  the 
  determination 
  is 
  made 
  by 
  finding 
  fragments 
  of 
  

   formations 
  moved 
  out 
  of 
  place 
  in 
  a 
  uniform 
  direction, 
  and 
  

   independent 
  of 
  the 
  slope. 
  It 
  is 
  a 
  favorable 
  condition 
  when 
  a 
  

   river 
  which 
  drains 
  a 
  part 
  of 
  the 
  glaciated 
  area 
  crosses 
  the 
  path 
  

   of 
  another 
  portion 
  of 
  the 
  ice, 
  as 
  the 
  gravels 
  are 
  taken 
  from 
  the 
  

   river 
  and 
  distributed 
  so 
  as 
  to 
  show 
  the 
  direction 
  of 
  the 
  move- 
  

   ment. 
  The 
  broad 
  valleys 
  of 
  the 
  Mauch 
  Chunk 
  (Upper 
  Sub- 
  

   carboniferous) 
  formation 
  are 
  especially 
  useful 
  in 
  marking 
  the 
  

   ice 
  front, 
  as 
  the 
  contrast 
  between 
  their 
  dark 
  red 
  country 
  rock 
  

   and 
  the 
  Pottsville 
  conglomerate 
  on 
  the 
  one 
  hand, 
  and 
  the 
  

   Pocono 
  (Lower 
  Subcarb.) 
  on 
  the 
  other, 
  is 
  so 
  distinct 
  that 
  the 
  

   erratics 
  are 
  distinguished 
  at 
  a 
  distance. 
  On 
  the 
  other 
  hand, 
  

   there 
  is 
  little 
  difference 
  between 
  fine 
  fragments 
  of 
  Mauch 
  

   Chunk 
  and 
  Catskill 
  when 
  weathered, 
  and 
  the 
  latter 
  when 
  

   leached 
  much 
  resembles 
  Chemung, 
  so 
  that 
  in 
  non-fossiliferous 
  

   localities 
  the 
  evidences 
  are 
  few 
  and 
  far 
  between. 
  In 
  the 
  event 
  

   of 
  finding 
  distributed 
  gravels 
  from 
  the 
  rivers, 
  they 
  are 
  uni- 
  

   formly 
  found 
  just 
  over 
  the 
  crest 
  of 
  the 
  ridges 
  opposite 
  to 
  the 
  

   direction 
  from 
  which 
  they 
  came. 
  * 
  This 
  is 
  well 
  shown 
  in 
  going 
  

   south 
  from 
  Danville 
  to 
  Shamokin. 
  In 
  ten 
  or 
  twelve 
  miles 
  the 
  

   evidences 
  of 
  glaciation 
  (as 
  shown 
  by 
  river 
  gravels) 
  can 
  be 
  com- 
  

   pressed 
  within 
  a 
  few 
  hundred 
  feet. 
  In 
  the 
  valleys 
  well-sections 
  

   were 
  very 
  useful, 
  and 
  especially 
  when 
  force 
  pumps 
  were 
  neces- 
  

   sary 
  from 
  depth 
  of 
  water. 
  Quicksands 
  are 
  found 
  against 
  the 
  

  

  