﻿Western 
  Pennsylvania. 
  211 
  

  

  terrace 
  rises 
  127 
  feet 
  above 
  the 
  Ohio, 
  according 
  to 
  a 
  lock-level 
  

   measurement 
  by 
  Mr. 
  Gilbert. 
  

  

  Similar 
  moraine-headed 
  terraces 
  occur 
  in 
  Ohio 
  on 
  the 
  Mus- 
  

   kingum, 
  Scioto, 
  and 
  Mad 
  rivers 
  and 
  their 
  tributaries, 
  and 
  

   seem 
  to 
  have 
  their 
  equivalent 
  in 
  terraces 
  on 
  the 
  lower 
  stretches 
  

   of 
  these 
  rivers 
  and 
  on 
  the 
  Ohio. 
  In 
  other 
  words, 
  there 
  is 
  a 
  

   general 
  system 
  of 
  deep 
  valley 
  gravels, 
  starting 
  from 
  the 
  mo- 
  

   raines 
  indicated 
  and 
  sweeping 
  down 
  the 
  valleys, 
  growing 
  pro- 
  

   gressively 
  finer 
  in 
  material. 
  Out 
  of 
  these 
  glacial 
  flood 
  deposits 
  

   a 
  system 
  of 
  terraces 
  has 
  been 
  cut 
  by 
  subsequent 
  erosion. 
  The 
  

   still 
  later 
  glacial 
  episodes 
  seem 
  to 
  have 
  introduced 
  modifying 
  

   elements, 
  but 
  these 
  are 
  unimportant 
  in 
  this 
  connection. 
  

  

  The 
  time 
  and 
  manner 
  of 
  origin 
  of 
  the 
  moraine-headed 
  terrace 
  

   planes 
  are 
  placed 
  beyond 
  question 
  by 
  their 
  morainic 
  connec- 
  

   tions. 
  They 
  are 
  clearly 
  the 
  products 
  of 
  the 
  streams 
  that 
  

   issued 
  from 
  the 
  glacier 
  during 
  the 
  moraine-forming 
  epoch. 
  

   The 
  carving 
  of 
  the 
  terraces 
  out 
  of 
  these 
  planes 
  was 
  chiefly 
  a 
  

   subsequent 
  work, 
  of 
  relatively 
  minor 
  importance 
  in 
  the 
  present 
  

   discussion. 
  The 
  coarseness 
  of 
  the 
  gravels 
  of 
  this 
  series 
  indi- 
  

   cates 
  vigorous 
  drainage, 
  which 
  in 
  turn 
  implies 
  an 
  open 
  valley 
  

   and 
  at 
  least 
  a 
  fair 
  gradient 
  below. 
  It 
  is 
  equally 
  evident 
  that 
  

   terraces 
  of 
  a 
  much 
  higher 
  level 
  and 
  different 
  gradient 
  could 
  

   not 
  have 
  been 
  formed 
  #t 
  the 
  same 
  time. 
  Minor 
  side-valley 
  

   terraces 
  might 
  have 
  been 
  formed 
  at 
  flood 
  stages, 
  but 
  only 
  to 
  

   the 
  height 
  of 
  the 
  maximum 
  floods, 
  and 
  these 
  must 
  have 
  had 
  

   the 
  same 
  slope 
  as 
  the 
  broad 
  flood 
  planes. 
  

  

  It 
  is 
  clear 
  that 
  the 
  upper 
  gravel-bearing 
  terraces 
  were 
  not 
  

   formed 
  at 
  the 
  same 
  stage 
  as 
  these 
  moraine-appended 
  ones, 
  for 
  

   they 
  are 
  not 
  only 
  of 
  a 
  different 
  type, 
  being 
  alluvium-covered 
  

   rock 
  platforms, 
  but 
  they 
  stand 
  high 
  above 
  most 
  of 
  the 
  morainic 
  

   heads 
  of 
  the 
  later 
  deposits 
  and 
  show 
  much 
  greater 
  antiquity 
  in 
  

   the 
  erosion 
  of 
  their 
  surfaces. 
  For 
  example, 
  at 
  Warren 
  the 
  old 
  

   gravels 
  have 
  an 
  altitude 
  of 
  1415 
  feet 
  above 
  sea-level, 
  with 
  a 
  

   terrace 
  at 
  1395 
  feet, 
  while 
  the 
  moraine-headed 
  flood 
  deposits 
  

   of 
  the 
  later 
  epoch 
  at 
  Russellburg, 
  eight 
  miles 
  upstream, 
  occur 
  

   at 
  about 
  1275 
  feet. 
  On 
  the 
  Beaver 
  river 
  the 
  moraine-headed 
  

   gravel 
  stream 
  has 
  an 
  elevation 
  of 
  about 
  830 
  feet, 
  while 
  along 
  

   the 
  valley 
  below 
  pebbles 
  referred 
  to 
  the 
  earlier 
  epoch 
  range 
  

   from 
  900 
  to 
  950 
  feet, 
  and 
  ten 
  miles 
  below 
  there 
  is 
  a 
  wide 
  rock- 
  

   based 
  terrace 
  at 
  about 
  885 
  feet. 
  But 
  these 
  higher 
  gravels 
  con- 
  

   tain 
  pebbles 
  of 
  granite 
  and 
  other 
  crystalline 
  rock, 
  whose 
  pres- 
  

   ence 
  is 
  only 
  to 
  be 
  accounted 
  for 
  through 
  glacial 
  agencies, 
  and 
  

   the 
  explanation 
  of 
  their 
  origin 
  must 
  embrace 
  that 
  element." 
  

  

  " 
  The 
  higher 
  glacial 
  gravels 
  antedated 
  those 
  of 
  the 
  moraine- 
  

   forming 
  epoch 
  by 
  the 
  measure 
  of 
  the 
  erosion 
  of 
  the 
  channel 
  

   through 
  the 
  old 
  drift 
  and 
  the 
  rock, 
  whose 
  mean 
  depth 
  here 
  is 
  

   about 
  300 
  feet, 
  of 
  which, 
  perhaps, 
  250 
  may 
  be 
  said 
  to 
  be 
  rock. 
  

  

  